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http://Awww.archive.org/details/cu31924003200106
THE STRUCTURE AND FUNCTIONS
OF
BACTERIA
HENRY FROWDE, M.A.
PUBLISHER TO THE UNIVERSITY OF OXFORD
LONDON, EDINBURGH, AND NEW YORK
THE
STRUCTURE AND FUNCTIONS
ele
BACTERIA
BY
ALFRED FISCHER
PROFESSOR OF BOTANY AT THE UNIVERSITY OF LEIPZIG
TRANSLATED INTO ENGLISH BY
A. COPPEN JONES
Orford
AT THE CLARENDON PRESS
1900
PRINTED AT THE CLARENDON PRESS
BY HORACE HART, M.A.
PRINTER TO THE UNIVERSITY
AUTHOR'S PREFACE
SEEING that there is at the present time no lack of treatises
upon bacteria, the publication of yet another needs some apology.
Such an apology is expressed in the title, Lectures on Bacteria*.
The lectures are intended to be an introduction to general bac-
teriology. They purpose to give a survey that shall collect and
condense the innumerable special researches into a connected
whole, and indicate in broad outlines the present position and
extent of bacteriological science.
Besides those medical aspects of bacteriology which in other
treatises occupy, rightly enough, the chief place, attention is drawn
to the importance of bacteria in agriculture, and to the parts they
play in the great fundamental processes of life—the circulation of
nitrogen and carbon dioxide. Furthermore, it seemed desirable to
point out and emphasize the advancement that general physiology
has received from bacteriological investigations. Finally, an
attempt has been made to remove the bacteria from the isolated
position to which their morphological and physiological pecu-
liarities had relegated them, and, by comparative studies, to
indicate their relations to other organisms.
A treatment of the subject on these lines that should be at the
same time not too bulky, seemed to me to be wanting. I there-
fore undertook the publication of the course of lectures I have
delivered for some years to students of biology, pharmacy, and
agriculture, with here and there among them—as it were like a
white raven!—a medical student.
ALFRED FISCHER.
LEIPZIG,
22 July, 1897.
* The title of the German work is Vorlesungen tiber Bakterien.
TRANSLATOR’S PREFACE
In offering to English readers a translation of Professor
Alfred Fischer's Vorlesungen iiber Bakterien, an Introduction is
rendered almost unnecessary, in the first place by the favourable
reception of the original German edition, and secondly, by the
fact that no work on Bacteriology of similar scope and mode
of treatment has appeared in England since De Bary’s classical
Lectures on Bacteria, But those Lectures were published in
1887, and the twelve years that have passed since then have
been years teeming with activity in every field of bacteriological
research. The present work epitomizes, in a comparatively small
space, the results of these investigations, and attempts to elucidate
them in their relations to the many-sided problems of Pathology,
and of Technical and Agricultural Chemistry, and the great
chemical processes of nature.
It should be stated that the few variations from the original
that occur have been made with the Author’s sanction, and that,
furthermore, the book has enjoyed the advantage of a proof-
revision by Professor Marshall Ward of Cambridge. The Index
is the work of Dr. Alfred J. Ewart.
ASC).
Davos PLatz,
March 13, 1900.
CONTENTS
CHAPTER I PAGE
INTRODUCTION: MORPHOLOGY
Form, Size, and Structure of the Bacterial Cell, Cell-membrane and Cell-contents
Morphology of the Bacterial Cell . . : : ‘ F a ‘ , ‘ 2 2
Finer Structure of the Bacterial Cell
CHAPTER II
MORPHOLOGY (continued)
Pigments, Intracellular Products ; Movement and Organs of Locomotion; Cell-division; Spore-
formation and Germination . ; é ‘ ‘ : ‘i , : : 4 ae
Movements and Organs of Locomotion . : ‘ : : ; ‘ ‘ 2 ‘ - 14
Reproduction of Bacteria by Fission . ' , ; : ‘ , ‘ » 16
Spores and Sporulation ‘ : : 4 ; ‘ ‘ : : » 19
CHAPTER III
TAXONOMY
The Question of ‘Species’ among Bacteria; Variability; Involution and Attenuation; the
Classification of Bacteria ‘ é : ‘ : ‘ ‘ , ‘ . ‘ « 24
CHAPTER IV
TAXONOMY (continued)
The Systematic Position of Bacteria; Other low Organisms with Pathogenic Properties . - 35
CHAPTER V
' DISTRIBUTION AND ORIGIN
Distribution of Bacteria ; Their modes of life; Spontaneous Generation . , ‘ : Ad
CHAPTER VI
PHYSIOLOGY OF NUTRITION AND GENERAL PRINCIPLES OF CULTURE
Chemical Composition of Bacteria ‘ 4 ‘ : F ‘ : F ‘ : 52
Food Stuffs of Bacteria . 5 : : ‘ Z ‘ 2 : : ‘ i 83
CHAPTER VII
RESPIRATION OF BACTERIA
Aerobiosis and Anaerobiosis; Light-producing Bacteria; Marine Bacteria; Sulphur and Iron
Bacteria : . . - F s 2 : : i ‘ . ; : . 60
CHAPTER VIII
INFLUENCE OF PHYSICAL AGENTS
Light, Electricity; Pressure, Temperature, Dryness and Moisture ; Disinfection by means of
Physical Agents. : s z 2 ' ; . . . : - 71
CHAPTER IX
THE ACTION OF CHEMICALS
Chemotaxis and Chemical Disinfection . ‘ ‘ 7 . : ‘i i ‘ . 78
vili CONTENTS
CHAPTER X PAGE
BACTERIA AND THE CIRCULATION OF NITROGEN IN NATURE
1. Introduction: The Assimilation of Free Nitrogen by the Bacteria of the Root Nodules of
Leguminous Plants, and by Bacteria inthe Soil . ; . i . » + 88
CHAPTER XI
BACTERIA AND THE CIRCULATION OF NITROGEN (continued)
2. The Liberation of Organic Nitrogen by Putrefaction, and its Mineralization by the Nitri-
fying Bacteria : F ; : ; : ‘ ‘ : : , : - 98
CHAPTER XII
THE CIRCULATION OF CARBON DIOXIDE IN NATURE
1. Introduction ; Organized Ferments and Enzymes; Races of Ferment Organisms ; Fermen-
tation of Alcohol and of Acids; Optical Activity and Fermentation : : : » 107
CHAPTER XIII
THE CIRCULATION OF CARBON DIOXIDE IN NATURE (continued)
2. Bacterial Fermentation of Carbohydrates . : ‘ : ‘ ‘ 3 . i . 116
Milk and other Dairy Products. ‘ : : ; : : : ‘ a ee) © 6
Butyric Fermentation . : ‘i ‘ F ‘ : : ; ; : . 12i
Cellulose Fermentation . . : : A : : : ‘ 7 c . 122
Mucilaginous Fermentation ‘ 5 3 . . 123
Fermentations in technical processes. . : ‘ : é : : : . 123
CHAPTER XIV
THE CIRCULATION OF CARBON DIOXIDE IN NATURE (continued)
3. The Yeast Fungi and Alcoholic Fermentation. Theory of Fermentation and Anaerobiosis.
Concluding Remarks on the Circulation of Nitrogen and Carbon in Nature. ‘ . 126
Theory of Fermentation and Putrefaction : ; . 133
CHAPTER XV
BACTERIA IN RELATION TO DISEASE
1. Diseases of Plants; Harmless ‘ Messmates’ in the Human Body; Pathogenic Bacteria ;
Points of Attack and Sources of Infection ‘ 5 é . . . ‘ : » 137
CHAPTER XVI
BACTERIA IN RELATION TO DISEASE (continued)
2. Description of some Pathogenic Species. . . . : : . : F « 147
CHAPTER XVII
BACTERIA IN RELATION TO DISEASE (continued)
3. Mode of Action of Bacteria, and the Reaction of the Organism, Serum Therapeutics and
Immunity. ‘ . ; ' : : , . : : 5 » 158
NOTES. : i 1% BR af wa ek > 6h a . . 169
INDEX ee Re rr) ee 185
*,* The Arabic figures inserted in round brackets in the text refer to the
Bibliographical Notes, pp. 169-184.
CHAPTER I
INTRODUCTION: MORPHOLOGY
Form, Size, and Structure of the Bacterial Cell, Cell-membrane,
and Cell-contents.
THE first historically recorded discovery of bacteria was made more
than two hundred years ago by Anton van Leeuwenhoek, the Dutch
naturalist and pioneer in the world of the infinitely little. Exploring those
then untravelled regions with home-made lenses of wonderful efficacy, he
found in the saliva of the human mouth
minute organisms to which he gave the
name of animalcula on account of their
power of movement,
In the description (1) and in the figures
he gives (reproduced in facsimile in Fig. 1),
curved and straight forms and long and
short rods are plainly recognizable. They
constitute the earliest reliable record of
bacteria, the study of which in later times
has revolutionized medicine and expanded
into a new science. From the year 1683
Leeuwenhoek’s observations stood alone
until, a century later, the Danish savant
Miiller made further investigations on bac-
teria. He classified them with the infusoria
and gave them names that are now familiar
to us all, Vibrio, Spirillum, and Bacillus.
SP Aas
) =——
of ES
—
=~
Fic. 1. Oldest known figures of genuine
bacteria (bacteria of the mouth) from Leeu-
wenhoek. 4 and F represent Bacillus buc-
calis maximus. B is perhaps Vibrio
buccalis; its movements were followed by
Leeuwenhoek from Cto D. Zis a species of
coccus, and G, no doubt, a Spzril/um sputi-
genum (compare with FIG. 26).
In 1838 Ehrenberg described in his great work on infusoria a large number
of bacterial forms, ranking them with his group Vibrionia, and from this
time forward the bacteria have never again drifted entirely out of sight.
It was not, however, until about the seventh decade of this century that
FISCHER B
av
2 INTRODUCTION: MORPHOLOGY
the subject was taken up by pathologists, who from that time to this
have contributed more than any others to the building up of the science of
bacteriology. It was after the publication of Robert Koch’s (2) first studies
on anthrax that the activity of those numerous investigators began, whose
incessant industry has so furthered our knowledge of these most minute
organisms and has accumulated a bulk of material such as the most
voluminous treatises (8) are inadequate to grapple with.
Before this period of expansion and rapid growth, a growth that has
been in some directions too luxuriant for strength or permanence, the
bacteria had been studied chiefly by botanists (Cohn, Nageli). Their work
had been both physiological and structural, and it is on the foundations
which they laid that the science in its modern form has been built up. For
the fundamental labours of these earlier years, that saw, too, Pasteur’s
brilliant researches into the physiology of fermentation, the student may be
referred to Loffler’s lectures on the history of bacteriology (4).
Morphology of the Bacterial Cell.
The vegetative phase of all the smaller bacteria consists of a single cell.
In the simplest types this has the shape of a sphere (coccus). If one diameter
is greater than the other, i.e. if the sphere becomes a cylinder, it is spoken
of as a rod or bacillus. In one group of cylindrical bacteria the cells are
more or less spirally twisted (Vibrio, Spirillum, Spirochaete). In Vibrio
(Fig. 2, ¢c) the twisting comprises only a quarter of a revolution or less, in
Spirillum (Fig. 2, a) one or more widely separated turns, while Spzrochaete
(Fig. 2, ¢) exhibits numerous closely-wound spirals like those of a corkscrew.
The simplest method of permanently fixing the shapes of the bacteria is to
let a small drop of water containing them dry upon a cover-glass. By this
means they are all flattened to the plane of the glass, the vibrio, for instance,
appearing, in spite of its three-dimensional curvature, only as a slightly
curved ‘comma’-shaped rod (Fig. 2, @). It was for this reason that Koch
called the vibrio of Asiatic cholera the comma-bacillus, but as a matter of fact
it has, apart from the curvature, no resemblance to a comma. A spirillum
dries in the form of a semicircle (Fig. 2, 4) and a spirochaete appears as
a sinuous line (Figs. 2,¢; 26, f), The spirochaetes often attain great length,
but it is uncertain whether they then consist of a single cell or of a connected
chain of several cells. The other types of bacteria—cocci, bacilli, vibrios,
and spirilla—are always unicellular. They may be grouped together as
Hfaplobacteria as contrasted with the many-celled true filamentous forms,
the Trichobacteria. In the members of this group (in the sulphur-bac-
terium Beggiatoa (Fig. 17, a), for instance) the vegetative phase is an un-
branched chain or filament of closely united cells, each one having the shape
of a bacillus and becoming free from the others and motile at the period of
MORPHOLOGY OF THE BACTERIAL CELL 3
reproduction. The wxbranched filamentous bacteria without definite sheaths
are generally classed together under the collective name of Leptothrix. The
most complex form is C/adothrix, an aquatic genus with numerous dicho-
tomous branchings. Here the cell-chains are enclosed in a separate sheath
and the lateral shoots arise by single cells pushing sideways through this
envelope and growing out in a new direction. The resulting branch is thus
only superficially connected with the parent stem (Figs. 2, f; 12). This
is spoken of as ‘false branching’
as contrasted with true branching,
such as occurs in the mycelium of
a fungus (Fig. 2,¢). In this case
one of the cells of the main stem
sends out a lateral evagination
which, continuing to grow in the
new direction, forms a branch.
This new member is a /ateral out-
growth of the parent cell itself, and
as intimately connected with the
cells of the stem as they are with
one another. Among the tricho-
bacteria this method of growth is
unknown.
We have now enumerated the
various morphological types occur-
ring among the bacteria, and there
remain to be considered the aggre-
gates and colonies which arise
among the haplobacteria by the
Fic.2. a, Spirillum undutla, living, with spiral twisting ;
, the same dried on the cover glass in semicircular shapes.
¢, Vibrio cholerae, slightly spiral ; d, driedincommaform, ¢,
Spirochaele Ober maieri from the blood in recurrent fever.
| Cladothrix dichotoma, a branched example, with sheath
and so-called ‘false branching’; above “a short branch of
two cells is just pushing through the sheath. g, Penicillium
multiplication of the single cell or
the proliferation of large numbers
in close proximity. The Anthrax
bacteria, for instance, often form
glaucum, a fragment of mycelium with true branching
(from Brefeld). Amplification: a@ and } 1500, ¢ and @
2250, € about 800, 7 600, # 120.
unbranched threads or chains which
are externally undistinguishable from those of the true trichobacteria
(Fig. 28). Unlike these, however, the anthrax filament may at any time
break up into its constituent cells irrespectively of the advent of the repro-
ductive process. Short threads of two or three cells, as well as single cells,
may also occur. These phenomena, both in bacilli and cocci, will be con-
sidered in more detail when the question of species is under discussion,
Very frequently the bacterial cells are found collected in masses of more or
less regular contour held together by a gelatinous substance. To these
masses the name of Zoogloea has been given. They occur both in liquids
and on the surface of solid nutritive substances, such as potatoes or gelatine.
Sometimes in liquids they form a skin or pellicle composed of very dense
BQ
4 INTRODUCTION: MORPHOLOGY
aggregates of individuals. In many cases the zooglocae and pellicles are
merely social aggregates of individuals*, just as a forest or a meadow is,
and, like these, cannot be considered as morphological units. In others, such
as the cloud-like mass shown in Fig. 3, or the delicate ramifying growth of
B. proteus (Fig. 22), we have a true ‘colony’ before us, whose shape is not
accidental, but a result of definite modes of growth and multiplication which
recurs regularly in every culture, the characters thus produced being
often of taxonomic value. In all such zoogloeae and pellicles each single
bacterium is independent of the others. The plant remains a single cell,
and there is never any division of labour such as occurs in the more complex
colonies of the lower plants (volvocineae) or animals (coelenterata).
The bacteria are the smallest of all
known organisms, the most bulky coccus
having a diameter of only about 24 (zg95 mm.).
Among the Staphylococci (the most widely
distributed pus-bacteria) the diameter is often
not morethan 0-8 u,and the volume accordingly
only j75500e000 Cub. mm. Inasmuch as the
cell-substance contains a large proportion of
water, the weight, too, is inconceivably small;
thirty billion would weigh only a gram. In
Fic. 3. Fragment of the botryoidal ag drop of water I c. mm. in size, seventeen
zoogloea of an aquatic bacterium (Zoo-
&lcca ramigera) of older authors. The fumndred million pus-cocci would have room
Tods are thickly aggregated at the peri-
hery, less so in the middle; they are and to spare. Even the comparatively large
eld together by mucus. Magn. 56. : F
compart ne Zoogioeae in Figs.17,4and anthrax bacillus is only 3-10 long and
1-1-2m broad, so that its volume would have
to be increased eight million times before it attained the bulk of a middling-
sized cigarette.
Finer Structure of the Bacterial Cell (5).
It would seem at first sight a hopeless task to attempt to gain an insight
into the architecture of so minute a thing as a bacterium. Nevertheless the
problem has been attacked, and, thanks to the efficacy of modern micro-
scopes, some new facts have been established. We might well suppose that
these organisms, standing as they do upon the very threshold of life, would
be of simpler structure than those elementary units, the cells, of which all the
higher plants and animals are built up. The first question then to be answered
is whether all those parts which constitute a simple cell, such as a plant-cell,
* The word ‘ growth-form’ is often applied to these aggregates. This literal translation of the
German Wuchsform is an unfortunate one, inasmuch as the words growth and Wuchs do not connote
precisely the same ideas, and, furthermore, the expression is used by different authors in slightly
different senses (cf..Goebel, Outlines of Special Morphology of Plants, p. 480).
PLASMOLYSIS 5
can be distinguished in the bacteria. The parts are: the cell-wall (w, Fig. 4,
a), the protoplasm (f, Fig. 4, a), the nucleus (4, Fig. 4, a), and the cell-sap
(s, Fig. 4,@). The cell-sap is contained either in spaces (vacuoles) scattered
through the protoplasm, or in a large central cavity that fills out the greater
part of the cell, the protoplasm being reduced to a thin layer coating the
cell-wall (primordial utricle). Now the relation of this protoplasmic lining of
the cell to the cell-wall is of great importance in the study of the structure
of bacterial cells, and it will be necessary to discuss in some detail the
conditions present in the case of plant-cells in general. It will then be seen
how far the observable facts are competent to elucidate the finer structure
of bacteria. The cell-sap of vegetable cells consists of water in which are
dissolved various substances, organic compounds and mineral salts. The
protoplasm, which forms a sac enclosing
the cell-sap, is during life endowed with
properties similar to those of animal
membranes, being readily permeable to
pure water, but almost or quite imper- ‘°°: }
meable to the substances held in solution
by the cell-sap. Asa result, these bodies
exert a strong pressure (osmotic pressure)
from within outwards upon the proto-
plasmic sac, stretching it and pressing
it tightly against the comparatively un-
yielding cell-wall. An explanation of
this is afforded by the modern Theory
of Solutions which shows that the mole-
cules of a substance dissolved in water
act as though they were in a gaseous
Fic. 4. Plasmolysis of a cell from a fine hair
of Echallium elatertum. a, distribution of the
cell-contents in natural state (mounted in water) >
w, cell-wall; #, protoplasm (primordial utricle); s,
cell-sap in central vacuole; 4, nucleus. 4, in 25%
common salt solution, medium degree of plasmo-
lysis; the protoplasm has rerteated from the cell-
wall and is Being constricted into two parts. c,
same cell after lying about half an hour in the
25 % NaCl; the contents form two separate
spherical masses. Magn. 300.
state and strive to fly outwards away
from each other. Consequently, if the membrane is in the form of a closed
sac surrounded by pure water and containing a solution, the substance
dissolved behaves as a gas, and its molecules, like those of hot air in
a bladder, expand and dilate the sac until the pressure inside is equal
to that without. In the case of the protoplasmic sac of plant-cells this
condition of equilibrium is unattainable, because unlimited dilatation is
prevented by the cell-wall, and the result is that the whole of the cell
is during life in a permanent condition of strain (¢rgor). It will be
observed that osmotic pressure can only originate when there is water on
both sides of the membrane, and this condition is present not only in water-
plants, but also in terrestrial vegetation, inasmuch as the cell-walls are always
saturated with water. If we now reverse the conditions and, instead of
placing the cell in pure water, lay it in a solution of greater osmotic pressure
than that of the cell-sap (5 per cent. saltpetre or 2:5 per cent. common salt)
6 INTRODUCTION: MORPHOLOGY
it is easy to foresee what will happen. The pressure on the outside of the
protoplasmic sac being now greater than that within, the protoplasm is
pressed inward away from the cell-wall. The sac shrinks until the concentra-
tion of the cell-sap is such that the osmotic pressure it exerts is just equal to
that of the fluid outside. Since the shrinking of the protoplasm commences
directly the pressure of the surrounding medium exceeds that of the cell-
sap, we have in this phenomenon, which is known as Plasmolysis, a means
‘of measuring the pressure within the cell. The gist of the whole matter
lies in the fact that the protoplasm is easily permeable to water both within
and without, but presents a more or less impassable barrier to the molecules
of the osmotic substances. As a result, the cells may lie for a long time in
the plasmolysing solution without returning to their original state. If they
be removed from the solution and placed in pure water again, the pressure
of the molecules inside the protoplasm immediately reasserts itself, and the
sac dilates until the original condition of turgescence is restored. All these
phenomena are only observable in the living cell. At death the protoplasm
loses its impermeability, and presents no hindrance to the molecules of
substances in solution. In spherical cells the protoplasm is contracted by
plasmolysis to a ball, but in long cylindrical cells, such as hairs for instance
(Fig. 4), or algal cells, the elongated sac generally becomes constricted
into two or three portions. These are at first connected by strands of
protoplasm (Fig. 4, 4),’but finally separate and form distinct masses. The
commonest case is where two round or oval lumps of protoplasm are left,
one at each end of the cell (Fig. 4,c). When, by transference to pure water,
the process is arrested and reversed, these masses melt together again
as soon as they touch and continuity is restored. This de-plasmolysation
must not be too rapidly performed, or the fragments of the protoplasmic
sac may burst, in which case the protoplasm invariably dies.
The application of plasmolysis to the study of bacterial cell-structure
may now be discussed, but it will be advisable first to consider the results
which other methods of investigation have yielded, notably those of fixing
and staining. If we examine bacteria under a very high power objective
(2,000 diams. and over) very little can be seen. The cell has a fairly sharp
outline, but it is not possible to distinguish a separate membrane enclosing
the cell-contents. The latter appears as a pale homogeneous mass with
occasional granules of stronger refringency, and in the case of some of the
larger bacteria (Spircllum, Cladothrix) sap-vacuoles differentiate themselves
from the protoplasm by their watery appearance. These meagre détails
are all that can be seen in the fresh unprepared state,—of the organs of
locomotion nothing can be seen,—but by the customary methods of fixing and
staining a number of other particulars can be made visible. If a minute.
speck of a pure culture be mixed with a fixing fluid (e.g. osmic or chromic
acid), spread upon a cover-glass, and allowed to dry, the bacteria adhere
FINER STRUCTURE OF THE BACTERIAL CELL 7
strongly to the glass and may by washing be freed from the fixative and
stained with haematoxylin or an anilin dye. Under this treatment all
bacteria, save the most minute forms, show the same structure (Fig. 5). The
membrane appears as a sharp outline enclosing a protoplasmic mass which is
smooth and homogeneous where it lines the membrane, but full of irregular
spaces (vacuoles) towards the centre. The protoplasm is uniformly stained
and shows no trace of finer details. Only the strongly refracting granules
already referred to take on a deeper tinge. Being similar in this respect to
the ‘chromatic substance’ of cell-nuclei, they are sometimes spoken of as
‘chromatin’ granules: a doubtful analogy at best. When the bacterial
cell contains only one such granule (Fig. 5, a, c, d, e) it certainly makes the
impression of a nucleus both as regards its size and its position in the cell.
But there are, just as often as not, several
granules in a single cell (Fig. 5, 4, d, 7), and
apart from the colour-reaction we have no
adequate reason for regarding them as nuclei.
They bear no relation to the process of cell-
division, and it is better to regard them merely
as granules of reserve food-stuff. All efforts to
detect a true nucleus have as yet failed, and it
is safer at present to look upon the bacterial
cell as being devoid of one. There is however
another view of the matter that has gained many
_ Fic. 5. Bacteria fixed with alcoholic
iodine solution and stained in various
adherents. When bacteria are stained with
anilin colours they appear to take up more of
the stain than does the protoplasm of ordinary
plant-cells, and to retain it with greater tenacity
in the face of decolourizing reagents such as
alcohol and weak acids. Since moreover the
ways. a and 4, Cladothrix dichotoma
with sheath and one (a) or several
{?) chromatin granules in each cell
haematoxylin). c, Typhoid bacteria
(methylene blue). d@, Vibrio cholerae
(methyleneblue). 7, Spirillum undula
(haematoxylin), All the figures show
the cell-structure described in the text.
Chromatin granules, black; vacuoles
(sap-vacuoles), white ; protoplasm, stip-
pled. Magn. a-e 2250, f 1500.
nuclei of all cells are distinguished by their
avidity for colouring matters there has arisen the idea of a certain group of
substances, xzclear stains, that possess a peculiar affinity to nuclear substance.
This affinity is mythical. Nuclei take up a// stains with more avidity than
the protoplasm does,.and this behaviour rests probably not on the chemical
nature of the nucleus but on its physical constitution, such as its great density
and power of absorption. It is to the failure to recognize these facts that
the statement must be attributed which we meet with in almost all books on
bacteriology, namely, that the bacteria have a particular affinity to ‘ nuclear
stains’ and that we are therefore justified in regarding them as primitive nuclei
devoid or almost devoid of protoplasm. Speculation followed speculation,
and, the bacteria being the simplest organisms known to us, the hypothesis
has been put forward that the first living things to arise on the earth were
similar naked nuclei, and that the protoplasm is the product of subsequent
8 INTRODUCTION : MORPHOLOGY
evolution. The tingibility of the cell-contents in bacteria is not so great
when one takes into consideration the amount of stain taken up by the cell-
membrane, and even though in some cases an unusual amount may be
absorbed there is certainly nothing that warrants us in assuming the existence
of a specific reaction between the anilin stains and nuclei. The bacterial cell
then, interpreted in the light of the above facts, is a simple protoplast
enclosed within a cell-membrane but devoid of a nucleus.
The phenomena of plasmolysis lend additional strength to this view.
To plasmolyse bacteria it is only necessary to place upon a slide a drop
of water containing bacteria, lay upon it a cover-glass (with a few fibres
of cotton-wool between to prevent crushing), and
allow the plasmolysing fluid to run in at the edge.
Spherical and very short rod-shaped bacteria become
under this treatment more highly refractive. This
increase of density is the outward expression of the
contracting of the cell-contents, and in these very
minute forms it is the only change perceptible. In
the larger bacteria, however, such as typhoid, cholera,
spirillum, cladothrix, &c., the details of the process
‘Fic. 6. Plasmolysis of bac-
teria. a, Vibrio cholerae from
an agar culture (Bouillon+1 7%
peptone+1 (- rape sugar) in
1-25 NaCl. They are plasmo-
lysed but still living ; the proto-
plasm is broken up into refrin-
gentgranules, 6, Thesame more
highly magnified, ¢, Vibrio cho-
Zevae plasmolysed with cilium.
@, Typhoid bacilli in 25 % NaCl,
stained ; cell-contents in various
positions, to the right a cell with
the protoplasm arranged as in
the plant cell (Fig. 4, 4). e,Spiril-
lum undula plasmolysed by the
evaporation of stagnant water ;
the structure of the clumps of
are readily followed. Solutions of 2-5 per cent.
saltpetre or 1 per cent. common salt (fresh blood
serum contains 0-7 per cent. NaCl) are sufficient
to set up plasmolysis. As soon as the solution
reaches the bacteria its effect is visible, and one
sees the protoplasm gradually raised away from the
cell-wall and pushed inward. Just as in elongated
plant-cells, it frequently breaks up into two or three
highly refractive rounded masses which expand and
fuse together again if the plasmolysing solution be
peed gerne ape oe replaced by pure water (Figs. 4 and 6). In short
Magn. a 300, b-e 1500. rods the protoplasm generally contracts to a single
spherical or egg-shaped mass which lies sometimes
in the centre of the cell, sometimes at one end. With a low amplifica-
tion plasmolysed bacteria often look as though they had broken up into
a row of granules (Fig. 6, a), but careful examination under a high power
reveals the delicate membrane of the cell-wall still intact (Fig. 6, 4).
Two facts of fundamental importance ate brought to light by these pheno-
mena. They show in the first place that the protoplasm of the bacterial
cell is not, as is the case with the ‘pellicula’ of infusoria, attached to the
cell-wall, but lies free within it just as does the protoplasm of plant-cells,
and furthermore that the osmotic pressure in the bacterial cell is com-
paratively low, only about half that of the cells of higher plants which need
solutions-twice as strong to plasmolyse them. The pressure inside the bac-
FINER STRUCTURE OF THE BACTERIAL CELL 9
terial cell is nevertheless considerable, namely from two to three atmospheres,
In stronger solutions (5 per cent. saltpetre) the plasmolysis is rapid but
ceases in a few minutes, and the protoplasm expands and fills out the cell
again. ‘This retrogression of the process is caused by the penetration of the
molecules of the salt. Even in weak solutions (2-5 per cent. saltpetre) this
takes place in a few hours, showing that the protoplasm of bacteria is far
more easily permeable than that of the higher plants, a peculiarity which is
shared by other low organisms such as flagellata, cyanophyceae, and florideae.
A most important result of this greater permeability is the ¢ase with which
bacteria are able to adapt themselves to changes of concentration in the
medium they live in, and it must greatly facilitate the absorption of food-
stuffs as well as the excretion of the products of metabolism such as toxins
and enzyms. Finally, it must be mentioned that the motile forms retain their
power of movement during plasmolysis, from which fact certain conclusions
can be drawn as to the nature of the organs of locomotion.
In the usual methods of preparation the liquids in which bacteria are
suspended contain a considerable amount of soluble salts (the ordinary nutri-
tive media hold o-7 per cent. NaCl), and when a drop evaporates upon the
cover-glass the liquid becomes more and more concentrated, so that before
the bacteria become dry they are plasmolysed. It is such plasmolysed cells
which we see in most cover-glass preparations, many species such as
2. typhi, Vibrio cholerae, and Spirillum undula frequently showing strongly
stained granules (Polar-granules) at each end of the cell, the central space
being empty (Fig. 6). The meaning of these appearances is evident. The
general conclusion to which all these observations lead us is that the
bacterial cell-contents are a mass of protoplasm representing an osmotic
system precisely like that of the cells of the higher plants, but, unlike them,
having no nucleus.
The membrane or cell-wall of the bacteria is in most cases thin, delicate,
colourless, and without a perceptible finer structure. Unlike the cell-wall
of plants, it consists of a protein substance (no doubt a modification of the
protoplasm) and not of cellulose *. It would seem therefore that the degree
of division of labour which characterizes the higher plants, where there
is an outer very permeable membrane of cellulose, and an inner more or
less impermeable protoplasmic layer, has not been reached by the bacteria
where communication between the organism and the outer world is regulated
by two layers of medium permeability.
As is the case in many algae and cyanophyceae, the cell-wall of some
bacteria secretes a jelly-like substance which appears optically as a delicate
* Analysis of the cell-wall of the B. ‘udercudosis shows that it contains a large amount of true
cellulose, so large indeed that it has been detected in tuberculous animal tissues, and was thought to
be a product of metabolism (Freund).
10 INTRODUCTION : MORPHOLOGY
clear space, sometimes narrower, sometimes broader than the cell it sur-
rounds (Fig. 7, c and d@). It may be rendered more visible by special
methods of staining. It owes its origin to the swelling up of the outer
layers of the cell-wall by absorption of water, the inner layers being con-
stantly built up anew from within. This gelatinized membrane may become
more and more liquefied until large numbers of cells are buried up in a
common jelly-like mass, and it is in this way that the various forms of
zoogloea arise. The majority of bacteria have no such envelope at all,
or at most a thin invisible mucilaginous coat. The formation is often largely
dependent upon the nature of the food-stuffs available (Fig. 7, 6 and c).
Inasmuch as ‘many bacteria secrete large quantities of mucilage without
forming a definite envelope, the word ‘capsule’ should be sparingly used
and restricted to clearly marked sheaths around the cell. Only in such
cases has it any classificatory value (Lezconostoc). Moreover, the clear
spaces visible around bacteria in
dried preparations are frequently
artificial products. These are par-
ticularly prone to arise in fluids
like blood and lymph, which con-
tain albuminous substances, the
albumen being seemingly precipi-
tated upon the bacteria, around
which it forms a coating that
easily takes up stain. When dry-
ing takes place the cells, being the
Fic. 7. Capsules and mucous sheaths.
a, Bac. an-
thraciswith so-called capsules in a dried streak-preparation
from the liver of a mouse; for the nature of these and of
the capsules of other pathogenic bacteria, see p. 10. dd,
L - :
‘cides (frog spawn fungus); 4, on
non-saccharine media, without sheath; ¢, with mucous
sheath on medium containing sugar (d-c from Liesenberg
and Zopf); @, older zoogloea mass with chains of cells
(from van Tieghem). Magn. a 1500, 4 and ¢ 1200, d 500.
last to give up their moisture,
shrink away from this envelope
and each one appears to be sur-
rounded by a capsule (Fig. 7, a).
That this is, however, nothing but
an artificial structure is shown by the fact that the same bacteria which
in the blood of an animal exhibit a capsule are, with some doubtful
exceptions, both in the tissues and in cultures, quite devoid of one. In
a mouse which has died of anthrax, the bacteria in the blood seem to have
a capsule (Fig. 7, 2), whilst those in sections of the kidney are totally devoid
of one. That this is not due to a difference of the modes of growth in the
blood and in the tissues is shown by the fact that the naked anthrax bacilli
from an agar culture become provided with an apparent ‘capsule’ if they are
rubbed up with a drop of blood from a healthy mouse and allowed to dry
upon a cover-glass.
Among the trichobacteria (Cladothrix, Crenothrix) and some of the
cyanophyceae (Tolypothrix, Lyngbya), the outer layers of the cell-wall
undergo a hardening process which leads to the production of a firm sheath
FINER STRUCTURE OF THE BACTERIAL CELL II
enclosing the cylindrical cells in such a way that they are freely movable
within (Figs. 2 and 5). At the period of reproduction the cells, them-
selves devoid of sheaths, become furnished with cilia, and, escaping from
the open end of the sheath, swim away and give rise to fresh colonies.
In this way whole forests of cladothrix are emptied of their cells and
there remain only the stiff sheaths which finally swell up and become
disintegrated. Occasionally the sheaths become fossilized as it were, by
the deposition upon them of oxide of iron. They are then very resistant
to decay, and accumulate sometimes in enormous quantities in chalybeate
springs and ferruginous marsh water (see Chap. VII). The term ‘sheath’
can only be applied when a distinct tube is formed in which the chain of
cells are enclosed; colourless spaces in stained preparations of filamentous
bacteria do not necessarily imply the existence of a sheath, being often the
result of plasmolysis.
CHAPTER II
MORPHOLOGY—continucd
. Pigments, Intracellular Products; Movement and Organs of Locomo-
tion; Cell-division; Spore-formation and Germination.
Most bacteria are individually colourless, and appear, even when
massed together in a pure culture, either white or of a yellowish tinge.
There are, however, a considerable number of species, the chromogenic or
pigment-bacteria, remarkable for the brilliant colouration of their cultures.
‘Some of the Sarcinae, for instance, have a bright yellow tint, Staphylococcus
pyogenus aureus is golden yellow or orange, B. brunneus yellowish brown,
Micrococcus agilis, Bacillus prodigiosus,and Spirillum rubrum each a different
shade of red, B. cyanogenus (the bacillus of blue milk) blue, B. violaceus
a deep violet ; and many kinds of water-bacteria, as well as the bacillus of
blue pus, give origin to brightly fluorescent pigments. The production of
all these colouring matters is very variable and largely dependent upon the
conditions of growth, upon the composition and reaction of the culture
media, and upon the influence of oxygen, light, and heat.
Most pigment-bacteria appear under the microscope to be colourless,
and the question at once suggests itself whether the colouring matter
is really contained within the cell. In B. prodigiosus, the cause of the
‘bleeding host,’ the pigment can be seen in the form of granules scattered
about between the colourless bacteria, so that in this case there can be no
doubt as to the seat of the colour of the cultures. The various fluorescent
substances also are secretions of the bacteria with which they are associated,
and they diffuse into the culture medium, which often fluoresces brightly
throughout its whole thickness. The pigment of B. cyanogenus also is
dissolved in the culture medium. In most pigment-bacteria similar con-
ditions prevail, that is to say, they are ‘chromoparous’ (6). Some species,
on the other hand, are actually ‘chromophorous, i.e. the protoplasm itself
is coloured. This is the case with the sulphur bacteria Chromatium and
Thiocystis, and with some sap-green species (B. virens). As regards these
last, however, it is undecided whether they are bacteria or minute algae (7).
BACTERIAL PIGMENTS AND CELL-CONTENTS 13
Finally, there are bacteria, B. violaceus for example, in which the pigment
is lodged mainly in the cell-wall; such forms may be termed ‘parachromato-
phorous,
There is conclusive evidence that the red colouring matter of the
sulphur bacteria, ‘ bacteriopurpurin, performs the same functions as the
chlorophyll of higher plants, assimilating carbonic dioxide under the in-
fluence of light and giving off free oxygen (see Chap. VII). Only the outer
layer of the cell contents is charged with pigment, the central mass being
colourless.
In all probability the colouring matter is of importance physiologically
only where it is bound up with the protoplasm. In all the ‘ chromoparous’
bacteria the pigment is an excretion merely, and, as might have been
expected, chemical and spectroscopic analysis fail to show that it has any
connexion with assimilation.
Some of the bacterial pigments are of the nature of lipochromes and
are related to the fats; others have a basic character and resemble the
ptomaines. Many seem to belong to the proteins, and the colouring matter
of B. cyaneo-fuscus has a composition similar to that of indigo.
Differentiated cell-contents, such as the starch or aleurone grains of
higher plants, are wanting in most bacteria. The protoplasm stains a
uniform golden yellow when treated with iodine. Some of the butyric acid
bacteria, however, as well as various species inhabiting the human mouth,
form an exception to this rule, and take on a blue or deep violet tint.
Very little is known about the substance to which this reaction (granulose
reaction) is due; it has been called granulose merely on the strength of
its behaviour towards iodine. Whether its chemical composition is similar
to that of the granulose of starch cannot be decided by this reaction alone.
It seems to be a carbohydrate, however, and carbohydrates as food are
necessary for its growth. The mouth bacteria obtain these of course from
our food, and the butyric acid organisms from the fermenting substances in
which they grow. The‘ granulose’ makes its appearance in the bacterial
cell in the form of very minute grains, so that after treatment with iodine
the yellow protoplasm seems full of black points. These granules become
considerably larger, and then seem to diffuse through the cell contents, so
that finally the whole cell stains blue or violet.
In the butyric acid bacteria granulose is at first absent, but is formed
when the time of sporulation draws near. In that part of the bacterium,
however, where the spore appears, none is deposited, the protoplasm staining
from first to last yellow with iodine. It would seem that we have here an
instance of division of labour, the swollen end of the bacterial cell being
devoted to sporulation, and the cylindrical portion serving as a manufactory
and storehouse for granulose, from which the spore may be nourished
(Fig. 11, c—/).
14 MORPHOLOGY
Absolutely unique in the whole of organic creation is the case of those
bacteria whose protoplasm contains free sulphur (Chap. VII). In these
‘sulphur-bacteria’ the cell is often crammed full of highly refracting
globules, which are soluble in alcohol, xylol, bisulphide of carbon, and
alkalies, and give other reactions of pure sulphur. It is in a non-crystalline,
soft, amorphous state, and when dissolved out by carbon disulphide leaves
delicate clear vacuoles in the protoplasm. Deposits of other substances
within the bacterial cell have not as yet been observed, with the exception
of fat or oil, which sometimes appear in the form of minute globules,
particularly in old cultures.
Movements and Organs of Locomotion.
All bacteria, whether spontaneously mobile or not, are, when suspended
in water, in constant motion. Examined under a high power, they are seen
to be shaken by a rapid oscillatory movement. This, the so-called Brownian
movement (8), is in no sense a vital phenomenon, but is common to all finely-
divided solid particles suspended in liquids. It may be observed in the
particles of gamboge or indian ink rubbed up with a little water. Its cause
is not well understood. As a general rule there is no danger of mistaking
the trembling motion for the spontaneous movements of bacteria.
There are two kinds of independent motion distinguishable among bac-
teria—the ordinary swimming movement, and the rarer ‘ oscillation’ peculiar
to some trichobacteria and similar to the movements of some cyanophyceae.
Among the cocci, Micrococcus agilis is the only one endowed with the loco-
motory powers, but of the bacilli a large number are actively mobile, such as
B. typhi, the butyric bacteria, and the majority of the putrefactive organisms.
The vibriones and spirilla, too, are good swimmers. The bacilli of tuber-
culosis, diphtheria, anthrax, many of the pigment bacteria, and the acetic
and lactic ferments are permanently immobile. When bacteria in active
movement are examined under a high power they seem to shoot across the
field of view with amazing speed; but this velocity is only apparent,
being due to the magnification of the distances they cover. Reduced to
actual figures, the absolute rate of progress is 10 cm. in 15 minutes, or about
3 mm. per second—in proportion to their size a very fair speed.
Locomotion is effected by special organs, the cilia or flagella. In the
fresh state, or in bacteria stained in the ordinary way with anilin colours,
the cilia are not visible, and special methods of preparation are necessary to
render them so. That worked out by Loeffler (9) has the claim to priority and
is the best. He employs in the first place a mordant composed of an iron
salt and tannin, which causes the stain to be thrown down not only within
the cell, but also upon its surface and upon the surface of the cilia, which
LOCOMOTION AND LOCOMOTORY ORGANS 15
then appear deeply coloured. Both bacteria and their cilia are by this
process made to look thicker than they really are, and appear therefore
more clearly.
According to the arrangement of the cilia upon the cell the bacteria may
be divided into three groups—monotricha, lophotricha, and peritricha (0).
The members of the first division bear a single flagellum at one end of the
cell (Figs. 8, 2, and 23); examples are the cholera germ and other vibrios,
and B. pyocyaneus. The lophotrichous bacteria have in place of the
single flagellum a brush or tuft of cilia (Spirdllum, many putrefactive
bacteria; Figs. 8,4; 22, a; 12) and the peritrichous forms have their whole
surface beset with cilia more or less thickly arranged, so that they appear
almost as though they had a shaggy coat (B. typhi, B. coli communis, some
butyric ferments, B. subtilis, B. pro-
zeus, and many others; Figs. 8, c, e, and
II, 13, 22, 24, 28). The arrangement
of the cilia is constant in each species,
and even the number of cilia united to
form the terminal tuft has a taxo-
nomic value. ;
Functionally and structurally the
cilia of bacteria correspond to the
analogous organs in other groups,
such as the swarm-spores of algae and
fungi, the infusoria, and the ciliated
. Fic. 8. Types of ciliation. @, monotrichous
cells of metazoa. They consist of long (1#6rio cholerae) ; 6, lophotrichous(Spirillum un-
< ‘i dula), c, peritrichous (typhoid bacilli); @, develop-
delicate threads of protoplasmatic sub- ment during fission of the cilia tuft of Spiril/uim
‘ a undula; eé, partial and (to the right) complete
stance, which vibrate to and fro and __ looping’of the cilia in Bac. swétilis. Magn. a-e
% 2250. In Figs. a, 4, c, the structure of the cell-con-
propel the bacterium through the water tents has been taken from iodine preparations
: (Fig. 5) in order to illustrate the structure of bac-
asa boat 1S propelled by oars, They teria as far as is known at the present time. In
Figs. dand e the contents are schematically shaded,
ro re) ] ] from the d for in these preparations (stained b effler’s
aed Uk 0! y bo 4 of method) the eecinitatian of colour on the cell-
the cell (Fig. 8, @), and are not re-__ surface conceals the structure. See also Figs. 11,
12, 13, 17, 22, 23, 24, 26 and 28, which give further
tractile. The shrinkage of the cell- examples of various ciliation.
contents during plasmolysis leaves the
cilia unaffected, so that they would seem to be fairly independent organs.
They receive their nourishment and vs viva from the protoplasm, with
which they are connected through minute pores in the cell-wall.
The cilia are very sensitive to injury, unfavourable conditions causing
them to be thrown off. When shed in this way they sometimes become
disintegrated and disappear in a few minutes. As a result a preparation
made from a culture containing actively mobile bacteria frequently shows
not a single cilium, and this is particularly the case when old cultures are
employed, the bacteria they contain being especially sensitive. Not infre-
quently the cilia become rolled up or looped before they are shed, and
16 MORPHOLOGY
peritrichous bacteria then appear to be surrounded by a mass of bubbles
(Fig. 8, ¢).
The speed of movement of the bacteria is subject to great variations,
the cilia being often paralyzed by noxious physiological conditions. Lack of
oxygen, insufficient nutriment, and the accumulation of acid in old cultures
are all factors which bring about this paralysis, and it is only necessary to
remove them (by neutralization of the acid, by removing the cover-glass, by
the addition of sugar, asparagin, &c.) to at once restore movement to the
cilia. That bacteria are motionless is not therefore a proof that they belong
to a non-motile species, and in some cases much experience is necessary for
decision.
The linear progression of bacteria through a liquid is always accompanied
by a rotation around the longer axis of the cell, as is the case with the
swarm-spores of the lower thallophytes. In the flagellate infusoria the end
of the cell bearing the flagellum is always in advance in swimming, and
probably the lophotrichous and monotrichous bacteria move in the same
way, a reversal of the path necessitating a revolution of 180° around the
transverse axis of the cell. The peritrichous bacteria have a similar mode
of progression to that of the members of the other groups, but in addition
they often exhibit curious tumbling movements, the cell hurrying across the
field of view and turning somersaults the whole way.
In Beggiatoa, one of the filamentous bacteria, remarkable slow oscilla-
tory movements of the threads are observed, isolated fragments of which are
also able to glide to and fro as diatoms do. Both these types of movement
occur too among the cyanophyceae, giving the name to one group of forms,
the Oscillariae. The cause of the phenomena is quite incomprehensible,
no organs of locomotion having been discovered, and the cell-wall being
apparently closed all round, so that the protoplasm cannot be perceptibly
extruded anywhere. It is, however, highly improbable that the motion is
independent of the protoplasm. More delicate methods of research will
perhaps enable us to clear up the mystery.
The term /lexile has been applied to filaments which, although fer se
rigid and stiff, are not straight, but twisted and thrown into curves. It is
supposed that such flexile filaments have somewhat less rigid cell-walls,
which yield to the strain exerted on them by the cell contents. These
flexile threads occur in all trichobacteria, but in the haplobacteria the
membrane is always rigid. Kinks and twists in filaments seem to arise
frequently by the mechanical displacement of neighbouring cells, The
subject of flexility needs further investigation.
Reproduction of Bacteria by Fission (11).
Given suitable conditions of nourishment, bacterial cells, like those of
all other organisms, divide into two when they reach a certain size. In
REPRODUCTION OF BACTERIA BY FISSION 17
the filaments of the trichobacteria cell-division means growth and increase
of length of the filament, and the process can only be called multiplication
when a cell detaches itself from its fellows and grows out into a new
filament. Among the unicellular haplobacteria, however, each cell-division
means duplication of the individual. ‘The procedure follows the same course
as it does in the tissues of the higher plants. The cell first increases in
length, and then becomes divided into two by a transverse wall. The
spherical bacteria (cocci) assume an ellipsoidal figure before dividing, and
the two new cells are at first flattened where they are in contact, rounding
off to perfect spheres again as they separate. Of the finer details of the
process nothing has been seen among
the bacteria, nor anything resembling 1
those changes in the arrangement of
the cell-contents which characterize
cell-division in the higher organisms.
The protoplasm is simply abstricted
into two parts separated by the in- |
growing cell-wall, as in Cladophora. | 4G :
In the filaments of this alga cell-
division is ushered in by the deposi- a.
tion of a ring of cellulose on the
Fic. 9. Transverse division of a multinuclear livin,
inner surface of the cell-wall, where
the new partition is to arise (Fig. 9, a),
By constant addition to its inner
edge this ring grows broader, until
at last, cutting through the proto-
plasm, it stretches right across the
lumen of the cell and divides it into
cell of an alga (Cladophora fracta). The new cell-
* wall (as in all multinuclear cells) arises independently
of the division of the nuclei. In Fig. @ the new trans-
verse cell-membrane grows out as a ring at right
angles to the sides of the cell and appears (in optical
section) as rod-like en beige from the latter, the
free ends being surrounded by granular protoplasm,
The large round bodies are starch grains. Fig, 6
represents an older stage, the new membrane is com-
lete with the exception of a small spot in the centre.
The figure is meant to give an idea of what probably
takes place during the fission of bacteria which are
too minute to allow the process to be followed. From
Strasburger. Magn. 600.
two equal parts. There seems little
doubt that division of the bacterial cell takes place in the same way, but
the details of the process are too minute even for our best microscopes to
follow.
Under the most favourable conditions of temperature and nutrition cell-
division takes place in a very short time. B. subtilis, the ‘ hay-bacillus,’
completes the process in half an hour. The cholera vibrio needs only
twenty minutes, so that in one day a single *comma-bacillus’ would rejoice
in a progeny of sixteen hundred trillions. This mass of bacteria would
contain one hundred tons of solid residue, and it would thus be necessary
to make experiments on a gigantic scale to allow even one single cell to
multiply with perfect freedom. In nature, of course, this increase in
geometrical progression can never take place: in the first place, because the
necessary supply of food is never present, not even in the diseased body, and
then again because many of the bacteria soon perish, and the accumulation
FISCHER Cc
18 MORPHOLOGY
of the excreta (bacterial products such as acids, &c.), particularly in pure
cultures, arrests development. As standards of comparison it may be
mentioned that a complete division of nucleus and cell in the staminal
hairs in Tradescantia takes from eighty to one hundred minutes, and that
the process may be effected in Asmoeba in from ten to twenty minutes.
The rate of multiplication among the bacteria is therefore nothing extra-
ordinary, particularly when we consider the simple structure of their cells
and the absence of a complicated karyokinetic process.
When a cylindrical bacterium divides into two it is immaterial, as far as
the result is concerned, whether the division is longitudinal or transverse ;
but, as a matter of fact, we find that it is always transverse, be the cylinder
straight or spirally twisted. This is in harmony with the generally observed
law in cell-division, that the new membrane is always formed in the
most economical manner; that is to say, in such a way that a minimum of
material is required. In cylindrical cells this is manifestly the transversal
position. When the cells resulting from such division remain connected,
chains and filaments arise, particularly in the case of non-motile species.
They occur sometimes even among motile forms, such as the cholera vibrio
(Fig. 28, 2), although in this case the moving cells are more easily separated.
Inasmuch as transverse fission is the only mode of increase among cylindrical
bacteria, it follows that chains or filaments are the only kinds of colony or
‘ growth-form’ that can occur, save where there is a subsequent shifting of
single cells (as in Cladothrix).
In the monotrichous and lophotrichous bacteria it is always the non-
ciliated end of the dividing cell which bears the cilia for the new individual
(Fig. 8, 2), the inner ends of the two rods never bearing them. In all cases
where the cell seems to have cilia at both poles there are in reality two
young cells still united by their inner ends. A curious possibility arises
out of this fact. At every division it is only one individual that is provided
with new cilia, the other cell bearing the old ones, and this process may be
repeated time after time. As a result, it may happen that of two bacteria
swimming about and not as yet disconnected one has brand-new cilia, and
the other a set which has lived through hundreds of generations.
In peritrichous forms it seems probable that, during the elongation of
the cell preparatory to fission, new cilia arise between the old ones along the
side of the cell.
Among the spherical bacteria it is manifestly, as far as the economy of
the cell is concerned, a matter of indifference in which direction the plane
of fission lies, because every plane that passes through the centre of the cell
represents a minimum. A predilection for one particular plane must there-
fore be the outward expression of hereditary morphological characters that
possess a classificatory value. The closest resemblance to the cylindrical
bacteria is shown by those spherical forms in which the planes of fission of
REPRODUCTION OF BACTERIA BY FISSION 19
successive generations are all parallel. In this case, if the cells remain
attached to each other, unbranched chains arise as in Streptococcus pyogenes
(Fig. 10, a), a pus bacterium, or as in Leuconostoc mesenteroides (Fig. 7, a).
If fission takes place alternately in two directions at right angles to one
another, flat tabular groups of cocci are produced, frequently quadrangular,
with four, sixteen, sixty-four cells and so forth. The red sulphur-coccus
————Lhiopedia and Micrococcus (Pediococcus) tetragenus (Fig. 10) are examples
of this mode _ of growth. Finally, the planes of division may be formed in
_ three different directions successively, at right angles to each other, giving
origin, when the cells remain adherent, to cubical packets. The genus
Sarcina (Fig. 10, c) offers a typical instance of these. It is, of course,
possible to determine the manner of fission among cocci only when the
cells remain’ adherent to each other, or where successive generations are
held together ‘by gelatinous exudations. If the cells separate after fission
their grouping gives no indication of the manner of division.
FIG. 10, Modes of fission among the coccaceae. a, Streptococcus pyogenes, planes of fission always parallel,
resulting in chains; 5, Ped/ococcus tetragenus (Micrococcus tetragenus), fission alternately at right angles in two
planes, resulting in plate- -like growths ; c, Savcéxa ludea, fission in three planes at right angles to each other, resulting
in cubes or bale-shaped growths.
There remains yet another mode of increase conceivable, in which the
planes of division arise without definite sequence or relation to previous planes.
This would result in all manner of irregular cell-groups, above all in rami-
fications in one or more planes; but there are no observations that would
prove the existence of such, and we can only conclude that irregular fission
does not take place. It seems likely that even in the great host of micro-
cocci (e.g. the Staphylococci of the pathologists) the splitting up of the
cells follows a fixed rule, but that their prompt severance from one another
prevents the formation of large groups. As far as the Staphylococci are
concerned, division probably occurs in the three dimensions alternately,
but not in regular sequence, so that after a few fissions in one plane a new
direction is taken at right angles to it, to be changed again soon for a
third. In this way we should get short chains, quadrate groups, and minute
cubical clusters side by side, and this is what actually occurs (Fig. 28, a).
Spores and Sporulation (12).
The bacterial cell, although able for a short time to resist the injurious
effect of an unfavourable environment (drought, changes of temperature) or
an insufficient food supply, cannot defy such influences for an unlimited
Ca
20 MORPHOLOGY
period, for months or years. It has, however, the ability to enter into a
resting stage, to assume a shape endowed with great powers of resistance—
the ‘spore.’ This power it shares with all low organisms such as algae or
fungi, which are periodically subject to dearth of nutriment or the inclemency
of the seasons. The term ‘spore’ is commonly applied to all these resting
forms, but must not be supposed to connote anything but a similarity of
function. The word ‘endospore’ has in addition a special morphological
significance, and is applied to the commonest form of bacterial spores.
Taking the sporulation of the anthrax bacillus as a type, we find the
process begins by a contraction of the cell-contents, which ball themselves
together into an egg-shaped mass as yet devoid of a proper membrane,
and lying loose in the otherwise empty rodlet: this is the young spore
Fic. 11. Sporulation and germination. a@, Anthrax bacillus with the cell-contents contracted to form the youn
spore, as yet without membrane; 4, ripe anthrax spore still enclosed in the rod, whose shape has not changed.
cand d, Clostridium butyricum (Prazm.) : ¢, vegetative Benenchacs stage ; d, ripe spore in the swollen spindle-shaped
cell, the contents of whichiare not quite used up in spore-formation. ¢ and f, Plectridium palud : e, unchanged
rod; 7, drum-stick or Lt form with ripe spore in swollen end. Fi Germination of spore in B. anthracis; the
young rod elongates itself in a direction parallel to the longer axis of the ellipsoidal spore (3, 4) (from Prazmowski).
A, Germination of spore of 2. subiz/is; the new rod grows out at right angles to the axis of the spore (3-5), and, as
in anthrax, separates from the spore brane (6) Seon Pr ki). 2B. leptosporus; the spore, surrounded
by a thin mucous coat (dotted 1-3), grows out into a rod without shedding a membrane; simplest form of germina-
tion (from Klein). Magn. a 2250, 5-/ about 1200, g-7 1000.
(Fig. 11, a). This contracts still further, becoming denser and more highly
refractive than when it filled out the cell, and there forms upon its surface
a firm coat, the proper spore-membrane, to the impermeability of which the
durability and resistance of the spore are due.. The spore lies now complete
within the cell-wall (Fig. 11, 4), which gradually decays and sets it free. The
free spore, which may be found abundantly in cultures two to three days old,
is a highly refracting, ellipsoidal, immobile body, considerably smaller than
the cell in which it arose. Under favourable conditions of temperature it
germinates in twenty-four to thirty-six hours. Rags of the cell-membrane
may often be seen still adhering to it (Fig. 11, g, 4, 2). The spores of
B. subtilis are formed in a like manner, the rods, as in B. anthracis, retaining
their cylindrical shape during the process (Figs. 11,4; 13,c). A less simple
type of sporulation is that where the rods change their shape, becoming
SPORES AND SPORULATION 21
spindle-shaped (Fig. 11, c, d), or swollen at one end like a drum-stick
(Fig. 11, ¢, f). The greater part of the cell-contents is withdrawn into this
inflated portion, and forms the spore, but some remains in the cylindrical
section of the cell, where it can be rendered visible by plasmolysis as an
extremely delicate layer lining the membrane. It is no doubt from this
remnant of protoplasm that the cilia derive their nourishment at the time of
sporulation ; for they are not drawn in during the process, but continue to
lash vigorously to and fro until the spore is set free and nothing but the
empty cell remains. The spindle-shaped butyric bacilli and the drum-stick
cells of some bacteria in bog-water form their spores in this manner, and it
seems that in all cases where the spore-bearing part of the cell alters its
shape some of the protoplasm remains in the cylindrical section.
It is certain, in spite of many contradictory statements, that the
alteration in shape has the value of a specific character, and can be used
for classificatory purposes. The significance of the process lies, of course,
not in the change of shape, but in the differentiation of the cell-contents into
two parts, one for the maintenance of life in the cell, the other subservient
to reproduction. It is a foreshadowing of that division of labour which
secures the distribution of spores and their transport to places suitable for
germination.
Endospores are still unknown in a large number of bacteria—in the
whole group of cocci, for instance, and in many pathogenic forms, such as the
bacteria of typhoid fever, tuberculosis, diphtheria, and cholera. That they
do form spores there can be no doubt, but our artificial methods of culture
do not offer them the necessary conditions, the determination of which is
one of the important tasks that bacteriology has before it. In the patho-
genic bacteria mentioned, as well as in many other forms, highly refracting
granules have been described as spores, but proof is wanting ; they have not
been seen to germinate. It is certain that in a number of cases what have
been taken for spores are merely ‘chromatin-granules’ and plasmolysed
lumps of protoplasm in disintegrated or decaying cells.
Ordinary methods of staining leave the spores untinged, and they
appear, so long as they remain within the body of the cell, as clear colourless
spaces. It must be remarked, however, that we are not for this reason
justified in regarding a// clear spaces in stained bacteria as spores. Many
methods have been elaborated for obtaining beautiful double ‘staining of
the spores within the rods, the impermeability of the spores being over-
come by boiling them in the staining solution, or by submitting them to the
action of substances like chromic acid, which render the spore-membrane
more penetrable, either by causing it to swell up, or more probably by
dissolving out certain constituents. The question whether any given
structure is a spore cannot, however, be decided merely by its colour
reaction—germination is the only reliable proof.
22 MORPHOLOGY
As soon as the spores are ripe they are capable of germination, and in
a dried state remain so for many years. This property is not peculiar to the
spores of bacteria: well-dried wheat germinates after being kept for twenty
years, and the spores of the smut-fungus may lie in a herbarium for eight
years and not lose their vitality (see Chap. VIII). Moistening with pure
water is not sufficient to cause germination of bacterial spores. A solution
capable of setting up a nutritive stimulus is necessary, and, of course,
a suitable temperature. .
The first stage of germination is the slow swelling up of the spore,
which gradually becomes less and less refractive (Fig. 11, g 2, # 2, 7 3).
In B. subtilis this phase lasts from one to three hours. Then the spore-
membrane bursts open at one point, and the contents, surrounded by
a delicate cell-wall, are protruded, and elongating soon assume the form
of a rod, the new bacillus (Fig. 11, 4 2-6). The end of this often bears
for a long time the ragged remnants of the spore-membrane. Germination
is now complete, the whole process having taken about four or five hours.
One peculiarity connected therewith is worthy of remark. The spores of
B. subtilis are in shape short ellipsoids, having the longer axis parallel
to the longer axis of the cell that gave them birth. The new rod, on
germination, bursts through the side of the spore, and stretches itself at
right angles to the latter (Fig. 11, % 5), so that the longer axis of the new
generation crosses that of the old. The spores of B. axthracis and of
Clostridium butyricum, on the other hand, burst open at the end, so that the
axes of the new and old generation are continuous. Both these types of
germination are common among bacteria, but they are constant for the
same species, and therefore can be used for purposes of classification.
The simplest germinative process is that shown by some harmless
saprophytic bacteria (e.g. B. leptosporus), where the spore itself grows
out without casting its membrane into the new bacillus (Fig. 11, z). In
this case the unchanged spore-membrane becomes the cell-wall of the young
rod, whereas in B. anthracis, B. subtilis, and Clostridium butyricum the
coat of the spore splits into two layers—an outer, which is left as the
empty membrane, and an inner, which ensheathes the protruding contents
of the spore and becomes the cell-wall of the new-born bacterium. The
spores of many fungi also germinate in this way.
De Bary (18) has described, under the name of arthrospores, structures
which have given rise to a great deal of misunderstanding. He gave this
name to certain cells in filamentous bacteria, such as Cladothrix (Fig. 12),
Thiothrix, &c., which detach themselves from their fellows and swim about
after the manner of swarm-spores, finally growing out into filaments again.
They are certainly reproductive cells (Gonidia), and may perhaps be termed
spores, in so far as a spore is subservient to reproduction. De Bary called
them arthrospores because they are formed from one joint or cell in the
SPORES AND SPORULATION 23
cell-chain, but they possess none of the essential properties of aspore. They
are simply detached vegetative cells without special powers of resistance
and destitute of any lasting germinative faculty. Another kind of arthro-
spore is said to occur in the case of Leuconostoc, where certain cells generally
larger than their neighbours surround themselves with a very thick mem-
brane, and without further preparation enter the ‘resting stage.’ Similar
cases occur among the blue-green algae, but are quite unknown among
ordinary bacteria. What have been described as arthrospores in bacteria (in
cholera cultures, for instance) are in all probability nothing but deeply-
staining granules from the detritus of old cultures. They have never been
seen to germinate. Bacterial arthrospores, if the term were justified, would
be of the same shape as the vegetative cells of the species to which they
belonged; the cholera arthrospore appearing as a curved refringent rod,
that of B. violaceus as a straight cylinder, &c.
Of the causes which bring about sporulation very little can be said.
As with other organisms, it seems to be induced by an unfavourable environ-
ment, arrest of food supply, or the accumulation of the excretory products
of the bacteria themselves. In pathogenic forms, at least in those cases
that have been carefully studied, spores are not produced so long as the
bacteria are enclosed within the diseased tissues. The anthrax bacillus
forms them only in places where there is free access of air—in the excre-
tions of diseased cattle, for instance, or on the surface of the carcase ; never
inside it, either before or after death (see also Chap. XVI).
CHAPTER III
TAXONOMY
The Question of ‘Species’ among Bacteria; Variability; Involution
and Attenuation; the Classification of Bacteria.
DESPITE the morphological sameness of the bacteria, the functions they
perform in the economy of nature are numberless. When their extra-
ordinary versatility first became known, the idea arose that they were beyond
the influence of many of the laws of life which govern all other organisms.
Any opinion regarding bacteria, however absurd it might be, was permitted,
and even the existence of definite species among the bacteria was denied.
The controversy on the species question has been long and heated, and only
in the last few years has it been decided once for all that the conceptions of
genera and species are as justifiable among the bacteria as among other
organisms. The subject-matter of the whole discussion may be summed up
in two words: pleogeny or mutability of function, and pleomorphism or
mutability of shape (14). The pleomorphists maintained that a coccus did
not necessarily remain a coccus all its life long, that it could, under certain
conditions, stretch itself and assume the shape of a bacillus, that this again
could become curved and change into a vibrio, to return again later on to
the coccus form that it commenced with. Words like Micrococcus, Bacillus,
Vibrio, Spirillum, which we now know to have a definite taxonomic value,
were in the eyes of the pleomorphists worthless designations of transient
changes of shape.
As an example of almost inexhaustible versatility the branched aquatic
bacterium Cladothrix dichotoma was advanced. But it has now been proved
to be not truly pleomorphic (15), Only at the season of reproduction, for
the purpose of securing new fields of growth, does the cell undergo change.
It loosens itself from its fellows in a filament, develops a tuft of cilia, and
emerges from the sheath (Fig. 12). The bacillus-like gonidia or swarm-
spores thus produced settle down sooner or later upon some solid body, to
which they adhere, and then grow out into new filaments. Neither coccus,
PLEOMORPHISM : INVOLUTION 25
vibrio, nor spirillum forms appear. The various twistings and distortions of
filaments, and aggregations of motionless gonidia, which have been described
as phases of pleomorphic development, are quite fortuitous.
Pus-cocci (Staphylococci) may be cultivated in any number of different
media, but they appear with unalterable persistence in the form of little
spheres (Fig. 28,2). The cholera vibrio occurs in the form of a slightly
curved rod, and it occurs in this form only. The only observable variation
in growth is that in some cultures isolated cells may prevail, and in others
chains (Fig. 28, 2). In a hay-infusion culture of B. subtilis, for instance,
we meet side by side motile and motionless single cells (Fig. 13, a, 4),
and, especially in the surface pellicles, motile
and motionless chains (Fig. 13, 4). In all these
the individual bacterium remains unchanged, a
unicellular, peritrichous-ciliate, actively motile
rod (Fig. 13, a). The motionless rods arise
from any causes which bring about temporary
paralysis of the cilia, and the motile chains
from the clinging together of successive genera-
tions of cells. In a fresh culture the fluid is
uniformly turbid, with actively moving isolated
rods only. These are afterwards driven by
lack of oxygen to the surface of the culture,
where they grow out into long, motionless, — Fic. 12. Cledothrix dicholoma,
spore-bearing chains devoid of cilia, the aggre- Seat ugg. Of the ele bean
: . i if has opened and gives exit to a swarm
gation of which forms a membrane or pellicle spore; in the right branch a whole
igi *: group of cells has changed into swarm
on the surface of the liquid. This completes spores, each with a lateral tuft of cilia.
in this side too the sheath is swollen
the cycle of forms. up, loosened, Magn. 1000.
These instances will suffice to show that
pleomorphism in its true sense does not exist. In all the simple bacteria
(haplo-bacteria) the ‘growth-form’ only changes. Single cells unite to chains,
clusters, or zoogloea, according to the substratum they live in, but the
form of the individual of the vegetative cell remains the same.
That proper and improper nutriment must have a great effect upon
the size of the cell is evident. Giant and dwarf forms occur among the
bacteria as among all other organisms, and have the same significance.
But for all species of bacteria there is an average size and form, deviations
from which are not more extensive than they are among other organisms :
always assuming that the conditions of existence are suitable, that the
bacteria are in a healthy condition, which is far less frequently the case than
we are apt to think. Imagine a few thousand animals cooped up together
in a confined space, plentifully supplied with food, but without provision for
the removal of their dejecta. A few hours would suffice to bring about
a frightful state of things. And these are just the conditions under which
26 TAXONOMY
the bacteria are living in agar cultures, in all our culture media in fact.
In such circumstances it is no wonder if some of the cells grow into mis-
shapen, feeble involution forms, with their physiological functions, such as
virulence or fermentative power, weakened or suppressed (16).
Bacteria, like all other living things, produce crippled and deformed
individuals when forced to live under unsuitable conditions (Fig.14). These
abnormal bacterial cells are known as ‘involution forms.’ The causes of
involution are of various kinds. Acetic bacteria, for instance (Fig. 14, cd),
produce monstrosities if their own fermentation product (acetic acid) accu-
mulates beyond a certain point, and also if the temperature exceeds the
optimum. In &. sudzzlis involution
forms arise if the relative propor-
tions of nitrogen and carbon in the
culture medium are not suitable—a
solution of o-I per cent. asparagin
and 10 per cent. sugar, for instance.
In other cases a high percentage of
neutral salts induces them. A curious
case of involution is that shown by
the bacteria in the root tubercles of
the leguminosae (see Chap. X).
The shapes that the cells assume
are very varied. Irregular, swollen,
or spindle-shaped rods and twisted
chains are common, and the protru-
sion of short lateral processes from
the cells often gives the growths the
Fic. 13. Bacillus subtilis in hay infusion. The
pale shows the fel cycle of forms. a, Peritri- appearance of a branched system
c ‘od;
ous motile short r: 4, non-motile rods and chains A
a motile chains; ¢, spores in non-motile rods and (Fig. 14). At the same time the
chains that unite on the surface of the infusion to form
2 cic aa pellicle ¢. Magn. a-d 1500, e from cel|-contents are reduced and stain
badly, a few granules being appa-
rently all that is left in the cell. Cells which show a high degree of
involution are dead, and cannot, even in the most favourable conditions,
be revived. They are common in old cultures, and particularly so in those
of strictly parasitic forms, such as the diphtheria and tubercle bacilli, that
even in the most nourishing media do not find quite the conditions they
require.
Such branched involution forms have been thought by some to show
that the bacteria of diphtheria and tubercle (Fig. 14, %, g) are fungi of
a higher order of growth than their ordinary rod form would indicate. It
has been suggested that the rods are merely a stage in the development of
a true filamentous bacterium or even a hyphomycete, and new names have
accordingly been given to the organisms, the tubercle bacteria being placed
INVOLUTION: ATTENUATION 27
in a new genus Mycobacterium or Tuberculomyces, and the diphtheria para-
site in another, Corynebacterium. In the writer’s opinion such an alteration
is not justified, for the branched forms of tubercle bacillus that arise in three
to six months old colonies are by no means common, and agree closely in
appearance with the involution forms in the root-nodules of leguminosae
Fig: 14, e—/, and of the acetic bacteria Fig. 14, c-d.
If the influences which give rise to involution forms be restrained
within certain limits, a general weakening or attenuation of the bacteria is
20 x
!
Fic. 14. Involution forms. a, Bac. sub¢lis from a four-days old culture containing 1 % sal ammoniac, 2% grape
sugar, and o-5 % nutritive salts (weakly acid). 4, Typhoid-like bacilli from water in hay infusion with 4 % sal
ammoniac; they are non-motile, without cilia, and suggestive of the bacteria of root-nodules (e and /). c, Bac-
terium acet? at 39°-41° (from E. Chr. Hansen). d, Bacterium Pasteurianum, 7 hours at 34° (from Hansen). ¢, Bac-
teroids from the root-nodules of Vécéa villosa ; the round spots are the still retainable remnants of cell-contents (from
Morck). a Bacteroids from Lupznus albus (from Morck; the four-armed one is from Vicia villosa). g, Tubercle
bacillus, branched filament from sputum (from Coppen Jones). 4, Diphtheria bacilli, so-called branched forms ;
they are certainly involution forms (from Bernheim and Folger). Magn. @ and 4 1500, c and d@ 100, e and fabout
1500, g 1250, 4 about 100.
effected. This condition arises spontaneously in long-continued cultivation
on artificial media ; virulence, fermentative power, and other functions being
diminished. They may be revived again, however, if the process has
stopped short of involution, by restoring the bacteria to a more natural
environment: in the pathogenic species by repeated passage through the
animal body, in the ferment organisms by giving them renewed oppor-
tunity for active fermentation.
The attenuation induced by prolonged artificial culture can be secured
in a much shorter time by exposure to more noxious agencies. The most
important form of enfeeblement, the loss of infectious power, can be induced
28 TAXONOMY
by almost all operations injurious to life, if only the proper degree of
strength be chosen. Anthrax bacilli are attenuated by a few hours’
exposure to direct sunlight or by the addition to the culture media of o-1-
0-2 per cent. phenol. Trichloride of iodine is effectual in the case of the
diphtheria and tetanus bacilli. By careful exposure to high temperatures,
Pasteur was able to produce a less virulent breed of anthrax bacilli which
retained their acquired characters for a long time (17). The change effected
was the same whether the cultures were maintained at 425°C. for twenty-
eight days, at 43° for six days, at 47° for four hours, or at 52° for fifteen
minutes. The temperature optimum for Bac. anthracis is between 30° and
37°, the maximum about 42° or 43°, and the lethal limit lies between 50°
and 60°, so that it is evident that attenuation is effected the more quickly
the nearer the temperature approaches the lethal limit.
That the decrease of virulence is only one phase of a general weakening
of all the functions is shown by the fact that the bacteria in attenuated cul-
tures have lost the power of forming spores: have become asporagenous (18).
At first sight this seems to be a result of the greatest importance. Spore
formation is one of the principal morphological features of the cell. If we
were really able to suppress it, and to suppress it so successfully that it never
made its reappearance even in the most vigorous cultures, we should indeed
have done a great thing, for we should have obtained nothing less than
the long desired proof of the hereditary transmission of acquired charac-
ters. But appearances, alas, are deceptive. It is no more possible to
obtain in this way a breed of sporeless anthrax bacilli than it is to rear
a tailless race of mice by incessantly cutting off their tails. All we do
is to hinder the production of fully ripe, resistant spores: rudimentary
spores are formed, We attain nothing more than a general enfeeblement
of the cell affecting all its functions equally. This is proved by the fact
that such ‘attenuated’ and ‘sporeless’ cultures get weaker and weaker
and finally die. By the inoculation of the weakened bacteria into suitable
animals and several passages through the body, in short, by a dracing treat-
ment, virulence is restored and resistant, fully ripe spores appear again.
The micro-organisms behave, in fact, just as sickly plants do when removed
to healthy surroundings. The importance of the experimental modification
of virulence for the attainment of artificial immunity will be considered in
a later chapter.
All these considerations show us that external agencies (in the brief period
which our experiments allow) are unable to bring about a lasting, hereditary
change in the structure of bacteria. The morphological characters remain
constant. The variety always throws back to the typical form, the concep-
tion of species and genera having the same value among bacteria as among
the higher organisms. Billroth’s views, according to which all the bacteria
occurring in an infected wound were different developmental stages of the
PLEOGENY 29
same species (Coccobacteria septica), can be no longer held. Like Zopf’s
far-reaching speculations on the cycle of forms among bacteria, they are
matters of history, and they are nothing more. The similar ideas of
Nageli have been unable to hold out in the face of more recent dis-
coveries, and there can be no doubt that the justification of Cohn’s original
conception of the possible systematic classification of the bacteria is now
generally recognized.
Less simple to decide than the question of pleomorphism is the question
of physiological change or pleogeny*. All bacteria possess to a certain
extent the power of living on different kinds of substrata, and the com-
position of these determines in a great measure the nature of the chemical
changes which the bacteria set up. According to the degree to which this
power is developed, we may conveniently divide the bacteria into two
classes, the ~onotrophic and the polytrophic. Among the members of the
first group the requirements of nutrition are specific and clearly defined.
The composition of the substrata may vary only within very narrow limits,
and, as a result, the metabolic products also of these bacteria and the func-
tions they are able to perform are specific and well defined. As typical
examples of the monotrophic group may be mentioned the sulphur bacteria,
the nitrifying organisms, all the truly parasitic bacteria, and those found
in the root-nodules of the leguminosae. Among the great host of ferments
and putrefactive organisms, too, there are monotrophic forms with specific
properties. Such are the bacteria of the acetic, lactic, and some butyric
fermentations, and the bacteria which decompose urine (urea ferment).
Some of the butyric bacteria, on the other hand, are polytrophic, and,
besides possessing specific fermentative powers, are able to break up
albuminous compounds; they are saprogenic as well as zymogenic. Others
of the same class are pathogenic also, and can live in the tissues of the
animal body (malignant oedema, quarter evil). We find, too, forms like
B. vulgaris with pronounced saprogenic (putrefactive) powers, which are yet
able to grow on non-putrefactive substances and cause them to ferment.
Again, many saprophytic bacteria are unable to exist in the animal
body, while others (typhoid, cholera) are decidedly polytrophic in this
respect, and multiply rapidly in the tissues. It is not necessary to give
parallel examples of differences among other organisms.
As might be expected, it is in monotrophic forms that the ‘ specific
characters,’ both morphological and functional, are most sharply marked.
But they undergo no lasting change even in those bacteria which are physio-
logically the most versatile; and the attainment in our cultures of new
varieties, varieties with acquired characters that are permanent and heredi-
tary, is not to be expected. We are able, it is true, to obtain cultures of
* Detailed instances will be found in Chaps. V, XI, XII, XIII, and XV.
3° TAXONOMY
pathogenic species with every desirable gradation of virulence, but they
are not new varieties. They are mere laboratory stocks, which revert at
once to the original type as soon as the original conditions of life are
restored*, It has not been hitherto possible to entirely suppress a single
biological character in any species. The supposed conversion of the anthrax
bacillus into the harmless B. subtilis was never realized, and, like most
reports of a similar character, arose from imperfection in the technique of
pure culture.
Although there is no longer any doubt that the various kinds of bacteria
constitute definite species and genera, it would be futile to minimise the
great difficulty that confronts us as soon as we try to arrange them in a
system of classification. The morphological similarity of the spherical
forms (cocci) and the close resemblance to each other of the rod-shaped
bacilli make a classification based on purely morphological data quite
impossible. Physiological characters have therefore been made use of as
auxiliaries—for instance: growth on various media and need for oxygen
or particular forms of nourishment; specific products such as pigments,
granulose, sulphur, fluorescence, phosphorescence ; specific chemical changes
such as fermentation, putrefaction, disease. A reliable description of the
species of bacteria is a task for the future, and will only be attained by the
united labours of pathologists, physiologists, chemists, and botanists (19).
Much has been done by arranging them in groups according to their most
prominent physiological characters. Examples of such groups are the
saprogenic or putrefactive, the zymogenic or fermentative, the chromogenic
or pigment-producing, the phosphorescent, the thermogenic, the nitrifying,
the iron bacteria, and the sulphur and purple bacteria. Such a classification
is not without value, but it is not justifiable to make generic names on this
principle, and use them as though the groups they indicate were equivalent
to true morphological genera. Such names as Photobacterium, Nitro-
bacter, Nitrosomonas, Granulobacter (for butyric bacteria with the granulose
reaction), Halibacterium (for marine forms), Gonococcus, and Proteus (for
several putrefactive bacteria) are names that should not be admitted in
a systematic classification. As working or trivial names they are useful
enough, but have no claim to equality with genera based on morphological
characters, which must, after all, form the ground-work of all systems of
classification. An attempt must be made, despite the dearth of material,
to obtain clear morphological definitions of the genera at least, even if it
be impossible at present for species. This was Cohn’s guiding principle
(20), and must be adhered to, The very factor to which the progress of
bacteriology is mainly due, namely the number and variety of its students,
has been a hindrance to the proper growth of a classification. Side by
* The various ‘ races’ of brewery yeasts have a different value to these ‘ attenuated’ bacteria.
BAR ec raaay cepa a I a
CLASSIFICATION 3I
side with the medically trained ‘ bacteriologist,; to whom the principles
and meaning of classification were unfamiliar, there have been investigators
from every other branch of biology—brewers, agriculturists, and even
chemists, all vying with each other in the manufacture of species. It is
not argued for a moment that botanists alone have the right to set up
a system; but it cannot be too often repeated that other investigators, if
their taxonomic schemes are to be of any value, must proceed on the
recognized and established principles of classification. As things are, there
is no agreement even as to the value to be attached to the few clearly
marked morphological features which do exist. The only point of unanimity
is the adoption of Cohn’s groups founded on the shape of the cell (coccus,
rod, spiral, or filament). The importance of the distinction between fila-
mentous bacteria and those forms which consist of a single cell deserves
more emphasis than it usually receives. The filamentous bacteria, zricho-
bacteria, constitute an order, separate from the haplobacteria, in which the
colonies of cells are mere ‘growth-forms’ or social aggregates of individuals,
such as, for example, the packets of Sarcinae, or the cobweb-like zoogloea
of B. vulgaris.
The power of movement or its absence has very properly been regarded
as a distinctive feature, but the constancy of the ciliation (mono-, lopho-,
peritrichous) has been undervalued. A cholera vibrio or a bacillus pyocyaneus
has only one flagellum (as a rare exception two); the typhoid bacillus,
B, subtilis, and many others are always peritrichous. Lophotrichous forms
like the spirilla and other aquatic bacteria, or the bacillus of blue milk
(B. syncyaneus), always have a polar tuft of cilia, the number of which is
approximately known and fairly constant. That their numbers in a stained
preparation are often irregular is due to the delicacy of the structures, which
are easily thrown off, and not to any original irregularity. In the motile
bacteria, as in the flagellate infusoria, the number and arrangement of the
cilia are morphological characters of fundamental systematic value. Another
is the form of the sporulating rod, which, although described by some as
variable, possesses in reality the constancy required of systematic characters.
The anthrax bacillus retains its cylindrical shape, the tetanus bacillus
becomes invariably swollen at one end, like a drum-stick (f/ectron), and some
of the butyric bacteria assume the shape of a spindle at the time of spore-
formation. Exceptions to the rule are generally deformed individuals, and
are very rare, not one ina hundred. A careful discrimination of these two
characters, ciliation and sporulation, enables us to arrange the chaotic multi-
tude of rod-shaped bacteria into a few well-defined genera and sub-genera.
It may be argued that in many cases the spores are unknown. This is true,
but it is no reason why we should not classify the better known forms into
‘good ’ genera, relegating the others provisionally, according to the shape of
their cilia, to genera where the sporulation is unaccompanied by change
32 TAXONOMY
of shape in the rod. The names of the genera might be formed con-
veniently in such a way that the root of the word indicated the shape of the
cell, and the termination the arrangement of the cilia. The root-words
might be daktron (rod), kloster (spindle), and plectron (drum-stick), and the
terminations -2zium for monotrichous, -2//um for lophotrichous, and -zdiam
for peritrichous types. The classification of the less numerous spirilla is,
as the table shows, a simpler matter. Among the Coccaceae the manner
of cell-division has already been employed as a generic character, but
greater weight should be attached to the contrast between two more
comprehensive groups into which they naturally fall. These are the
Homoccocaceae, where the planes of fission follow a definite sequence, and
the AJdlococcaceae, in which such regularity is not found (see pp. 18 and 19).
The different meanings attached to the words Bacillus and Bacterium
deserve notice. In two of the most recent systems of classification (21) the
senses in which they are used have little in common. Lehmann and
Neumann apply the word Bacterium to all rod-shaped forms in which spores
are unknown (a point in regard to which any day may bring a change in
our knowledge), the genus Bacilius embracing those in which spores have
been found. Of the manner of ciliation no notice is taken. Migula, on
the other hand, uses the term Bacterium for all non-motile rods, and Bacillus
for peritrichous species, the remaining motile forms, both lophotrichous and
monotrichous, constituting a new genus, Pseudomonas. This classification
very properly regards the presence or absence of spores as unimportant ;
but the shape of the spore-bearing rod is neglected, and no distinction is
made between monotrichous and lophotrichous species. Perhaps the best
plan would be to drop altogether the word Bacterium as a generic term,
seeing that it is now used as a collective name for the whole group of micro-
organisms. The word Bacz//us might then be used, in memory of Koch’s
first work, for all those species which, like the anthrax parasite, are non-
motile and retain their shape during sporulation.
At present we are not justified in regarding the presence or absence of
capsules as sufficiently important to constitute a generic character, although
very useful for the differentiation of species,
The trichobacteria are so few in number that it will suffice at present
to unite them in a single family representative of another order.
Based on the characters that have been under consideration, a classifi-
cation of the bacteria might be arranged as follows :—
Order 1. HAPLOBACTERINAE.
Vegetative phase unicellular, spherical, cylindrical, or spirally twisted; isolated or,
united in chains or clusters.
Family 1. COCCACEAE.
Vegetative cell spherical.
CLASSIFICATION 33
Sub-family 1. Alococcaceae.
Planes of fission without definite sequence; no pronounced colonies or growth-
forms, cells isolated or in short chains or irregular clusters.
Genus Micrococcus, Cohn. Non-motile. Includes most cocci, the pathological
‘staphylococci,’ &c.
Genus PLANOCOCcCUS, Migula. Motile.
Spb-family 2. Homococcaceae.
Planes of fission in definite sequence.
Genus SARCINA, Goodsir. Three planes of division at right angles to each other,
Cubical colonies, non-motile. :
Genus PLANOSARCINA, Migula. Similar to Sarcina, but monotrichous, ciliate, and
motile,
Genus PEDIococcus, Lindner. Two planes of fission, alternate and at right angles.
Micrococcus tetragenus, Thiopedia (a sulphur organism), and probably some species
usually termed micrococcus,
Genus STREPTOCOCCUS, Billroth. Planes of fission parallel, giving rise to chains;
the pathological S¢reptococct and Leuconostoc.
Family 2, BACILLACEAE.
Vegetative cell straight, cylindrical, ellipsoidal or egg-shaped; very short forms
difficult to distinguish from cocci. Fission always transverse.
Sub-family 1. Bacilleae,
Spore-forming rods cylindrical, unchanged.
Genus BACILLUS, Cohn. Non-motile. 2. anthracis, B. diphtheriae, &c.
Genus BACTRINIUM, A. Fischer. Motile, monotrichous, with terminal cilium:
includes provisionally all monotrichous rods whose spores are as yet unknown, e.g.
Bac. pyocyaneus,
Genus BACTRILLUM, A. Fischer. Motile, with lophotrichous ciliation. Includes
provisionally Bac. cyanogenus, and many other sporeless forms.
Genus BACTRIDIUM, A. Fischer. Motile, peritrichous, in some spores as yet un-
known. Very numerous representatives, e.g. B. subtilis, B. megatherium, B. vulgaris
(old genus Proteus), B. tyfhz, and B. colz.
Sub-family 2. Clostridieae.
Rods spindle-shaped during sporulation.
Genus CLOSTRIDIUM, Prazmowski. Motile, peritrichous; includes some of the
butyric bacteria. Genera with monotrichous and lophotrichous ciliation are unknown
as yet.
Sub-family 3. Plectridieae,
Rods drum-stick-shaped during sporulation.
Genus PLECTRIDIUM, A. Fischer. Motile, peritrichous; some butyric bacteria, the
parasite of tetanus and a methane ferment.
FISCHER D
34 TAXONOMY
Family 3. SPIRILLACEAE.
Vegetative cell cylindrical but spirally twisted. Fission always transverse,
Genus VIBRIO, Miiller and Léffler. Very slightly curved rods, ‘comma’ shaped;
motile, monotrichous. Vibr.o cholerae astaticae and numerous other vibrios of fresh and
salt water.
Genus SPIRILLUM, Ehrenberg. Cylindrical cells twisted in an open spiral ; motile,
lophotrichous. Spirillum undula, Sp. rubrum.
Genus SPIROCHAETE, Ehrenberg. Cells long and attenuated, spirally twisted with
numerous turns; cilia unknown; the cell membrane is perhaps yielding. Spirochaete
Obermaieri (remittent fever).
Order 2. TRICHOBACTERINAE.
Vegetative phase an unbranched or branched filament or chain of cells, the indi-
vidual members of which break off as swarm-spores (gonidza).
Family 1. TRICHOBACTERIACEAE.
(2) Filaments non-motile, rigid, enclosed in a sheath.
Genus CRENOTHRIX, Cohn, Filaments unbranched and devoid of sulphur granules,
Genus THIOTHRIX, Winogradsky. The same, but containing sulphur granules,
Genus CLADOTHRIX, Cohn. Filaments branched, false dichotomy (includes
Sphaerotilus).
(4) Filaments motile, with oscillating and gliding movements, and devoid of
a sheath.
Genus BEGGIATOA, Trevisan. Containing sulphur.
For the genus Streptothrix see next chapter. Further particulars as regards the
discrimination of genera and species will be found in the works quoted in Appendix
(No. 3).
CHAPTER IV
TAXONOMY (continued).
Tho Systematic Position of Bacteria; Other low Organisms with
Pathogenic Properties.
A QUESTION often asked is, whether bacteria are animals or plants?
Now the terms ‘animal’ and ‘plant’ are collective terms invented by
laymen to describe familiar living things, insects and elephants, mosses and
oak trees, and they date from a time when such minute beings as bacteria
were quite unknown. It is therefore as superfluous as it is futile to attempt,
as many have done, to detect the distinguishing characters of the ‘ animal’
and ‘vegetable’ kingdoms among organisms for which these terms were
never intended. For this reason Haeckel and others have proposed to
establish a third domain, that of the Protista, which shall include all those
forms in which differentiation has not been pronounced on the lines
of either animal or plant development. The new group would take up
Radiolarians, Flagellata, and Infusoria from the animal side, and the Cyano-
phyceae as well as some low forms of Algae and Fungi from the plants.
The border line between protista on the one hand and plants and animals
on the other, is, it must be confessed, artificial. To these protista, which
embrace approximately all those forms of life we commonly call micro-
organisms or microbes, the bacteria belong.
Another question almost as common as the first is, whether bacteria are
fungi, as the synonym Schizomycetes or fission-fungi would seem to’ imply.
As far as the processes of life are concerned the bacteria and fungi agree
in every detail, for, with very few exceptions *, the members of both groups
are unable to derive their nourishment from inorganic compounds. That is
to say both bacteria and fungi are metatrophic, are restricted in their food
to substances fabricated by the higher organisms. Some of them are even
* The saltpetre bacteria and others.
D2
36 TAXONOMY
paratrophic, that is, are able to exist only upon living animals or plants. But
in spite of the many points of physéological similarity between fungi and
bacteria there are very wide morphological differences. In every fungus, be
it a mushroom, a morel, a mildew (Fig. 15, c), a smut-fungus, or an animal
parasite, like Herpes, we can always recognize two distinct parts, the mycelium
or vegetative section, and, arising from this, the spore-bearing or reproductive
section. The mycelium is in all fungi an irregularly felted mass of branched
filaments, the Ayphae, which in many cases (e.g. Penicillium, Fig. 15, c) consist
of cylindrical cells set end to end, each cell having much the appearance of
a rod-shaped bacterium. The fructification is of varying degrees of com-
plexity, from isolated cells or chains of cells (Fig. 15, c) among the lower fungi
Wi SS
ATS Se
pale wr
Fic. 15. A, Oscéllaria tenuis (one of the Eyenaphiyeras!, fragment of filament; cZ, hollow cylindrical chromato-
phore ; ¢, so-called central body, finely vacuolated protoplasm with deeply-staining granules (black). 3B, Polyfoma
xvella, flagellate with two anterior cilia; v, contractile vacuole; &, nucleus; 4, membrane, cell contents filled with
assimilation products pee) Cc, Penicillium glaucum (true fungus, mycomycete), mycelium has arisen from
te ao @; on aerial hyphae brush-like fruits with chains of conidia. Magn. a 2250, 4 about 600, ¢ (from
refeld) 120.
up to the highly complex sporophores of the toadstools and mushrooms.
Upon a suitable soil the mycelium is able to vegetate for long periods,
continually producing new reproductive cells or fruits. Among the bacteria
such a differentiation is nowhere to be found. Their vegetative phase is
either a single cell or a cell filament on which special reproductive organs
are not developed but which, like Cladothriz, breaks up entirely into
gonidia. When the reproductive organs, the spores, appear, the vegetative
cell as such ceases to exist. As in the myxomycetes and many other
protista, it is entirely used up in the formation of the reproductive cell.
The bacteria may therefore be termed holocarpous, as contrasted with the
fungi which are excarpous, that is to say, able to produce several successive
fructifications from the same vegetative thallus.
Failing to find the kindred of the bacteria among the fungi we must
seek them among those low organisms which we have named collectively
CYANOPHYCEAE 37
the protista. Of these, two groups in particular deserve careful attention,
the blue-green algae or Cyanophyceae, and the Flagellata.
The Cyanophyceae present in their outward configuration many points
of resemblance to the bacteria. We find spherical cells (Chroococcus), rods
(Aphanothece), \ong cell chains or filaments (Oscillaria), and spirally twisted
unbranched forms (Spirulina). As among the bacteria, too, colonies or
‘ growth-forms’ occur; some like sarcina (Glococapsa), others in flat plates
(Merismopoedia), while sheath-bearing bacterial species like Cladothrix find
their parallels among the Scytonemeae (Tolypothrix) with their dichotomous
false branching. But it must not be forgotten that these striking similarities:
to the bacteria are not confined to the Cyanophyceae; the chlorophyll-
bearing algae offer a like series of parallels. The powdery green coating seen
on the north side of tree-trunks, &c., consists of minute, free-living spherical
cells (Pleurococcus), pools and ditches are often coloured green by myriads
of rod-shaped species (Stichococcus), the curved vibrios have their analogy
in the gracefully-formed Raphidia, and examples of sheathed filaments and’
capsulation are not wanting. This is not surprising, for free-living cells
must have the form either of spheres or cylinders, and the simplest cell
unions must be filaments, flat plates, or packets. The similarity of external
configuration is only a superficial similarity and in no way justifies a
systematic approximation of the two groups.
The cells of Cyanophyceae, whether free-living (Chroococcus, Aphano-
thece) or united to form filaments, multiply like all other cells by division,
just as do the bacteria. Just as these, too, do the isolated Cyanophyceae
‘split off’ from their sister cells. And for this reason, supported by the
not less superficial one of resemblance in shape, the bluish-green algae
(‘ Schizophyceae ’) have been placed alongside the ‘ fission-fungi’ (‘ Schizo-
mycetes’) to form a separate class of plants, the ‘Schizophyta.’ But the
‘fission’ of these organisms is not peculiar to them, for it must always occur
when isolated unicellular organisms divide; and the opinion that the bacteria
are a series of colourless organisms parallel to the schizophyceae is not
well founded.
The differences between the two groups are as numerous as the similari-
ties. The Cyanophyceae, if we except the gliding movements of the
Oscillariae, are non-motile, whereas a large proportion of the bacteria are
actively motile and possess special locomotive organs, the cilia. These are
moreover present not only at the time of reproduction, but persist throughout
the life of the cell, The sporulation of the two groups is also different.
The Cyanophyceae form not endospores but arthrospores, which arise by
the differentiation and enlargement of a whole cell. In the finer structure
of the cell again there is but one detail which the Cyanophyceae and bacteria
have in common, the absence of a nucleus. In other respects the cells of the
Cyanophyceae are much more highly differentiated than those of bacteria.
38 TAXONOMY
The bluish-green colouring matter is borne bya special organ, the chromato-
phore (Fig. 15,a@: ch), generally having the shape of a hollow cylinder or
sphere, forming the outermost layer of the cell-body and enclosing the
remaining cell contents (Fig. 15, a,c). The central mass stains deeply, and
thus has somewhat the appearance of a nucleus. We are, however, no more
justified in describing it as a nucleus than in calling the strongly-stained
granules in the protoplasm ‘nuclear chromatin.’ As in the case of the
‘chromatin grains’ of bacteria, the nature of these substances is unknown.
In no bacteria, not even in the chromogenic species, is such a differentiation
of the cell contents to be observed.
If we now turn to the Flagellata we find among them also many points
of similarity to the bacteria. In a species like Polytoma uvella (Fig. 15, 4),
for instance, often found in stagnant waters, we have an egg-shaped cell
enclosed in a definite membrane (%), and provided with a pair of permanent
polar cilia or flagella. Other forms, such as Jonas, are ‘monotrichous,’ others
again, like Tetvamitus, have a brush or tuft of cilia at the anterior end of the
cell. The process of ‘encystment,’ too, closely resembles the formation of
endospores, the cell-contents contracting and secreting a new membrane,
the cyst or spore thus formed being set free by the disintegration of the
membrane of the mother cell just as are the endospores of bacteria. But
notwithstanding these points of likeness the Flagellata differ from the bac-
teria in one fundamental feature: ¢hey possess a definite nucleus (Fig. 15,6: k)
like that of the cells of higher organisms. It is therefore evident that we
have as little right to assume a close genetic relationship between bacteria
and Flagellata as between bacteria and Cyanophyceae. The view most
in harmony with observed facts is that which looks upon the Bacteria as
a distinct group of protista, simpler in structure than the rest, and showing
affinities to both Cyanophyceae and Flagellata. We might regard the bac-
teria as the more direct descendants of the ancestral stock from which these
arose, the differentiation of a chromatophore leading to the Cyanophyceae
and the development of a nucleus as well as the general attainment of motile
power giving rise to the flagellate phylum. Among the arche-bacterial
types there may well have been both motile and non-motile forms which
would have been respectively the starting-points of the two branches, whilst
capsulation, the production of cell-chains or filaments, and other colonial
or growth forms were primitive acquirements which have reappeared
again among the Cyanophyceae and Flagellata and undergone greater
specialization.
The protista (sicro-organisms), the simplest living things, among which
we have tried to indicate the place of the bacteria, are not only manifold
in form, but have very different modes of life and exercise a great variety of
different functions in the economy of nature. Asa general rule, these physio-
logical features are not so conspicuous among the other groups as they are
SACCHAROMYCES: AMOEBAE 39
among the bacteria, but they become so when vigorous growth and suitable
conditions of nourishment allow them to multiply rapidly. Such instances
as the yeasts and the mould-fungi, with their energetic chemical effects,
suffice to remind us that the bacteria are not alone in their performances.
Some of these low organisms (22) are, like the bacteria, capable of pro-
ducing pathological effects, but it is very rarely that such cases present the
characters of a genuine infectious disease. Only within the last few years
has it become known that the yeast-fungi (Saccharomycetes, see Chap. XIV)
are capable of developing pathological properties. Injection into animals
of pure cultures of different species of brewery and distillery yeasts gives
rise to processes having all the characters of true parasitic diseases, ending
in many cases with the death of the animal. The germs spread to distant
parts of the body and are found in all the organs as well as in the blood,
but the lesions produced have not been recognized as those of any known
disease in human beings. Still, an infection by yeast cells is just as con-
ceivable as by any other organisms. It has recently been suggested that
the various forms of cancer and similar neoplasms may be due to parasitic
saccharomycetes, and some investigators have announced the discovery of
such in stained sections. Unimpeachable results have however not been as
yet attained, and it has been shown in some cases that the supposed para-
sites were nothing but fragments of cells and nuclei of the tissues themselves.
The parasitic nature of tumours is indeed denied by some pathologists.
The thrush fungus (Saccharomyces albicans of some authors) found
on the buccal mucous membrane of children belongs possibly to the
saccharomycetes. Its elongated cells have much the appearance of
a yeast and they multiply by budding in the same way, giving rise to
mycelium-like masses which cover the culture fluid with a thick pellicle.
They set up, too, in suitable media (beer-wort) weak alcoholic fermentation.
Whether these masses are true mycelia or not is uncertain, and it is
therefore doubtful whether the fungus should be classed among the yeasts
or with such hyphomycetes as Ozdium and Monilia candida,
Some of the closely allied group of Flagellata occasionally occur as
parasites or messmates in the animal body (e. g. Mastigophora) which, as we
have seen, have alliances to bacteria, but it has not yet been demonstrated
that they are ever pathogenic. Zyrichomonas vaginalis is sometimes found,
associated with other micro-organisms, in the mucous secretion of the
vagina. th of
a milligram of solid matter. Rain-water is contaminated with bacteria which
it has taken up in its passage through the air; one sample contained thirty-
five germs per litre. The water of rivers, lakes and springs contains very
varying numbers of micro-organisms, their abundance depending largely
upon the amount of organic matter present. As soon as water is rendered
impure by the influx of such substances as are carried into rivers by sewers
and drainage canals, it ceases to be merely a liquid in which bacteria can
retain their vitality for a long time unimpaired, and becomes a nutritive
medium in which they proliferate with great rapidity. The water of the
Spree for instance contained, above Berlin, on an average 6,140 bacteria
per cubic centimetre, that taken below the city no less than 243,000.
The bacteriological analysis of water is a very simple process, If the
microscope shows that bacteria are not too numerous, a measured quantity
of water (generally 1 c.c.) is mixed with nutrient gelatine and poured out on
to a glass plate or shallow dish. The germs distributed through the gela-
tine are allowed to develop into colonies which can then be counted in the
usual way. The mere enumeration of the bacteria in water is of far less
importance than the determination of their nature and properties. The
majority of forms found in rivers and springs are of an entirely innocent
character ; they are bacteria whose natural habitat is water, and they per-
form a useful task in consuming the dissolved organic impurities. But-when
by any means pathogenic species obtain access to. such waters these may
easily become a source of infection, particularly when, as is the case with
cholera and typhoid organisms, the bacteria belong to species which can
live and multiply side by side with the ordinary saprophytic forms (see
Chaps. XIII and XIV)*. When the water to be analysed contains a very
large number of bacteria it must be diluted, before mixing, with an equal
quantity of sterilized water. For the detection of isolated pathogenic forms
in comparatively pure water, that is to say in water containing very few
bacteria of any sort whatever, a method of ‘nursing’ or ‘enrichment’ is
employed. A sterile nutritive solution (peptone-sugar) is added to the water,
so that the few germs present may multiply before the gelatine is added.
This method involves ofcourse the drawback that the non-pathogenic species
multiply also, and the danger that they may outnumber and overpower the
less abundant bacteria we are looking for.
Numerous aquatic bacteria are able to resist freezing for long periods,
and ice sometimes contains enormous quantities of them (e. g. 2,000 per c.c.).
[Even glacier ice is not free from them.]
Of all the natural habitats of bacteria there is none in which they are
* For marine bacteria, see Chap. VII.
CLASSIFICATION ACCORDING TO NUTRITION 47
found constantly in such numbers as in the soil. They occur both as spores
and in active vegetation (e.g. nitrifying bacteria). In the soil, as in water,
the presence of organic matter is favourable to their increase, and they are
most numerous in cultivated earth. Garden mould seldom contains less than
. 100,000 germs per c.c., among them being almost always some pathogenic
forms such as the bacilli of tetanus and malignant cedema. Putrefactive
and pigment bacteria are also generally present as well as fermentative
and nitrifying organisms. For the ordinary purposes of hygiene the culti-
vation upon nutrient media of the bacteria of earth, water, and the atmo-
sphere, and the detection and determination of pathogenic species, are
sufficient ; the latter task being, however, by no means easy. Still more
difficult and tedious is the systematic investigation of all the species present,
necessitating as it does the employment of a number of different culture
media varying in composition according to the nature of the species to be
cultivated. It is not possible, for instance, to isolate the nitrogen-fixing
bacteria of the soil upon a gelatine suitable for the tetanus parasite. In
each case the character of the culture media must correspond to the require-
ments of the organism to be cultivated.
/ ~~ Tt has been customary to divide the bacteria, according to their mode of
life, into two great physiological classes, the Saprophytes and the Parasites.
Both groups are dependent upon other organisms for their sustenance ; both
are unable either to build up their protoplasm from inorganic bodies, or
to obtain, from the decomposition of such, the energy necessary for their
vital processes. Those bacteria which can live and grow only within the
tissues of living organisms are called parasites, those which are satisfied
with the secretions or excretions of living, or the substance of dead, organisms,
are called saprophytes.
But this classification no longer corresponds to facts. Being a corollary
of two axioms of general physiology, it could be held good only so long
as their validity was unimpaired. The first of these axioms was, that of
all organisms the green chlorophyll-bearing plants alone* are able, with
the aid of sunlight, to assimilate the carbon dioxide of the atmosphere, and
from it to build up carbohydrates. All other living things, that is to say,
plants devoid of chlorophyll, bacteria, and all forms of animal life, were
supposed to be dependent, for their carbon, indirectly or directly, upon the
carbon compounds already formed by green plants. The integrity of this
principle has now been destroyed by the discovery of certain bacteria in the
of the atmosphere. The second postulate was that no organisms of any kind
could fix and utilize the free nitrogen of the air. All organic nitrogen was
thought to be derived from the nitrates of the soil; green plants being
* The red and the brown seaweeds also.
48 DISTRIBUTION AND ORIGIN
again the only forms of life able to take it up in this state, other organisms
receiving it from them at second hand in the shape of highly complex
nitrogenous bodies. But this theory too has had to be abandoned since
bacteria have been found, semi-parasitic in the tissues of plants, that can
assimilate and store up free nitrogen (see Chap. X), The two groups of
bacteria which necessitate a qualification of these two fundamental principles
are, in a sense, complementary to one another; on the one hand, the earth
bacteria which get their nitrogen from soluble nitrates, and their carbon
dioxide from the air; on the other, the bacteria living on certain Legumi-
nosae which derive their carbon dioxide from the tissues of the plant, and
their nitrogen from the air.
These facts force us to recognize the existence among bacteria of a
special group of forms characterized by an extremely primitive metabolism,
a physiological humility which shows them to occupy the very lowest rung
of the ladder of life *.
Such organisms as these cannot possibly be called saprophytes, and
placed side by side with organisms which, like the putrefactive bacteria, have
complicated physiological requirements. A better classification would be
to divide the bacteria, according to their mode of life, into three biological
groups, prototrophic, metatrophic, and paratrophic (24). Prototrophic species
are those which either. require no organic compounds at_all for their
nutrition (nitrifying bacterium), or which, given the smallest quantity of
organic carbon, can derive all their nitrogen from the atmosphere (bacteria
of root-nodules). With them may be classed those forms that are able,
with a minimal supply of organic matter, to break up specific inorganic
bodies and derive energy from the process (sulphur and iron bacteria). In
no case are the chemical changes involved clearly understood, and there
apparently exist forms exhibiting gradations between the purely proto-
trophic and the metatrophic habit.
The metatrophic bacteria, under which heading most known species
must be placed, cannot live unless they have organic substances at their
disposal, both nitrogenous and carbonaceous. They flourish in every place
where these are accessible, in impure water, on foodstuffs of all kinds, and
in all the cavities of the body which are in communication with the exterior
and offer nourishment in the form of secretions or particles of food; the
mouth, for instance, and nose, the alimentary canal, and the vagina. Many
of the metatrophic bacteria bring about profound changes in the chemical
composition of the media in which they grow: fermentation (zymogenic
bacteria), or putrefaction (saprogenic bacteria). Others break up the sub-
stratum in a less conspicuous manner, but develop particularly in media
* Tt seems more than probable that further investigation will result in the discovery of other low
organisms (protozoa) with similar functions.
CLASSIFICATION ACCORDING TO NUTRITION 49
which have already been decomposed by saprogenic species, and contain
a mixture of organic compounds which serve as nutriment. Such bacteria
may be termed saprophile. :
In the case of some metatrophic species the chemical change effected
by their growth varies according to the nature of the medium they grow in
(polytrophism), while the action of others is restricted to some one specific
process (monotrophism, see Chap. III). The ability to grow in the living
tissues is possessed in various degrees by different species, they are faculta-
tive parasites (cholera, anthrax, and perhaps typhoid bacilli), but many are
totally unable to exist under such conditions (obligatory saprophytes, obligatory
metatrophic bacteria). .
Finally, the paratrophic group includes all those bacteria that can
exist only within the living tissues of other organisms. Paratrophic bacteria
occur either not at all outside the animal body (gonococci), or else only as
dust-borne spores or resting cells (tubercle, diphtheria). Their culture is
possible only under conditions closely resembling those of the living tissues
(on serum at blood-heat, for instance), and where there is freedom from
competition with rapidly growing metatrophic species. Some paratrophic
bacteria seem to flourish well even outside the animal body, if their growth
- on culture media is any criterion, but the conditions here are so very
special that no conclusions can be drawn therefrom as to their behaviour
in nature.
Using then the modes .of nutrition as a basis for classification, we
‘distinguish the following groups :—
I. PROTOTROPHIC BACTERIA.
Nitrifying bacteria, bacteria of root-nodules, sulphur and iron bacteria ; occur only in
the open in nature, never parasitic, always monotrophic.
II. METATROPHIC BACTERIA.
Zymogenic, saprogenic and saprophile bacteria; occur in the open and upon the
external and internal surfaces of the body, sometimes parasitic (facultative parasites),
monotrophic or polytrophic.
III. PARATROPHIC BACTERIA.
Occur only in the tissues and vessels of living organisms, true (obligatory) parasites.
’
It is worthy of remark that not only the bacteria but all other organisms
can be arranged in these three biological divisions. All green plants from
the simplest unicellular alga up to the largest forest trees are prototrophic;
all fungi and animals are metatrophic, except parasitic forms, which are
paratrophic.
_ Seeing that the spores of metatrophic bacteria are ubiquitous, it is not
‘surprising that all liquids offering suitable conditions of nourishment should,
if left uncovered and exposed to the air, soon become turbid from the
FISCHER E
50 DISTRIBUTION AND ORIGIN
development of bacteria. But although it seems to us nowadays self:
evident and unavoidable that such turbidity must owe its origin to the
development of the ubiquitous and invisible germs—omune vivum ex ovo—
this occurrence was, until not so very many years ago, a profound mystery.
It appeared absolutely certain that these living ‘animalcula’ must have
arisen spontaneously from the non-living constituents of the liquid. ‘ Spon-
taneous Generation’ (generatio aeguivoca) seemed, on apparently good
evidence, an indisputable fact.
The question as to the origin of life—an older problem even than that
of evolution—is the natural outcome of the Kant-Laplace theory of the
origin of the earth, according to which our planet was at first a molten mass
that had to cool down before living things could exist upon its surface.
Whence came these first forms of life? Were they precipitated from
other planets through space upon our globe, or did they arise de xovo from
the non-living substances that then alone existed? The first hypothesis
has little to support it, and would only throw the question one step further
back. It seems much more probable that the first organisms did really
arise from inorganic matter by ‘spontaneous generation,’ and that, as the
evolutionary theory assumes, all other living things have been gradually
evolved from them in continued series. The doctrine of evolution is,
indeed, incomplete without the assumption of such a commencement. Its
possibility once granted, there is no @ priord reason to deny that it stil]
takes place, and that side by side with the ceaseless evolution of new species
there may actually arise new organisms by spontaneous generation. Since
we can imagine only the very simplest forms of life to be produced in this
manner, it is natural that evidence for their occurrence should have been
sought among bacteria and other fermentative organisms. Experiment
showed that in some cases boiling for many hours did not prevent the
development of bacteria in nutrient solutions, and since it was contrary to
all experience that any living things could have survived such temperatures
spontaneous generation seemed to be beyond dispute. When, as frequently
happened, such organic infusions remained sterile after boiling, the advo-
cates of the theory explained matters by saying that conditions had been
changed, and the fluid was ‘no longer suitable’ for the genesis of life.
They also showed that passing air through it apparently removed these
unsuitable conditions, bacteria appearing again. If, however, the air
had been raised to a high temperature first, or passed through sulphuric
acid, or even through cotton-wool, the sterility of the liquids sometimes
remained unimpaired, but not always. Contradictory results abounded
and difficulties arose both for partisans and opponents of the theory. It was
long before the controversy could be regarded as satisfactorily settled, but
settled it was at last, and we know now that at the present day a spontaneous
generation of organisms does not take place in this way. We know that
SPONTANEOUS GENERATION 51
in all those cases where bacteria appeared in liquids after protracted boiling
they arose from highly resistant spores, whose vitality the heat had been
unable to diminish, and we know further that those experimenters were
right who sought to render air sterile by passing it over sulphuric acid or
through heated tubes, and that it was for this reason that the infusions
sometimes remained clear.
Infusions of organic substances, if they be only boiled long enough, will
remain sterile for years, even in vessels closed only by a plug of cotton
wadding. A spontaneous generation of new life never takes place; for
bacteria, like all other organisms, can only originate from pre-existing
germs. The fantastic theories that spring up from time to time regarding
the derivation of bacteria from the disintegrated protoplasm of the higher
creatures are apocryphal and, like the stories of organisms that consist
only of carbon, oxygen, and hydrogen, must be received with the scepticism
they deserve (26).
Bacteriological research has thrown light from another side also upon
the irrepressible theory of spontaneous generation. Before the discovery
of prototrophic micro-organisms it was inconceivable how the first forms
of life to appear upon the earth could have obtained their nourishment,
for all organisms, including the infusoria and other protozoa, seemed to
be metatrophic. Now, however, that we are acquainted with the meta-
bolism of the prototrophic bacteria, particularly the nitrifying bacteria, we
can form some conception of the mode of nutrition of the earliest terrestrial
organisms,
CHAPTER VI
PHYSIOLOGY OF NUTRITION AND GENERAL PRINCIPLES
OF CULTURE
Chemical Composition of Bacteria (27).
THE substance of bacteria, like that of all other living organisms,
consists for the main part of water, which constitutes about 85 per cent. of
their weight (aman body 65-70 per cent., green plants 60-80 per cent., algae
about go percent.). The high proportion of water (shown also in the abun-
dance of sap-vacuoles) is due to the fact that they are aquatic, not terrestrial
organisms, inhabitants of liquid media. Analysis of the substance of pure
cultures freed as far as possible from the culture medium has given the
following results :—
(NENCKI) (KaPPEs)
Putrefactive bacteria (a Bacillus
mixture of several species). prodigiosus.
Water . . . ° Fi ‘ 83-42 < 7 85-45:
Protein substances . . . «. 1396 . « 10-33
Fat. . ‘ . - F . 1-00 3 3 70
Ash . : : 6 é i é +78 “ - 1-75
Residue (not analysed) . . . 84 8. . 1:57
These two analyses give, of course, only a general idea of the composition
of the bacterial cell. Other cases would probably show greater differences,
for bacteria are, like all other organisms, dependent in their composition
on the nature of the food-stuffs' available. Like these also they are no
doubt capable of selecting their food to a certain degree.
The quantity of the ash will be larger where the surrounding medium
is rich in salts, and there will be a higher percentage of proteids in bacteria
from a nutritious peptone culture than in those nourished on glycerine and
ammonium chloride. Still, on the whole, their composition is evidently not
widely different from that of other organisms, as the above analyses show.
CHEMICAL COMPOSITION OF THE CELL 53
Nencki (27) isolated the proteid constituents of the bacterial cell by pre-
cipitation with boiling hydrochloric acid, removal of fats with ether and
alcohol, solution in caustic potash and final precipitation with chloride of
sodium. The substance thus obtained (mycoprotein) agrees pretty closely
in composition with one prepared by Schlossberger from yeast, and contains
52°39 per cent. carbon, 7°55 per cent. hydrogen, 14-75 per cent. nitrogen,
and about 25 per cent. oxygen. Even if we assume that this body (free
from sulphur and phosphorus) is an unaltered constituent of the bacterial
cell and not a decomposition product from more complex proteins, it is
not necessarily the essential constituent of the protoplasm. If it were we
should have to place the bacteria chemically as well as morphologically
below all other organisms, where, as far as we know, the phenomena
of life are inseparably bound up with the presence of highly complex
bodies like nuclein and nucleo-albumen rich in phosphorus. Since, how-
ever, such compounds do undoubtedly occur in some bacteria, further
investigations are necessary before we can form an opinion as to the nature
and importance of mycoprotein.
Closely related to the proteid bodies (using the word in its widest sense)
are those poisonous substances known as toxalbumines. The constitution
of their molecule is, however, as yet quite unknown *.
Carbohydrates are probably present in all bacteria, but they certainly
do not play such an important part as in the cells of higher plants, for, as
already mentioned, the cell-membrane of bacteria consists not of cellulose
but of a proteid substance. Apart from the not very clear indications
afforded by the ‘granulose reaction’ in some few species (p. 13) there are
no instances of differentiated carbohydrate cell-contents.
The gelatinous substance produced by Leuconostoc (see p. 10) and
by some of the mucilaginous species that occur in wine and beer is probably
a carbohydrate (dextrane, [C,H,,O,],) similar to cellulose and the slimy
secretions of some gelatinous algae. As special temporary cell constituents
must be regarded also those products of fermentation which, although
formed in the cell, are not stored up but excreted as metabolic products (see
Chaps. XI-XIV). The percentage of ash given in the various analyses of
bacteria cannot be taken as indicating in any degree the value of salts in
the economy of the cell, because the nutritive media were not prepared with
this end in view.
Food Stuffs of Bacteria (28).
Mineral substances are as necessary for the Bacteria as for all other
organisms, albeit in a much smaller quantity. One milligram of living
bacteria (i.e. about thirty milliards of individuals) with 1 per cent. of ash
* For the relations of the toxalbumines to infectious diseases, see Chap. XVII.
54 PHYSIOLOGY OF NUTRITION
would contain only ;3, milligram of mineral salts, so that it is evident that
‘very minute quantities of such salts are sufficient for nutritive media, say
from o-I to 0-2 per cent.
Of the elements may be mentioned as indispensable: sulphur, phos-
phorus, calcium, magnesium, potassium, and sodium, with traces of chlorine
and iron. Whether other alkalies such as rubidium or caesium can be
substituted for potassium and sodium, or other alkaline earths, such as barium
or strontium, for calcium, is doubtful. Recent experiments on the metabolism
of mould-fungi render it improbable.
Full details as to the most suitable proportions of mineral salts in nutri-
tive media will be found in practical treatises. In the following brief account.
of the nutrition of bacteria, the words ‘necessary salts’ will be understood
to mean o-r per cent. K, HPO,, 0-02 per cent. Mg SO,, and o-o! per cent.
CaCl, Sodium and iron are also necessary, but are always present in
traces in the other chemicals used unless these have been specially purified ;
they are likewise present in tap-water. For pathogenic forms the addition
of from o-1 per cent. to o-7 per cent. NaCl is advisable ; but if the media
have been prepared with infusion or extract of meat it is not necessary.
These few mineral substances will be found sufficient probably for all
bacteria, be they prototrophic, metatrophic, or paratrophic. The require-
ments in carbon and nitrogen compounds, on the other hand, are widely
different for each group.
The prototrophic nitrifying bacteria grow well in a fluid of the following
composition :—
Water, H,O 2 F . a . F 100-
Potassium Nitrite, KNO,_ : : 05
Potassium Hydrogen Phosphate, K,HPO, F +02
Magnesium Sulphate, MgSO, . - . 3 03
Sodium Carbonate, Na,CO, . ‘i ‘i : 05
Sodium Chloride, NaCl .. ms 4 . 05
The nitrous acid supplies the nitrogen, and the carbon is derived, not from
the Na,CO,, but from the carbonic acid of the atmosphere. Other proto-
trophic soil bacteria are able to fix atmospheric nitrogen, but require some
organic compound as a source of carbon; sugar for instance.
The growth of various metatrophic bacteria upon solutions of different
carbon and nitrogen compounds is illustrated by the following table. All
the solutions contained the same amounts of mineral substances, and, with
the exception of three cases, had an alkaline reaction; free acid, as the
figures show, generally retarding development. The cultures were kept at
the most suitable temperature in each case and were watched for fourteen
days, so that delayed growth might not be overlooked. The symbols
employed have the following meanings :—
FOOD STUFFS 55
+++ = very luxuriant growth, liquid turbid with bacteria or a pellicle upon the
surface (cholera, B, szb¢z/is), or deposit of bacteria at bottom of vessel
(anthrax).
++ = growth moderate, faint but distinct turbidity or a delicate pellicle.
+ = feeble growth, liquid opalescent or turbidity visible only on shaking.
++? = scarcely any growth.
O = no growth.
Source of Source of ~ | Reac- | B. An- B. B. coli vibr. B. B. pyo-
Nitrogen. Carbon. tion. | thracis. | typhi. » CO". | cholerae.| subtilis. | cyaneus.
1| 1% Peptone {1% Grape sugar] alk. |++-+|+-++|+++|+++|+++ [+++
2) ,, Peptone » Peptone alk. | ++ | +4+ | ++ |] ++ + |4+4++
8| ,, Asparagine | ,, Grapesugar| alk. ce) + J+4+4+/4+4+4+/++4+/+4++
4| ,, Asparagine} ,, Grapesugar| acid fo) +? |4+4+4+/] 0 ++ |+4¢+
5| ,, Asparagine | ,, Asparagine | alk. fe) fe) ++) ++ + +
6| ,, Asparagine | ,, Asparagine | acid oO fe) + ° + +
7! ,, Ammonium] ,, Glycerine | alk. fe) fe) ++ + {+++ |) +++
tartrate i
8| ,, Ammonium| ,, Ammonium] alk. ie) ie] +? fe) fo) +?
tartrate tartrate
9| ,, Ammonium| ,, Glycerine alk. (e) o }+4+4+) ++] ++ | ++
chloride
10) ,, Ammonium| ,, Glycerine | acid ° Oo j++4)] 0 ++ +
chloride
11] ,, Potassium » Grapesugar| alk. oO ° + +? |} +4+ |+4++
nitrate
12| ,, Potassium | ,, Glycerine alk. fe) fo) fe) (e) Oo {+++
nitrate
13 none oy Sugar alk, (o) fe) fe) O° fe) Oo?
14| ,, Potassium none alk. O° fe) ° ° oO ce)
nitrate.
.The most conspicuous result that the table shows is the great difference
between B. anthracis and B. pyocyaneus. The former prospers only where
peptone is available as a source of nitrogen (1 and 2), the latter grows just
as well and exhibits its magnificent fluorescence upon nitre (12). In other
words B. anthracis is a ‘peptone bacterium, B. pyocyaneus a ‘nitrate bac-
terium’ approaching the members of the prototrophic group in its physio-
logy. The latter, however, needs a special source of carbon, not being
able to assimilate the carbon dioxide of the air (14). There is another
numerous group of metatrophic forms which, given carbon in a suitable
form, can utilise the nitrogen of ammonia compounds, and thrive as well
as they do upon peptone. B. coli is one of these ‘ammonia bacteria; and
also V. cholerae and B. subtilis. Others, B. typhi for instance, are ‘ amido-
bacteria’; they cannot assimilate the nitrogen of ammonia, but grow fairly
well upon amide bodies, such as asparagine and leucine. One fact must
not be lost sight of if the bacteria are classified in this way, according to
56 PHYSIOLOGY OF NUTRITION
their nitrogen requirements: the power of utilising the nitrogen in different
compounds is largely dependent upon the form in which the carbon is
supplied. Thus, the ammonia bacteria can utilize the nitrogen of saltpetre
if sugar is present, but they are unable to do so if the carbon is presented
to them in the form of glycerine. No bacteria can dispense with nitrogen
entirely, and the feeble growth of B. pyocyaneus in sugar solution alone (13)
was probably due to the presence of impurities, or to the absorption of
ammonia from the air of the laboratory.
The first attempts to characterise bacteria according to their behaviour
towards different nitrogen compounds were made by Nageli, who also
examined yeasts and mould fungi in the same way. The subject has been
still more recently studied by Beyerinck (28).
In medical bacteriology, fluid culture-media are now very rarely ~
employed, having been driven out by the stereotyped recipes for peptonised
gelatine and agar. That this has been disadvantageous there can be no
doubt. The case of 2. typhi and B. coli, the one an amido-bacterium, the
other an ammonia bacterium, shows the value of fluid media of varying
composition for the differentiation of species. Not less important are the
conclusions which may be drawn from such investigations as to the
occurrence of pathogenic bacteria in nature. This subject will be more
fully discussed in later chapters.
The form in which nitrogen is offered affects greatly the influence
which the reaction of the culture medium has upon the growth of the
bacteria. The more assimilable the nitrogen compounds, the less sensitive
are the bacteria to the deleterious effect of free acid. 2B. pyocyaneus, as the
table shows, is, when it gets its nitrogen in the form of asparagine (5-6),
unaffected by the acidity of the medium which palpably retards growth
when ammonium chloride is the only source of nitrogen. The extremely
sensitive V. cholerae is totally overpowered by free acids (4 and 6),
while B. sudtilis, which grows well on weakly acid hay-infusion, is less
marked in its behaviour, and B. colz seems to be quite uninfluenced by the
reaction.
The formation of pigment too, in B. pyocyaneus, as in other chromogenic
bacteria, is largely dependent upon the reaction of the medium and the
nature of the nitrogenous bodies it contains. The organic compounds of
nitrogen, although they all of them contain carbon also, are not in themselves,
sufficient for vigorous growth, as a comparison of Nos. 1, 2, 3 and 5 of the
table shows, The carbon of ammonium tartrate (8), for instance, is
absolutely useless to B. subtilis, and even B. coli and B. pyocyaneus are
half starved when they have no other carbon-containing compound to draw
upon. The value of a special source of carbon, so clearly shown in the
table, is twofold: the excess of organic carbon is used in the building up
of proteids; and, more important still, the breaking down and oxidation of
FOOD STUFFS 57
the carbon compound provides the necessary energy for the full utilisation
of inferior nitrogenous substances (11 and 12).
The organic carbon compounds are of very different degrees of import-
ance as food-stuffs, their value depending mainly but not exclusively upon
their heat of combustion. Grape sugar and sugars in general are the best,
then glycerine and other polyacid alcohols, such as mannite and dulcite,
and finally compounds like tartaric and succinic acid, benzoic acid, and the
monacid alcohols and their derivatives such as fatty acids and amines *.
Compounds, in which an oxygen atom is linked by two valencies to the
carbon atom, as in urea CO(NH,),, or oxalic acid (COOH),, are useless as
sources of carbon, as are also the cyanogen comtpounds. It would appear,
therefore, that the carbon is in its most utilisable condition when it is linked_
with hydrogen in the form Ce being less valuable as CH, still less so as
CHOH, and not at all in the CO or CN radicles. Exceptions to this rule,
however, must not be forgotten.
Speaking generally, the best data for the discrimination of a bacterial
species from the standpoint of nutritional physiology are those afforded by
its behaviour towards nitrogenous compounds, the demands made for carbon
being less narrow and well defined.
The paratrophic bacteria are, as our table shows, able to grow only in
those solutions that contain peptone. They approach in this respect most
closely to the metatrophic forms, but their requirements are often much more
special. The gonococcus, for instance, grows only upon albuminous media,
such as coagulated blood serum. Upon this too the diphtheria bacillus
thrives best. The tubercle bacillus, which we are obliged for the present to
regard as a Strictly parasitic species, is able to grow upon inferior substrata,
even on those of the ammonia bacteria (see Chap. XVI).
For the culture of bacteria there are commonly used, besides bouillon
and other fluids, the so-called solid media prepared with gelatine or agar.
The commonest recipe is an infusion of meat (1 lb. meat to 1 litre water),
with about 1-2 per cent. peptone and sugar, to which Io per cent. gelatine
or 1-2 per cent. agar is added, the whole being boiled and filtered hot.
This gives a clear gelatinous mass, in which the nutritive substances are
equally distributed throughout the non-nutrient gelatine or agar. The
introduction of such culture media (29) has been an important factor in the
rise of bacteriological science, for it is only by the use of easily melted
and easily congealed substrata that we are able to isolate bacteria without
difficulty, and obtain them in pure cultures,
The nutritive fluids mentioned in the table on p. 55 may of course be
mixed with gelatine or agar, and the advantages of several methods united.
* For details, a re-examination of which is desirable, see Nageli ; appendix No. 28.
58 PHYSIOLOGY OF NUTRITION
If it is desirable to exclude all organic compounds, colloid silica’ may be
used as a matrix instead of gelatine. Great latitude is permissible in the
composition of the peptone-sugar-meat solutions that are used. Almost
every laboratory has its own special recipes, dictated either by whim or by
the results of long experience. In many cases the media are unnecessarily
overloaded with nutriment. Besides meat, hay, straw, plums, potatoes and
many other substances may be used to make nutrient infusions. The wort
of beer is often used, and, as an opaque solid culture-medium, potatoes are
often employed. For practical details the technical handbooks must be
consulted.
Apart from characters such as pigmentation or the production of gas,
the modes of growth of different bacteria on the same substratum are
sufficiently varied to be of some use in the differentiation of species. Too
much importance, however, must not be attached to these features. In
fluid media, two chief types of growth are seen. In the one the liquid
remains clear, in the other it becomes turbid. When the fluid remains
clear we know that we have to do with non-motile bacteria with a tendency
to filamentous growth. They collect on the walls and bottom of the tube
in flocculent masses, which may be dislodged by shaking (Anthrax, Strepto-
coccus). When such non-motile forms need much oxygen (B. tuberculosis
for instance), they form a thick membrane, sometimes smooth, sometimes
wrinkled, over the surface of the clear liquid. All isolated, non-filamentous
forms, particularly the motile species (Cholera, Typhoid), cause the medium
to become uniformly turbid. The turbidity varies from a dense milky
opacity to a faint opalescence only visible on shaking. Strongly aerobic
species form a pellicle on the surface of the fluid (V. cholerae, B. subtilis).
By culture upon gelatine we are able to divide the bacteria into two
groups, those which, by means of a peptonising enzyme, liquefy the gelatine
and those which do not. The great majority of known forms belong to the
former class; B. typhi, B. coli, Streptococcus, Lactic bacilli, &c., are examples
of the latter. Further differences of growth on gelatine are brought out by
the methods of ‘plate’ and ‘stab’ culture. In plate culture the melted
gelatine is inoculated with a minute quantity of bacteria, or of substances
containing bacteria, and then poured out in a thin layer upon glass. There
it coagulates, and the germs it contains multiply. The germs being fixed
produce around themselves sharply-defined colonies, each colony having
arisen from a single germ or from a few stuck together. The young
colonies in particular show many characteristic features; colour, form, and
contour, consistence and lustre, and the structure as seen under a low-
power lens are all useful points. A detailed account will be found in
Lehmann and Neumann’s atlas (80), and is needless here. The following
description of the colonies of two well-known bacteria will show the nature
of the differences involved better than an enumeration of all the known
MODES OF CULTURE 59
varieties of growth. The two species selected are the cholera and the
anthrax bacteria.
The anthrax colonies liquefy the gelatine slowly ; they are whitish and
round ; under a low power the edge of the colony appears filamentous, in
‘tresses’ and loops, not smooth; in older colonies an irregular skein or
clump lies in the fluid, almost transparent, gelatine. The cholera colonies
liquefy the gelatine rapidly ; they are yellowish white, and magnified show
a granular texture and an undulating, not filamentous, edge; the liquefied
gelatine immediately round the disintegrating granular clumps is turbid.
It must not be forgotten that many variations occur, and that closely
similar forms, such as the cholera vibrio and the aquatic vibrios or typhoid
bacilli and B. colz, are not distinguishable by their colonies alone.
The gelatine stab-culture is made by thrusting vertically into a tube
of coagulated gelatine a platinum wire charged with the substance of a pure
culture. Species which need oxygen grow only in the upper parts of the
gelatine, anaerobic forms only in the deeper parts. Growth in filaments or
chains shows itself by delicate threads growing out into the surrounding
gelatine so that the path of the needle appears feathery or moss-like,
e.g. Anthrax. Bacteria whose cells are isolated are confined in their
growth to the path of the needle: Among species which liquefy the
gelatine great importance is laid upon the shape of the liquefied tract.
This may be equal all along the path of the needle, and have the appear-
ance of a straight liquid tube in the middle of the solid gelatine, or, if
liquefaction is more rapid towards the surface, a funnel-shaped well is
produced. Here again considerable variations take place, and closely allied
species are not distinguishable by these data alone.
The surface of agar or gelatine that has been allowed to solidify in
tubes in a slanting position is used for ‘streak’ cultures. These are made
simply by smearing the surface of the substratum with a platinum wire
charged with bacteria. The differences in colour, contour, lustre, and con-
sistence are similar to those of the colonies in plate cultures. Agar is not
liquefied by any bacteria.
The modes of nutrition of particular biological groups among bacteria
will be considered in the subsequent chapters.
CHAPTER VII
RESPIRATION OF BACTERIA
Aerobiosis and Anacrobiosis; Light-producing Bacteria; Marine
Bacteria; Sulphur and Iron Bacteria.
THE old and now obsolete term ‘vital air’ applied to oxygen meant
that without it life was impossible. Of all the axioms of physiology, none
has ever seemed to be so unassailable as that which asserted the necessity
of oxygen to every living thing. It appeared to be a law absolutely
without exception that all animals and all plants must ‘breathe, that is
to say, must take up free oxygen in order to oxidise and break up the
organic compounds of their food or of their tissues, and thus obtain the
energy necessary for the processes of life. Deprivation of air seemed
inevitably followed by cessation of life. Here again the study of yeasts
and fermentative bacteria has overturned the old views and revolutionized
our ideas regarding the chemistry of life.
In 1861 Pasteur (31) discovered that many zymogenic bacteria could live
and even set up active fermentation in the absence of oxygen; and he gave
them for this reason the name of anaerobes. His statements, romantic and
improbable as they at first appeared, were soon corroborated, and the
general recognition of their accuracy has resulted in the division of the.
bacteria into two groups, the aerobes and the anaerobes. In the aerobic
bacteria the process of respiration is the same as in all ordinary organisms,
They absorb oxygen, and with it break up non-nitrogenous bodies such as
glycerine or sugar into carbonic acid and water. They are also able, like
plants and animals, to assimilate nitrogenous substances such as peptones
and amido compounds, although with less gain of energy and less easily
than they can carbonaceous bodies. Many of the aerobic bacteria are
totally unable to live without oxygen, and when deprived of it die as would
a mouse in pure hydrogen. They are exclusive or obligatory aerobes. In
rarefied air or in artificial mixtures of gases their vitality is proportional to
the amount of oxygen present. Under an air-pump, for instance, they
AEROBIOSIS AND ANAEROBIOSIS 61
ceas2 to grow long before a vacuum is reached (acetic bacteria, B. subtilis).
Contrasted with the obligatory aerobic bacteria, we have the obligatory
anaerobic forms which thrive only in the absence of oxygen, small traces of
this gas being sufficient to inhibit growth (B. ¢e¢anz, some butyric bacteria,
bacillus of malignant oedema). Between these extremes there is a great host
of bacteria representing every gradation between the two modes of life.
These are the facultative anaerobes, which, while growing best with a
plentiful supply of oxygen, are nevertheless able to exist with a very small
amount, and even with none at all, although in this case their vitality is often
much impaired. Anaerobic bacteria, both obligatory and facultative, are found
everywhere in nature where the air cannot penetrate, or where it is replaced
by other gases—in the deeper layers of the soil, for instance, in the mud of
rivers and standing waters, or the ooze of the sea bottom, and in manure.
In all such places anaerobic bacteria are the principal and often the only forms
of life, and by the fermentative and putrefactive processes they set up they
effect the disintegration and removal Of dead animals and plants.* As be-
longing to the group of facultative anaerobic bacteria may be mentioned the
lactic acid ferment, the bacteria of typhoid and cholera, many staphylococci
and streptococci and most putrefactive bacteria. The power of anaerobic
growth varies, even in the same species, according to the source whence
the germs were derived and the mode of culture. Deprivation of oxygen
affects the bacteria in many ways. Some of the pigment bacteria (B. vio-
laceus, for instance) give rise to colourless cultures if oxygen be excluded ;
others, like Spirdllum rubrum, form pigment most plentifully under these
conditions, though here again, as in so many other cases, exceptions occur.
Many obligatory anaerobic bacteria (e.g. some of the butyric ferments)
are motile, the energy necessary for movement being derived from the
partial breaking-down of the molecules of the fermentable substances on
which they grow. Access of oxygen stops the motion at once, just revers-
ing the conditions found among the aerobic forms, where the cilia work
the more vigorously the more oxygen is present, and become paralyzed at
once if it be cut off. These movements, minute as they are, represent an
expenditure of energy that is considerable when the size of the organisms
is taken into account.
Those aerobic bacteria which possess the power of movement are
attracted by oxygen. If a drop of water containing them be placed upon
a slide and covered with a thin glass slip, they crowd to the edges of the
preparation and around air-bubbles in the water. This property of being
attracted by oxygen has been utilised by Engelmann (82) for the detec-
tion of minute traces of free oxygen. His ingenious method, which
* The chemical aspects of the phenomena of putrefaction as well as the theoretical explanation
of anaerobiosis are discussed in Chap. XIV.
62 RESPIRATION OF BACTERIA
employs aerobic bacteria as living reagents, has given us what has hitherto
been unattainable, a micro-chemical test for the gas. It is a well-known
fact that there is in green plants, going on side by side with the ordinary
respiration of the cells, a second process of gaseous exchange accompany-
ing the assimilation of carbon dioxide and also often incorrectly called
‘respiration.’ The carbon dioxide is split up in the green parts of the
plant by the energy of the sunlight, the carbon being retained, and the
oxygen thrown off into the atmosphere. Now it is not all rays that are
equally powerful to cause this chemical change. The absorption spectrum
of chlorophyll shows that the red rays between Frauenhofer’s lines B and
C are the most strongly absorbed by green plants, and after them that
part of the spectrum just beyond F. It is just these regions that are most
active in causing the evolution of oxygen, and Engelmann demonstrates
this inthe following way. By means
ob ¢ 2 45 i of an arrangement of prisms in the
substage of a microscope he pro-
jects a spectrum upon green algal
filaments or moss-leaves lying in
the field of view, the water in which
= they are mounted containing aerobic
i ll bacteria. If, now, the illumination
be strong, and all extraneous light
be carefully excluded, it can be
seen that the aerobic bacteria crowd
Fic. 16, Detection of oxygen by means of bacteria. sound the filaments just where the
The vertical lines are the Frauenhofer lines of a spectrum :
thrown on the field of the microscope. In the spectrum active rays fall, the chief swarm
a
lies a filament of the alga Cladophora, and around it, at
B, C, and F, the bacteria swarm (see text). Magn. 200. lying between B and C, and an-
other smaller one near F (Fig. 16).
Around these parts of the alga only a few scattered organisms occur, show-
ing that the evolution of oxygen is at a minimum. The application of
this bacterial method for the detection of minute traces of free oxygen is
an operation of great delicacy, and the results which it gives must be
interpreted with great care because the movements of bacteria are very
often rendered more active by other substances beside oxygen, particularly
by food-stuffs, as will be explained in the section on chemotaxis. The
attraction by oxygen is in fact only a special instance of chemotaxis.
The great amount of energy derived from the combustion of food-stuffs,
and from the oxidation of the tissues in higher organisms, is not all expended
in the form of mechanical work, but applied in part to raising the tempera-
ture of the tissues. In animals this is conspicuous in the case of ‘ warm-
blooded’ creatures, and in plants we have instances in the germinating grains
of cereals and in the flowers of the Aroideae, which are often sensibly warm to
the touch. Fermenting and putrefying substances also (hay, manure, cotton-
THERMOGENIC AND PHOSPHORESCENT BACTERIA 63
waste) frequently become heated, the temperature inside the mass being often
raised to 60° or 70°C. This spontaneous heating, which may increase to
actual ignition (spontaneous combustion), is due to the respiratory activity of
aerobic bacteria (thermogenic bacteria of Cohn), which set up fermentation
and putrefaction.
Cohn (83) found in damp cotton-waste a micrococcus which, when well
supplied with air, raised the temperature of the decaying mass to 67° C., if
care were taken to prevent radiation, carbonic acid and trimethylamine
arising as respiratory products.
Light is another form in which the surplus energy of respiration exhibits
itself both in terrestrial and marine plants and animals. The existence of
phosphorescent insects is a matter of common knowledge. The mysterious
light often seen in old willow-trees is given off by the mycelia of certain
parasitic fungi. Among marine organisms there are very many kinds which
are luminiferous. The phosphorescence of the sea is mainly due to light-
producing infusoria, hydrozoa, and ascidia, and in our latitudes above all to
bacteria (34). The various kinds of phosphorescent bacteria have been
collected together to form a ‘ biological’ genus Photobacterium. They include
actively motile straight rods and vibrio-like forms, but very few have been
completely described, and names like B. phosphorescens, B. luminosus, Vibrio
albensis must not be regarded as designating ‘good’ species. The faint
glow seen upon decaying haddocks, mackerel, and other sea fishes, is pro-
duced by these micro-organisms. Whether any phosphorescent bacteria
occur in fresh water is doubtful: but all with which we are as yet well
acquainted are from the sea. Being marine organisms, the phosphorescent
bacteria need in their culture-media from two to three per cent. NaCl,
besides the usual salts and peptone, and to obtain successful cultures
of phosphorescent bacteria the media must contain these substances,
besides some other source of carbon such as sugar, glycerine or asparagine.
Phosphorescent bacteria would seem therefore to be ‘ peptone’ bacteria,
saprophytic in the sea upon dead animals and plants, from which they
are washed off in countless numbers by the waves. The phosphorescent
bacteria of the North Sea and Baltic flourish best at about 18° C., but
can grow quite well at much lower temperatures, even down to o° C.,
resembling in this respect the other inhabitants of northern waters. In the
absence of oxygen they are indeed able to grow slowly, but are then not
phosphorescent, the production of light being an exclusively aerobic pheno-
menon. This is evident, too, when we watch the phosphorescence of the
sea, for perfectly smooth water does not shine at all, while the brilliancy of
the light is always greatest upon the crests of the waves, or where the
water is churned up with air in the wake of a ship. That the respiratory
exchange (oxidation) of the bacteria is the cause of the phenomenon is
proved by the fact that the death of the micro-organisms or exclusion of
64 RESPIRATION OF BACTERIA
oxygen quenches the light at once, whilst a more abundant supply of
highly combustible food-stuffs like carbohydrates increases it. We know
of no specific substance (/uciferinz) to which the luminosity could be due,
and the phenomenon is of quite different nature from that exhibited by phos-
phorescent bodies like calcium sulphide, for in the bacteria the production
of light is quite independent of previous insolation.
Phosphorescent bacteria may easily be obtained by placing the flesh of
fresh haddocks or herrings in a solution of two to three per cent. of NaCl
and keeping at a low temperature (5°-10°C.). In one or two days not
merely the fish, but the whole of the liquid in which they lie, give off
a pale-greenish light which becomes much more brilliant if a little sugar or
glycerine (i. e. respirable material) be added. The bacteria may be readily
isolated by the usual methods upon peptone-sugar-gelatine prepared with
an infusion of fish and common salt. The pure cultures thus obtained are
so strongly phosphorescent that by protracted exposure they may be
photographed in their own light. The rays given off are only the more
refrangible, from DtoG. This is evident, too, from the bluish colour of
the light.
A few words may be said here regarding other marine bacteria (85).
The investigations of the German Plankton Expedition show that the
prevailing forms are actively motile rods and vibrios, cocci being less
numerous. Their distribution is determined mainly by the proximity of
land, for the algal vegetation which is richest near the shore attracts
myriads of marine organisms that provide the bacteria with a rich variety
of nutriment. The influence of the shore extends four or five miles out to
sea. The degree of illumination seems without influence, and no regularity
could be observed in the distribution, but the number of bacteria per c.c.
varies immensely, however far from land the sample be taken, as the
following figures show:
Plymouth Docks (ebb) . . ; - 7 : 13,320 per c.c.
1 mile from shore (flood) 5 : ‘ . < 3,960. +s
240 miles from shore (Gulf Stream) 3 : F 645 is
450 miles from shore (Sargasso Sea) : 20, 200, 206, 168 a
These numbers include the spores of mould-fungi. In 54 per cent. of all
samples about 100 germs per c.c. were found; at depths of from 800 to
1,100 metres below the surface only from eight to twelve bacteria were
present.
Specimens of ooze from the ocean bed at depths varying from 1,523 to
2,406 metres were sterile; others again from 4,099 to 5,250 metres contained
from one to four bacteria per c.c. This paucity is the more remarkable
when we consider that the temperature was from 2° to 5° C., and the
conditions altogether such that foraminifera and radiolaria occurred abun-
dantly. Possibly the culture-medium (peptonized fish-broth gelatine) was
SULPHUR BACTERIA 65
at fault, since it affords nourishment only to metatrophic forms, not to
prototrophic, and it is just these latter that we might expect to find on
the ocean floor. It is not at all unlikely that there are species characterized
by an extremely simple type of metabolism, which would throw new light
upon the mysteries of marine life. Russel has discovered aerobic bacteria
that reduce nitrates in samples of mud from the sea-bottom.
In the aerobic bacteria which we have been considering, respiration
consists essentially in the oxidation of complex organic substances, the
energy thus set free being necessary for the processes of life. In the case
of the anaerobes the energy developed, although smaller in amount, is
likewise derived from the splitting-up of fermentable and putrescible organic
substances. Both classes, aerobes and anaerobes, are metatrophic in this
respect. There is, however, in certain prototrophic bacteria a process going
on comparable to respiration, but consisting in the oxidation of izorganic
compounds. The micro-organisms in question are the nitre bacteria
(see p. 105) and the remarkable
sulphur bacteria, the classical
examples of prototrophic re-
spiration.
The sulphur bacteria (36),
Thiobacteria (p. 13), whose
cells are often crammed full of
‘spherical refringent masses of
pure sulphur, occur in nature
in places where free sulphury gil 17.,.Sclphur Bacteria, sre, Beggiatog al of sul
etted hydrogen is present, s2u%e waters ¢ almost te from sulphur after another day or
Such are sulphur springs where Dectesim; s, piece of a mesh like sooglosa of Lamprsietis
the SH, is principally of mineral ®*84"% 4 ¢ fom Zopf-
origin, and the mud of standing
waters and of the sea-bottom, where it is set free by the putrefaction of dead
plants and animals *. The bacteria were formerly thought to produce the
SH,, and therefore to play an important part in the origination of sulphur
springs, but the splendid researches of Winogradsky have shown that the
micro-organisms need the gas as a food-stuff, and are unable to live without
it. Thiobacteria can be found at any time of the year, but are most
abundant in the early spring and late autumn, periods when the remains
of the summer's vegetation in’standing waters are plentiful, and when the
putrefactive processes set up by other bacteria cause an evolution of SH,.
The sulphur bacteria frequently form a snow-white furry coat on the rotting
vegetation, with here and there pink or pale puce-coloured patches.
SR
oo)
Taye aeK
pean
* The ‘dead-ground’ of the Bay of Kiel and the ‘ Limanes’ of the Black Sea are instances.
FISCHER F
66 RESPIRATION OF BACTERIA
Colourless and coloured species are almost always found in close proximity
to one another—the colourless forms on substrata of all kinds, the coloured
ones in spots exposed to a strong light. The white furry coating generally
consists of filamentous forms, particularly of the unbranched non-motile
filaments of the delicately-sheathed Zhéothrix, anchored by their bases to
the bodies on which they grow. Here and there among them one finds the
free-living, slowly-oscillating filaments of Beggiatoa (Fig. 17 a-c) ; unicellular
colourless species also occur. The coloured forms (Erythrobacteria*)
present a greater variety. The genus Chromatium (particularly C. Okenit)
consists of short, plump, rod-shaped or ovoidal cells which are often present
in such numbers as to give the water a reddish tinge. Thiospirillum is
a corkscrew-shaped form. The dirty pink scum that coats stones and
vegetation in sulphurous waters consists generally of a mixture of non-motile
species, small tablets of spherical cells (TAéopedia), cylindrical species, and
zoogloeae (Lamprocystis, Fig. 17 e). Winogradsky gives descriptions of
nine genera.
The very fact that the red sulphur bacteria collect at those spots where
the illumination is strongest would seem to indicate that there is some
relation between the light and their physiological processes. To understand
this connexion, however, we must first try to gain some conception of the
physiology of the colourless forms. In these it is only the part played by
the sulphuretted hydrogen that is clearly understood, but it is highly
probable that they are prototrophic in habit. The waters in which they
naturally occur contain only traces of organic matter}; they will not grow
at all upon highly nutritious media like peptonised gelatine. Asa source
of carbon, minimal quantities of formic or propionio acid suffice, and for
nitrogen, traces of ammonia, all substances which are common products
of putrefaction. They are strictly aerobic, and do not need light. They
grow best in water containing 100 milligrams SH, per litre, but are killed
by a saturated solution of the gast. If filaments of a sulphur bacterium
already loaded with sulphur be placed in pure water they lose it all in from
twenty-four to forty-eight hours (Fig. 17 @-c), and finally die from ‘ sulphur-
hunger.’ But if, before it be too late, they be replaced in water containing
SH, they recover and continue to grow, the SH, being oxidised and the free
sulphur stored up in the protoplasm of the cells. The disappearance of the
sulphur when the bacteria are placed in pure water is due to a further
oxidatory process by which sulphuric acid is produced. This unites with
the lime dissolved in the water to form gypsum. Other bacteria common
in marshy places, such as Cladothrix and Spirillum, as well as mould-fungi,
* This term is suggested as perhaps the best equivalent of the German word ‘ Purpurbakterien,’
+ In Weilbach water only 0.0048 gram per litre, yet sulphur bacteria grow vigorously in it.
= 4-56 grams SH, per litre. The Stachelberg spring contains 0.073 gram.
SULPHUR BACTERIA 67
are not able to live in water containing SH,; they perish where Chromatium
or Beggzatoa thrives.
Seeing that a solution of SH, in water is very easily decomposed by the
oxygen of the air* and free sulphur deposited, the sulphur bacteria would
be able to take advantage of this oxidation for their life-processes, even if
they merely had the power of existing in H,S solution. The H,S entering
their cells would be oxidised by absorbed atmospheric oxygen, and the
sulphur set free would represent an abundant source of energy for further
oxidation. To explain the facts observed we need only assume further that
the protoplasm increases the oxidizing power of the atmospheric oxygen
and renders it ‘active. The amount of energy gained is very consider-
able. The oxidation of the dissolved SH, gives seventy-one calories, and
the further conversion of the freed sulphur into sulphuric acid no less than
2,109 calories.
The fact that the thiobacteria can dispense entirely with organic com-
pounds which might be burnt up to CO,, together with their inability to
live without sulphur, shows that it is the oxidation of the sulphur alone
which takes the place of the respiratory processes of other organisms.
The two great physiological processes, the assimilation and storage
of respirable material on the one hand, and the liberation of energy by
its oxidation (respiration) on the other, may be represented in a tabular
form. The following scheme shows the processes in green-plants, meta-
trophic bacteria and sulphur bacteria :
ASSIMILATION OF RESPIRABLE MATERIAL.
INTAKE. STORAGE. OUTPUT.
GREEN PLantTs | Carbonicacid avd water | Carbohydrates Oxygen
plus the energy of sun-
light
METATROPHIC Organic substances (e. g.
BACTERIA sugar, which is not
stored but burnt up at
once)
SULPHUR Bac- | Sulphuretted Hydrogen | Sulphur Water
TERIA and Oxygen
* In presence of cotton-wool or some other porous body it is even oxidized to H,SQ,.
68
RESPIRATION OF BACTERIA
RESPIRATION OR LIBERATION OF ENERGY.
SOURCE OF RESPIRATORY GAIN IN
ENERGY. Gas. BEE ENERGY.
GREEN PLants | Carbohydrates Oxygen Carbonic acid | more than 6,000
and water calories
METATROPHIC | Organic sub- Oxygen Carbonic acid | more than 6,000
BACTERIA stances (e.g. and water calories
sugar)
SULPHUR Bac- | Sulphur Oxygen Sulphuric acid | 2,109 calories
TERIA
These two tables give, of course, only a general view of the broader
outlines of the processes. Real and apparent exceptions will occur to the
student, but they do not impair the validity of the schemes as a whole.
The green plants derive from the sun the enormous amount of energy
necessary for the formation of carbohydrates out of carbonic acid and water,
and set it free again in the process of respiration. The metatrophic bacteria
make use of highly combustible organic compounds which they oxidize at
once, and the sulphur bacteria are able at little cost to store up sulphur.
The oxidation of this sulphur represents a very abundant supply of energy,
a supply that is probably far more than sufficient to cover the wants of the
organism living under the conditions already described, and building up its
protoplasm from the minute traces of fatty acids and ammonia contained in
the water it lives in, The question therefore naturally suggests itself whether
the bacteria have not some other use for the surplus energy. We know that
the nitrifying bacteria are able, without sunlight, by means of the energy
they obtain from the oxidation of nitrogen, to seize and assimilate the CO, of
the atmosphere, and perhaps the Thiobacteria are able to do the same with
the much more abundant energy derived from the oxidation of sulphur.
Should this supposition turn out to be true, we should have in the colourless
sulphur bacteria a group transitional as regards their physiology between
the Erythrobacteria and bacteria of ordinary metatrophic habit (37).
The Erythrobacteria or coloured sulphur bacteria are unique in possess-
ing the power of assimilating CO, in the presence of sunlight by means of
a special red colouring-matter (Bacteriopurpurin) which exercises a similar
function to that of the chlorophyll of green plants. Engelmann has shown
that the spectrum of Bacteriopurpurin is of a very peculiar character,
having in addition to an absorption band between B and C another
very broad one in the region of the invisible ultra-red, where the wave
length is from 0-8 to og p. By the bacterial method he was able to
prove that oxygen was evolved in both these regions, showing that the
invisible heat rays are utilized for the assimilation of CO, just in the same
IRON BACTERIA 69
way as those of the visible spectrum. The Erythrobacteria have therefore
a double source of energy, the oxidation of sulphur and the absorption of
sunlight, a great advantage to them in the struggle for existence, inas-
much as they would be able to subsist in places where, from want of
sulphuretted hydrogen, the colourless Thiobacteria would perish. What
products arise from the assimilation of the CO, is not known; starch has
not been detected.
The Erythrobacteria are among the most sensitive phototactic organisms
known, the slightest diminution of brightness repelling, the slightest increase
attracting, them. The actively motile chromatia can be led about under the
microscope at will by suitable manipulation of the light.
The importance of the part which the sulphur-bacteria play in the
economy of nature lies in their behaviour to sulphuretted hydrogen. They
take this gas—a useless and injurious product of organic decay—and work it
up into sulphates, substances which are assimilated by other organisms, and
used to build up new life.
Hardly less remarkable in their nutrition than the sulphur bacteria are
the ‘iron-bacteria’ (ferrobacteria, 38), which resemble them somewhat in
their prototrophic respiration. Our knowledge of these organisms is still
very fragmentary. Stagnant pools in marshy places are often covered by
a greasy-looking brownish scum, that consists mainly of ferric hydroxide,
Fe(HO),, together with organic matter and some phosphate of iron.
The ferruginous matter is deposited as bog iron ore. By the action of
reducing substances arising from putrefaction, the ferric compounds,
particularly ferric hydroxide, are reduced to ferrous compounds, which
are dissolved by and unite with the CO, in the water to form ferrous
carbonate. The atmospheric oxygen alone is sufficient to convert this
back slowly again into ferric hydroxide, and cause its precipitation, but
Winogradsky has shown that the change is not purely chemical, and that
here, too, bacteria step in and accelerate the process. In the iridescent
deposits of the pools enormous numbers of short brittle tubes are found.
These are the broken sheaths of an unbranched filamentous bacterium, that
may be called for the present Lepiothrix ochracea. These yellowish-brown
sheaths are coloured blue by hydrochloric acid and ferrocyanide of
potassium; they contain ferric hydroxide. Specimens of bog iron ore
from Siberia, Sweden, and North Germany contained, in thirty-three
samples, only three with large amounts of these sheaths.
Besides the empty sheaths vigorously growing mats of Leptothrix are
always found, some with the cylindrical cells still 2 sdtz, others where they
have already swum away in the form of gonidia (as in Cladothrix). By
placing the bacteria in water containing CO, the iron may be dissolved out
of the sheaths, leaving them colourless. If these bleached filaments be then
put into a weak solution of ferrous carbonate of a concentration correspond-
7o RESPIRATION OF BACTERIA
ing to that of the pools, the sheaths become coloured anew. It is, however,
only those which contain living cells that take up iron; the empty sheaths
remain colourless. It is evident from this that the living bacterial cell
hastens the oxidation of the ferrous carbonate, just as it does the oxidation
of SH, in the Thiobacteria. In the iron bacteria, too, some energy, albeit
a very small amount, is gained in the process.
Since the sheaths and cell membranes of other aquatic plants (e.g.
Cladothrix and Crenothrix among the bacteria, Conferva (Psichohorrmium)
among the filamentous algae) are also turned yellow by the deposition of
iron oxide, it has yet to be seen whether the iron bacteria form a separate
biological group. Their nutrition as regards carbon and nitrogen also needs
further investigation. They are doubtless prototrophic.
It would be justifiable from some points of view to consider the acetic
bacteria in this section, but they can be more conveniently treated of with
the other fermentation bacteria.
CHAPTER VIII
INFLUENCE OF PHYSICAL AGENTS
Light, Electricity ; Pressure, Temperature, Dryness and Moisture ;
Disinfection by means of Physical Agents.
THE Erythrobacteria are the only forms in which nutrition is influenced
by light, as in the higher plants. It is the predilection for light which
causes Chromatium and other motile species to seek out the brightest parts
of the vessels they are kept in, forming a reddish coating on the side of the
glass.
All other pigment bacteria, most of which are only chromoparous, are
quite destitute of the power to assimilate and split up CO,, although they
naturally absorb certain rays of light. The pigment is in fact only an
‘accidental’ by-product of bacterial life; it is formed just as abundantly in
the dark as in the light, and as a result none of the chromogenic bacteria
become bleached or ‘etiolated’ when light is excluded. This alone shows
that the pigment does not play the part of the chlorophyll of higher plants.
But whilst bacteria grow just as well in darkness as in weak diffused daylight
it is necessary that the illumination should remain within certain limits, the
overstepping of which leads, in our cultures at all events, to the injury and
death of the cells. Very numerous experiments have been made to show
the deleterious effect of light on bacteria (39), but it must not be forgotten
that the conditions under which bacteria occur in nature are not those of
our artificial cultures, be they on solid media or liquid. In the transparent
gelatine and agar it is impossible for them to obtain any shade whatever,
whereas in nature, in ponds and rivers for instance, the minutest algal cell
or particle of mud offers abundant shelter. It seems therefore certain that
diffuse light in nature is innocuous, and even bright sunlight can only
destroy a limited number, and drive the motile forms to seek shelter from
72 INFLUENCE OF PHYSICAL AGENTS
the direct rays. We are certainly not justified in supposing that the
sunshine causes disinfection on such a scale as would lead to the self-
purification of rivers (40).
In cultures, both spores and vegetative bacterial cells are killed by
exposure to direct sunlight for two or three hours, and it is to the actinic
rays, not to the heat, that the deleterious effect is due. All parts of the
spectrum, however, are not equally efficacious, for if the light before falling
upon the bacteria be allowed to pass through coloured solutions, such
as bichromate of potash or ammonium cupric oxide, we find that it is
only the more refrangible rays that have the power of arresting growth.
It has been shown that freshly inoculated broth cultures of typhoid bacilli,
exposed for eight hours to light that had passed through a solution of
bichromate of potash, grew well, the broth becoming turbid with the
organisms. If instead of bichromate an ammoniacal copper solution was
used, the bacilli would not grow at all, the broth remaining clear even after
five days. Leaving aside the question as to whether the culture medium
itself is injuriously changed, it is evidently the blue end of the spectrum
—the photochemically active rays—that destroys the bacteria. These
rays too inhibit the formation of spores in the common mould-fungus
(Botrytis cinerea) during the day. Only at night is it able to develop the
reproductive cells, In some fungi, on the other hand, light is necessary for
fructification. Such are the little (1-2 mm. high) P2/obolus that projects its
ripe sporangia more than a metre into the air, and Coprinus, both forms
common on horse-dung. These become etiolated in the dark just as would
a chlorophyll-bearing plant. Such examples show that it is not possible
to formulate definite laws as to the action of light on colourless fungi (41).
Possibly there are other ‘photophile’ bacteria besides the pigmented
Erythrobacteria and their highly sensitive ally, the B. photometricum of
Engelmann. Asa general rule it is necessary to keep bacterial cultures in
the dark, or protect them at least from intense light. Weak diffused light
does no harm.
For practical purposes of disinfection on a large scale, light (even
bright sunshine) is unsuitable.
Strong electric currents (44) are fatal to bacteria, their protoplasm being
killed and no doubt altered in the same way as that of other plant cells is,
although some of the deleterious effect may be due to the electrolytic
dissociation of the nutrient media, and to the increase of temperature that
the current may cause. These drawbacks are a great hindrance to the
employment of electric currents for the disinfection of food-stuffs.
Attempts have been made (43) in distilleries to destroy by this means
the harmful bacteria without injuring the yeast cells. Currents of about
5 amperes were employed, but the practical difficulties of the process are
very great, and have not yet been entirely overcome.
EFFECT OF HIGH PRESSURE. ON BACTERIA 73
Weak currents act on bacteria presumably in the same way that they
do on infusoria and other motile micro-organisms (44). These are ‘ galvano-
tropic’; they collect around the kathode (negative pole). If the direction
of the current be reversed, the infusoria also turn round 180 degrees, place
themselves parallel to the current, and swim towards the other (now
negative) pole. Experiments with bacteria. have not yet been made,
and they would be very difficult on account of the small size of their
cells.
Very numerous and thorough experiments have been made as to the
effect of Rdxtgen rays upon bacteria, and it has been conclusively shown
that they are not even inhibitory, let alone fatal to the cells (45). The
higher plants behave in the same way, and the hopes that were entertained
of being able to disinfect the diseased body by means of Rontgen rays have
not been realized.
High pressure (46) seems to be.absolutely without influence upon
bacteria. The processes of putrefaction and alcoholic fermentation go on
without interruption under a pressure of from 300 to 500 atmospheres, and
the spores of the anthrax bacillus are unchanged after exposure for
24 hours to 600 atmospheres. In considering such facts as these it must
not be forgotten that the weight supported by a single bacillus (say of
5 « by 1 w) is extraordinarily small (even at 500 atmospheres only about
80 milligrams), so that even at the bottom of the deepest ocean at a depth
of 7,086 metres a coccus of 2 « diameter would only have to withstand
a pressure of go milligrams. In the face of these facts we are hardly
justified in saying that bacteria are less sensitive to pressure than other
organisms, because the behaviour of the larger plants and animals seems to
be in this respect incommensurate with that of bacteria.
The force of gravity exercises upon bacteria no effect comparable with
the geotropism of other plants.
Bacteria, like higher plants and cold-blooded animals, are organisms
whose temperature corresponds to that of their environment, falling and rising
with the external temperature ; they are pockilothermic (47). The relations
of the cell to temperature are expressed by specifying three points on the
thermometric scale, the maximum, minimum, and optimum. These three
cardinal points hold good for all organisms. The different functions of Jife
are performed best at different temperatures, and the temperatures most
favourable to growth, to movement, to fermentative power, to virulence, to
sporulation and to germination are probably different in each case, but
a general average, coinciding with the most rapid multiplication of the cells,
is spoken of as the ‘optimum.’ For all species these three cardinal points
vary slightly within certain limits.
The following cardinal points are those which hold good for growth.
The maximum is that temperature which cannot be exceeded without
74 INFLUENCE OF PHYSICAL AGENTS
cessation of growth, the minimum is the lowest temperature at which
growth even of the feeblest kind is possible. ;
Minimum. Optimum. Maximum,
Germinating wheat . ‘ 5-7°C. 29°C, 42-5°C.
» gourd. =. = 13-7" 33-7° 46-2°
Bacillus anthracis . . 14° 37° 45°
» tuberculosis . 30° 38° “42°
», thermophilus . 42° 63-70° 72°
» subtilis . . 6° 30° 50°
9» fluoresc. liquefac, 5-6° 20-25° 38°
» phosphorescens fe) 20° 38°
The table shows that wheat seedlings and free living metatrophic
bacteria like B. subtilis and B. liguefaciens need approximately the same
temperature, while the gourd plant, an inhabitant of warmer zones,
requires more, as do also the cholera and anthrax germs, 30° to 40° being
the optimum. The phosphorescent bacterium from our cold northern seas
has the lowest minimum of any, and at the other end of the scale we find
the representative of that curious group the thermophile bacteria. The
tubercle bacillus has the narrowest temperature range of all, only twelve
degrees separating maximum and minimum. It is, to use an expression
borrowed from animal physiology, s¢exothermic, as are all the strict parasites
on warm-blooded anjmals. The metatrophic bacteria, on the other hand,
are eurythermic, and can endure great variations of temperature with a
difference of as much as 30° between maximum and minimum.
Bacteria which are pathogenic for warm-blooded animals, and at the
same time eurythermic (e.g. B. anthracis), are very probably metatrophic
in our climate.
The thermophilic bacteria (48), singular as it may seem, are very widely
distributed even in our temperate climate, and a large number of species
have been isolated from the soil and from sewage. It is difficult to see
where these bacteria (mostly non-motile aerobic rods) find the necessary
conditions of existence. Strong sunshine sometimes heats the surface of the.
soil to 70° C. and might permit the development of such forms, but they
must be subject to long periods of quiescence. Manure and other substances
that become heated during fermentation are their most likely habitats,
In B. thermophilus the optimum reaches the temperature of coagulation
of most proteids and the maximum exceeds this. We might be inclined to
regard the protoplasm for this reason as different from that of ordinary cells,
but we must not forget that in all proteid bodies the temperature of coagu-
lation varies very much according to the reaction of the protoplasm, the
amount of salts present and other factors, so that we are not justified in
looking upon the thermophile bacteria as being miracles of nature. Besides,
we know of other organisms that live at very high temperatures. In
the hot springs of Ischia and around the ‘fumaroli’ at Naples micro-
EFFECTS OF HEAT AND COLD ON BACTERIA 75
organisms are found in water at 60°C. and more. In the drainage from
the Carlsbader Strudel (54°) there is a thick growth of Leptothrix with
green Oscillariae, and in other thermal springs crustacea and insect larvae
are known to occur at 60°.
Bacteria may be roughly divided according to the optimum temperature
into two great groups, those which grow best at about 20°C. (B. fluorescens,
B. phosphorescens, B. prodigiosus, and many other metatrophic forms), and
those which need a higher temperature. For the maintenance of a regular
high temperature special incubators are used. These secure great uniformity
of any desired degree of warmth by the use.of some form of thermostat,
and are subject to variations of 0-1 to o-5° only. Still better is a whole room
maintained at a constant temperature. Practical details will be found in
the works cited in No. 3 of Notes (p. 169).
When the temperature at which bacteria are grown approaches either
the maximum or the minimum, not only is growth arrested, but all the
functions of the cell are enfeebled. Continued cultivation under these
circumstances, particularly at temperatures near the maximum, effects
injuries from which the cell recovers only very slowly when the optimal
conditions are restored. (See Lects. III. & XVII.)
Exposure to extreme cold has, apart from arresting growth, very little
ill-effect upon bacteria. In this respect they resemble other poikilothermic
organisms, falling into a lethargic state (Azbernation). Sporeless anthrax
bacilli can endure a temperature of — 26-8° for twelve days without injury,
whilst the spores show no decrease of vitality or virulence after exposure
for twenty hours to —130°. Long imprisonment in ice and even repeated
freezing and thawing can be endured for weeks and months, bacteria be-
having in this respect like algae and other water plants. The cold of our
winters is therefore quite inadequate to kill bacteria, and the application of
artificial low temperature is equally useless as a means of disinfection (49).
In sharp contrast to this indifference to cold is the behaviour of bacteria
towards heat. If the maximum be exceeded death speedily follows (largely
as a result of the coagulation of the protoplasm), and consequently a tem-
perature of 50° or 60° C. for ten minutes is sufficient to kill the sporeless
vegetative cells of all but the thermophile bacteria. At 70° five minutes is
sufficient. It is on this principle that the preservation of tinned foods is
now so largely carried on and the ‘ Pasteurization’ of wine (heating for half
an hour to 70°) is another instance of its application. In ‘fractional sterili-
zation’ as applied to culture media like blood serum, which cannot be heated
to 100°, the same result is aimed at, namely the destruction of the vegetative
cells. Spores which have resisted the first heating are then allowed to ger-
minate and are killed by a second heating, the process being repeated five or
six times if necessary until sterility is secured. At the present time hundreds
of such sterilizers are in daily use in the service of antiseptic surgery.
76 INFLUENCE OF PHYSICAL AGENTS
The spores (47) are always far more resistant than the vegetative cells,
and their resistance is proportional to their dryness. This must not be looked
upon as a peculiarity of bacterial spores, for all resting protoplasm is resis-
tant in consequence of the small amount of water it contains. Grains of
corn rendered as dry as possible in a desiccator can endure from 100° to
110° dry heat for hours without losing the power of germination. They are
in this respect not far inferior to anthrax spores, which succumb to 140° in
three hours.
For many objects disinfection by dry heat is impracticable, as they
would be injured by the high temperature. The method can, however, be
advantageously used for disinfection of the glass ware used in bacteriological
work, but for surgical instruments and dressings boiling water or hot steam
is to be preferred.
Spores are more quickly destroyed by moist heat, although if the
temperature of boiling water be not exceeded some hours are necessary to
destroy such resistant spores as those of B. swbézlzs and its allies. Anthrax
spores are generally killed by boiling water in from two to five minutes, but
one must always reckon on finding a few of much greater resistance that
need ten or twelve minutes.
Moist seeds are much more quickly destroyed, being -killed below
boiling point. The cause of the greater resistance of bacterial spores is not
known, but probably both the impermeability of the membrane and the
durability of the protoplasm playa part. If the impermeability of the
membrane be the cause, it is easy to conceive how the spores can remain
some time in water before the protoplasm can take up moisture. They
would even in boiling water be exposed at first to a dry heat. This view
gains in probability when we consider that the spores of 2. subtilis germinate
very slowly, many hours passing before they swell and lose their lustre by
absorbing water. This stage is passed through more quickly ifthe spores be
boiled for five minutes. It seems as though the spore membrane were at
first very impervious to water. We know too that this is the case with the
resting spores of algae and fungi. The resting cells of other low organisms
such as amoebae, infusoria and flagellata, although not yet investigated,
are doubtlessly similar in this respect to bacterial spores.
The sterilization of fruits and preserves by boiling in closed vessels
is familiar to all and has been in use since the last century. A discussion
of the various kinds of disinfectors, of the Koch steamer, and the autoclave
that by means of superheated steam kills the toughest spores in one minute
at 140° would be out of place here. They are used for the sterilization of
culture media. The uses to which these physical methods of disinfection are
put in public sanitation will be found in text-books of hygiene.
All plants, those of our climates as well as those of the steppes and
deserts, have to protect themselves against dryness in times of drought.
EFFECT OF DESICCATION 77
Mosses and lichens on naked rocks shrivel up to dry friable masses, falling
into a resting condition in which they remain for weeks without losing the
power of growing again as soon as they receive moisture. The algae of
ponds and those that live on soil that is only periodically wetted can also
withstand drought for months. In all these cases it is the whole plant that
falls into a resting-stage, in which it can retain its vitality for long periods,
although not for an unlimited time. Among animals, too, many instances
are known (rotifers, tardigrada, anguillulae) in which the whole organism
can remain dried for weeks and months, and then be recalled to life by
moisture.
Organisms which are provided with a morphologically specialized resting-
stage (spores, cysts, or seeds) can still better resist the effects of dryness.
The spores of the Smut fungus, Ustilago carbo, germinate when placed in
water, even after lying for ten years in an herbarium; and wheat grains
sprout after ten or even twenty years if they have been carefully protected
from moisture. This suspension of vitality cannot, however, go on for
unlimited periods, and the miraculous stories of ‘ mummy wheat’ germinat-
ing after thousands of years are certainly fabulous.
The spores of bacteria (50) can also remain dormant for a very long time ;
those of Anthrax, for example, germinate after ten years’ quiescence. Thus
the resting-stage among bacteria is of similar duration as among other organ-
isms, and is shortened in the same manner by moisture or damp air. Many
bacteria can resist drying for long periods even in their vegetative condition.
Dry tubercle bacilli retain their virulence for weeks, as do also the bacilli of
diphtheria and typhoid fever and pus cocci. True aquatic bacteria, on the
other hand, such as the cholera vibrio, are killed by drying in from two
to five hours *.
For the purposes of practical disinfection drying is not applicable, but
it seems to play an important part in the destruction of bacterial cells in
nature. Still, if dried bacteria, during short periods of moisture such as
continually occur, can find suitable nutriment they revive again, and multiply,
only to fall again into the resting-stage. For this reason it is not to be
expected that disease products, or earth contaminated with organic substances,
will be spontaneously disinfected by mere exposure to the air.
* For the relation of these phenomena to infectious diseases see Chaps. XV. and XVI.
CHAPTER IX
THE ACTION OF CHEMICALS—CHEMOTAXIS AND
CHEMICAL DISINFECTION
EXAMINATION of a drop of stagnant water under a medium-power
objective shows that the bacteria and infusoria it contains are not evenly
distributed through the liquid. They gather in clouds around particles
and flocculi of decaying organic matter, to which they seem to be attracted
as fish are attracted by bait, or ants by aphides. ‘Instinctive’ is the term
(favoured of anthropomorphists) applied to such actions when performed by
the higher organisms, and yet we see them here carried out with unfailing
accuracy by creatures whose whole body consists of but a single cell. Are
then the bacteria too to be credited with instinct? That would indeed be
a surprising conclusion !
This capability shown by unicellular organisms of being attracted by
food-stuffs was first carefully studied by Stahl (51). He showed that the
naked protoplasmic masses which form the plasmodia of the myxomycetes
were affected by a nutritive body placed in their neighbourhood. They turned
towards it on whichever side it was placed, suggesting by this action the
term Trophotropism as a suitable descriptive name for the phenomenon. At
about the same time Pfeffer (52) investigated from a more general chemical
standpoint the effects of such stimuli upon the movements of bacteria,
protozoa, and the spermatozoids of the higher cryptogams, and was able to
show that the nutritive value was not the sole and only determinant, but
that other more recondite factors were involved, factors based on the chemical
constitution of the substance employed asa stimulant. For this reason he
introduced the now generally adopted expression Chemotaxis.
Pfeffer’s method for the study of the chemotaxis of bacteria is simple
and efficient. A capillary tube, sealed at one end and about 5-10 mm. long,
is half filled with the liquid to be tested, e.g. a 5 per cent. slightly alkaline
solution of Liebig’s Extract, or of peptone, in such a way that a bubble of
air remains in the closed end. The outer surface of the glass is carefully
cleaned from any traces of the broth, and it is placed in a drop of water
containing bacteria in such numbers as to be slightly turbid. In five or
ten seconds it can be seen that the bacteria are more thickly congregated
‘around the open end of the capillary than elsewhere, and in a few minutes
CHEMOTAXIS 79
they form a dense swarm there, and begin to enter the tube. The move-
ments of the bacteria in the drop become more lively as soon as the
diffusing peptone reaches them, and at the entrance of the tube they
are a whirling, ‘buzzing’ mass, like hiving bees. The potential energy
of the food-stuff has been converted into the kinetic energy of the
vibrating cilia. If now a cover-glass be laid upon the drop another kind
of chemotaxis can be observed. The bacteria which have entered the
tube move upward, attracted by the air in the blind end, and in about
half an hour that section of the capillary immediately below the bubble
is plugged by a thick mass of organisms. But these phenomena, the
attraction by food and the attraction by air, might as well be called tropho-
tropism as chemotaxis, the latter being shown in
its purest form when solutions of salt are employed. e@ ®
For instance, a 1-9 per cent. solution of KCl has
a powerful effect, and draws the bacteria into the q yl
tube just as peptone does, being still feebly at- mee
tractive even ina dilution of o-o19 percent. Among a;
the alkalies, potassium is chemotactically the most
powerful, then sodium and rubidium. The alkaline ABR
earths are less effective. The influence of a salt ‘ee hat be
is attributable mainly to its electropositive con- Sl a
stituent, the acid radical acting much more weakly. Fic. 18 Chemotaxis. A part
Further details of this interesting subject will $.a0? fronene Moma
efa-
: : ; ill
be found in the works of Pfeffer already cited. eens; and 3 party! fein
: : : ae t.weakly alkaline pept
Among organic substances with a high nutritive [Clution; at Y an airbubble.
eee : About 4 minutes after introduc-
value, asparagin and peptone may be mentioned as __ tion of capillary, the bacilli have
co 5 = collected round the open end of
being strongly chemotactic, whilst sugar, one of the tube, positive chemotaxis.
. 3-4 hr. later, the bacilli, driven
the best food-stuffs and richest sources of energy, by want of oxygen, have collected
5 ‘ 4 a 4 close to the air-bubble. From
has but little attractive power. Glycerine is in nature. Magn. 50.
all cases, as far as is known, inactive.
Contrasting with the phenomena just described, is the power which
certain substances have of repelling bacteria. This is known as negative
chemotaxis. Free acids and alkalies have this effect, and capillaries filled
with their solutions invariably remain empty. Alcohol too is ‘ instinctively ’
despised by micro-organisms. In some salts the action of the acid radical
and that of the base neutralize each other (e. g. carbonate of ammonia 1-76
per cent., phosphate of potash (monobasic) 3-48 per cent.), and the bacteria
then take up a mean position at a certain distance from the mouth of the
tube.
In the case of negatively chemotactic compounds, the poisonousness of
the substance is no more a criterion of its repellent power than the food
value is an index of the attractive power in positively chemotactic bodies.
For instance, a solution of 0-019 per cent. potassium chloride plus o-o1 per
80 CHEMOTAXIS AND CHEMICAL DISINFECTION
cent. mercuric chloride attracts bacteria by reason of the potash it con-
tains, but they rush into the tube only to meet their death from the mer-
cury salt. It would therefore seem that chemotaxis, useful as it must be to
bacteria in the search for food, may lead them to destruction, although of
course they are not exposed in nature to the temptations of such fatally
seductive capillaries.
If the facts gained by these experimental observations are to be
employed to elucidate the life-history of bacteria in their natural habitats,
in water, in the soil, or in the tissues of the diseased body, a number of con-
siderations must be borne in mind. In the first place, chemotaxis can only
take place in media which permit free movement, in liquids. Secondly,
different kinds of bacteria by no means react in the same way to the same
substances. Furthermore, it must be remembered that the sphere of
influence of such a capillary is very small. It is not possible to entice
into it all the bacteria swimming in the drop. It would not be possible,
even if we could renew the solution in the capillary as fast as it diffused so
that it could continually give off a diffusion-stream, as perhaps the decaying
flocculi in stagnant water do. For the bacteria would be already stimulated
by the diffused substance, and to enable them to again react chemotactically
a much higher concentration would be necessary. Pfeffer has shown that
“Weber's Law’ (Psycho-physical law of Fechner), which formulates the
relation between strength of stimulus and intensity of sensation, holds
good for the chemotactic movements of bacteria. Weber’s law is to the
effect that an external stimulus just sufficient to give rise to a sensation
must be increased in a definite ratio in order to awaken that sensation
again. For instance, a weight of one ounce laid upon the hand gives rise
to the sensation of pressure upon a certain spot. If now upon the weight
a further load of #5 oz. be laid no further sensation is aroused. Not
until the original weight has been increased by one-third (i.e. to 1-33 0z.) is
the sensation of pressure aroused again. Had ten ounces been the weight
originally employed it would have had to be raised to 13-33 ounces. For
thermal stimuli the increase must amount to jth, and for visual stimuli to
ziath of the original stimulus in order that it should again cross the threshold
of sensation and enter our consciousness. Precisely the same law governs
the phenomena of chemotaxis. In the case of one of the putrefactive
bacteria the stimulus has to be increased five times before chemotactic
movements are again set up. If the organism be already in a or per
cent. solution of extract of meat the capillary must contain a o-5 per cent.
solution, if it be in 1 per cent. the tube must be filled with 5 per cent.
bouillon before a reaction follows, while to start very active movement we
should have to fill the tube with a broth ten or twenty times as strong as the
surrounding fluid. These facts should not be forgotten when we attempt
to explain by chemotaxis the behaviour of bacteria in the living body, or the
CHEMOTAXIS 81
emigration of leucocytes towards a bacterial focus (53). An exact knowledge
of the conditions involved is impossible, inasmuch as the composition of the
fluids in which the tissues are bathed and the amount of chemotactic
substances they contain are entirely unknown. For this reason considerable
discretion must be exercised in the use of the word chemotaxis if it is not to
become a mere shibboleth (see Ch. XVII).
In the case of some substances, peptone for instance, the absolute
amount sufficient to cause perceptible reaction is very minute. Pfeffer
calculated that a capillary filled with o-or per cent. peptone solution,
a strength just sufficient to attract bacteria in water, contained only
guv0deee0 Of a milligram of peptone. And yet compared with the size of
the bacterial cells this quantity is not disproportional.
Like all other processes that depend ultimately upon the activity of the
living cell, the phenomenon of chemotaxis is extremely obscure. This
much however we know, that the effect of the substances diffusing from
the capillary is to cause the bacteria to turn (hence chemo-/azis) in such
a way that their longer axes are parallel to the direction of the diffusion
stream. They then swim either against the stream (positive chemotaxis) or
with it (negative chemotaxis). Why one substance should act positively
and another negatively is at present quite inexplicable. A more detailed
discussion would be beyond the scope of this treatise, and would be at best
fragmentary. Those substances which in weak solutions exercise a positive
action on bacteria have in some cases (KCl 19 per cent.) the same effect
when the solution is concentrated ; in other cases the bacteria are repelled.
Neutral salts such as KCl and NaCl can be endured in very high concen-
trations. The hay bacillus, for instance, grows well in an infusion containing
9 per cent. NaCl, 5 per cent. sal ammoniac, 11 per cent. KCl, or 10 per
cent. KNO,. Such substances are not poisonous, and arrest growth finally
by the osmotic pressure they set up.
Of great interest are those poisonous compounds which even in minute
quantities destroy the life of the cells. They are not by any means
especially poisonous for bacteria. A one-tenth per cent. solution of corrosive
sublimate (HgCl,) kills tubercle bacteria in ten minutes, and the cells of
algae are destroyed just as soon, if not sooner. In a I per cent. solution
of carbolic acid both tubercle bacilli and ordinary plant cells are killed in
one minute. With few variations and exceptions the protoplasm of all
organisms is destroyed in about the same time by the more powerful poisons.
The destruction of bacteria by poisons—chemical disinfection (54)—is
employed in all cases where the application of the methods described in
the last chapter is impossible on account of the injury caused by the heat.
The resistance of bacteria to chemicals is not only different in different
species, but varies also according to many external circumstances. It is
greatest when the organisms are growing under the most favourable
FISCHER G
82 CHEMOTAXIS AND CHEMICAL DISINFECTION
circumstances, when temperature, food, and other conditions are all
optimal. Bacteria are, in short, like all other organisms, strongest and
most resistant when they are in the best health. There is always a
great difference between spores and the far less resistant sporeless vege-
tative cells, so that a disinfectant can only be considered effective when it is
able to kill spores. In pr.ctice, circumstances may sometimes arise when
this requirement may be dispensed with.
Every disinfectant must be tested on the following three points :
I. In what concentration must it be added to a given substratum in
order to prevent the development and multiplication of bacteria without
killing them? This is the coefficient of inhibition.
11. What is the shortest time in which sporeless bacterial cells in water
at 20°-25° C. are killed by a substance in medium concentration (i.e. not
sufficient to cause plasmolysis or other injury)? This is the zzferdor lethal
coefficient.
111. What is the shortest time in which, under the same conditions, the
spores are killed? This is the superior lethal coefficient.
Laborious investigations without number have been made with the
object of determining these three values for all kinds of organic and
inorganic compounds, and we have now a number of carefully-chosen
substances that are suitable for use as disinfectants. The following tables
give a few examples. Further details will be found in the works cited
in Notes 3 and 54.
I. COEFFICIENT OF INHIBITION FOR ANTHRAX BACILLI IN
OX-BLOOD SERUM. From Behring (54).
The numbers indicate the number of cubic centimetres of serum in
which one gram of the solids or one cubic centimetre of the liquids was
dissolved ; for instance,‘ Corrosive Sublimate, 10,000’ = 1 gram HgCl, in
10,000 c.c, serum :
Cyanin and malachite green. ‘i . : : 40,000
Nitrate of silver . F : ‘ ‘ i if 30,000
Corrosive sublimate . é a ‘ a é : 10,000
Trichloride of iodine ‘ 3 a : z . 1,500
Caustic soda. é . ‘ . . ‘ i 1,500
Cadaverin (bacterial toxine) . F 3 : F 1,500
Quinine hydrochlorate . . . ‘ 5 3 500
Carbolicacid . . . : : . e : 500
Thymol . . Sa 3 . : : 3 250
Salicylate of soda. ‘ ‘ ‘ é 3 é 150
Alcohol. ‘ : ; ‘ : . : 15
Common salt . : . . F : . ‘ 15
The extraordinarily small quantities which are in some cases sufficient to
COMPARATIVE EFFICACY OF DISINFECTANTS 83
prevent putrefaction do not kill the bacteria, they only arrest growth and
multiplication.
II, LETHAL VALUE FOR SPORELESS TUBERCLE BACILLI.
+ Inferior lethal coefficient; the time is given in which tubercle bacilli
(from cultures, not in sputum) are killed. From Yersin (54).
Carbolic acid . . : . . 5% 30 seconds
5 ane 7 ‘ . ‘ 1% 1 minute
Absolute alcohol. , : 7 pie 5 minutes
* Todoform 3 F ‘ 4 é 1% 5 35
Ether . . . . . beaks Io ,,
Corrosive stiblimate 5 ; 0-1% 10,
Thymol . : z 3 . : 03% 3 hours
Salicylic acid . a 7 : s 0-25% 6,
To kill the bacteria in sputum, where they are embedded in mucus,
a much longer time is necessary, e.g. 10 per cent. lysol for twelve hours.
The figures in the table were obtained by mixing tubercle bacilli from
a growing culture with the fluids, taking out samples from time to time,
and sowing them on culture media. The values given may be regarded
as being good for most sporeless bacteria, the sensitiveness of which is
illustrated by this example.
III. LETHAL VALUE FOR ANTHRAX SPORES.
From Paul u. Krénig (55).
Substance. Concentration. Time (Temperature 18° C.).
Corrosive sublimate Z 1-7 % ( 16litres) 12-14 minutes
” . s . 084% ( 32» ) 24-30 ”
” : . : 0-42% (64 5 ) 45-60 ”
af . , o2 % (128 ,, ) 60-80 e
35 ei . or % (256 ,, ) more than 120 minutes
Nitrate of diver : ‘ 425% ( 4 » ) , 15-16 minutes
0-08% (200 ,, ) not in 10} hours
Sulphate ‘of copper . : 16 % ( 1 4 ) notin 10} days
Sugar of lead . ‘ 7 325 % ( I 5 ) notin7 95
Sulphuric acid . é 49 % ( 2 4, ) not in 30 hours
Caustic potash ‘ ‘ 56% ( 1 4 ) 18 hours
Permanganate of potash . 395% ( 4 5 ) 40 minutes
Bichromate of potash. 74% 4 5 ) notin 4 days
Permang. of potash, 8 litres, plus 8 litres HCl 5 minutes
Chlorine water 7 . 022% ( 32litres) 2 ,,
Bromine water : fs 05% (32 » ) 2 55
Carbolic acid : : 5 4 agence not in 24 hours
Formaldehyde . 4 5 # sical 120 minutes
The lethal concentration for spores is determined in the following way.
A suspension of the spores in water is dried upon silk threads, pieces of
glass, or, better still, upon carefully-cleaned garnets, which are then laid in
the disinfectant. Specimens are taken out at intervals and sowed in nutri-
G2
84 CHEMOTAXIS AND CHEMICAL DISINFECTION
tive media. Before this is done, however, the threads or garnets, as the
case may be, must be freed from any adherent disinfectant. Washing with
distilled water is not enough for this, they must be rinsed with ammonium
sulphide, and the soluble metallic salts precipitated and rendered harmless.
Only in this way is it possible to obtain reliable results, for the small
quantity of disinfectant adherent to the spores, although not sufficient to
kill them under ordinary circumstances, might do so during the stage of
imbibition, and would certainly destroy the germinating rod, thus giving
a false idea of the disinfecting power of the substance. In the examples
given in Table III, from 15,000 to 20,000 spores were killed in the stated
times. The concentration is given in per cents. and also in terms of
‘molecular dilution’; for instance, ‘Corrosive Sublimate, 16 litres,’ means
that sixteen litres of the solution contain the molecular weight of HgCl,
(i.e. 271) expressed in grams, so that 100 c.c. of the solution contains
#43 grams =1-7 grams. This method of calculation, so much used-in
modern chemistry, is given because it permits an easy comparison of
different salts—a comparison of molecule with molecule, so to speak.
According to Koch absolute alcohol, concentrated glycerine, concentrated
NaC] solution, and distilled water do not destroy anthrax spores, even after
acting on them for months. The table shows that the halogen elements
(chlorine, bromine, iodine) are the most powerful, and, among the metallic
salts, corrosive sublimate. Nitrate of silver has some little effect, but sulphate
of copper and acetate of lead are powerless. Free acids or free alkalies must
be very strong to act as disinfectants. Chromate of potash, too, although it
is a powerful oxidizing agent, has little germicidal power, whilst potassic
permanganate, far less effective as an oxidizer, is a powerful disinfectant.
Tables II and III show the great difference between spores and vegeta-
tive cells in their behaviour to disinfectants. Five per cent. carbolic acid,
for instance, which kills tubercle bacilli in thirty seconds, does not destroy
anthrax spores in twenty-four hours; with o-1 per cent. sublimate, the times
are ten minutes and sixty or eighty minutes respectively. Absolute alcohol,
powerless against anthrax spores, kills tubercle bacilli in five minutes. The
resistant power of the spores depends, no doubt, principally on the great im-
permeability of the spore wall to dissolved substances of all kinds. This
property is shared by the cysts and membranous coatings of all low organisms
in the resting-stage, and by the seeds of plants. A resting-stage of long
duration is indeed inconceivable without some such protection. In plant
seeds and algal spores the impermeability is caused by the deposition in
the membrane of fats and resinous bodies, and possibly the spore wall in
bacteria is protected in the same way. Another cause of the durability of
spores is the density of the protoplasm, which contains less water than it
does in the vegetative cell.
The great efficacy of corrosive sublimate as a disinfectant seems at first
DISSOCIATION OF SALTS 85
sight to be only a special case of the toxicity of all mercury salts, and formerly
it was thought that they were equally poisonous if only equal amounts of the
metal were present. Equimolecular solutions therefore were supposed to be
necessarily of equal disinfectant power. Recent investigations, however (55),
based upon the modern theory of solutions, have proved this to be wrong,
and rendered it probable that the toxicity of a poisonous salt varies with
the degree of dissociation. The dissociation theory (56) has shown that
in a solution of a salt there are present not only the unaltered molecules of
the dissolved substance, but also a certain number of disintegrated or ‘ dis-
sociated’ molecules. Ina solution of HgCl,, for instance, a certain propor-
tion of the HgCl, molecules are split up into their electrically active
components or ‘ions,’ the positive metallic ions (kations) Hg, and the
negative ions (anions) Cl]. The degree of dissociation, that is to say,
the proportion of dissociated molecules to unchanged HgCl, molecules,
changes with the concentration of the solution, the temperature, and
other conditions, and different salts of the same metal are dissociated in
different degrees. Now many of the physical properties of a solu-
tion, such as electrical conductivity, freezing-point, boiling-point, and
osmotic pressure, depend upon the degree of dissociation, and it seems very
probable that toxicity does also. Since different salts of mercury are very
differently dissociated in watery solutions, we might expect that their toxic
powers would be different also, and as a matter of fact we find this to be
the case. A sixteen-litre solution (1-58 per cent.) of mercury cyanide,
a salt which is but very slightly dissociated, does not kill staphylococci in
three minutes, whilst a solution of corrosive sublimate only a quarter the
strength (i.e. sixty-four litres, 0-4 per cent.) does. Anthrax spores left for
twenty minutes in this solution of sublimate were almost all killed (seven
colonies grew), whilst after a sixteen-litre solution of the cyanide, acting for
the same time, innumerable colonies sprang up.
A comparison of differently dissociated mercury salts shows clearly,
therefore, the relation between dissociation and toxicity. The connexion
becomes more striking still when we compare the same salt in different
degrees of dissociation.
The proportion of the dissociated molecules to the unaltered molecules
in a given solution of a salt is constant. For example in a solution of HgCl,
the proportion of Cl ions to unchanged Hg Cl, molecules is constant. By
adding other chlorine ions, for instance by adding the more strongly disso-
sociated NaCl, the number of dissociated Hg Cl, molecules can be reduced,
the reduction being dependent on the ratio between the amounts of disso-
ciation of HgCl, and of NaCl. Let us suppose a sixteen-litre solution
of Hg Cl, to contain x Cl ions and y unchanged molecules, then, broadly
speaking,” = c (a constant). If, now, there be added enough solid NaCl,
86 CHEMOTAXIS AND CHEMICAL DISINFECTION
which is more readily dissociated than Hg Cl,, to make a sixteen-litre solu-
tion, there will be present ++ Cl ions derived from the NaCl. For the
pure HgCl, solution += cy, but for the HgCl, plus NaCl solution
a+(4+m)=cy or x = aoa In other words, the number of Cl ions
derived from the Hg Cl, is reduced, its degree of dissociation is lessened,
and some of the free Hg and Cl ions are built up again into complete
molecules. The following table shows that as the degree of dissociation
is lessened the toxicity is reduced. The figures give the numbers of
colonies that arose from approximately equal quantities of spores after
these had lain for six minutes in the solutions (55).
Number of Colonies.
16 litre Hg Cl, solution . . : . 8
35 4 plus1 NaCl. 7 32
3 a a 2 es ; ; 124
2” 3 ” 3 0 282
” oe ” 4 ” *: . 382
” ” ” 46 ” . . 410
” ” ” 6 ae x * 803
” ” 33 10 ” . . 1,087
The gradual diminution in germicidal power is unmistakable and needs
no comment.
These facts must be borne in mind in experiments on the disinfection,
by sublimate, of liquids such as serum or bouillon which contain 0-7 per cent.
(eight litres) of NaCl. It is evident that more sublimate will be necessary
than in fluids free from salt. The presence of peptone or albumen will
necessitate a still further addition of sublimate, because some of it will form
insoluble compounds with the proteids whereby its toxicity will be reduced.
Since the degree of dissociation is dependent upon the nature of the
solvent and also upon the temperature of the solution, these factors are of
influence on the disinfecting power. The increase of toxicity which accom-
panies increased temperature is not, however, solely the effect of augmented
dissociation.
Although the technique of disinfection has not as yet been affected by
these recent discoveries, it does not need much penetration to see their high
scientific interest and importance.
There are many other bodies besides those mentioned that could be
used for disinfecting purposes—anilin dyes, such as methy] violet, ethereal
oils, and many aromatic compounds. New disinfectants spring up every day,
and are widely advertised, only to disappear again very soon.
Gases, such as carbonic acid, carbonic oxide, hydrogen, nitrous oxide,
nitric oxide, sulphuretted hydrogen, sulphurous acid, and coal gas, arrest
the growth of bacteria in agar cultures exposed to a slow stream of the
gas, but the method has no practical value. The ozone in the air, too,
DISINFECTION OF THE TISSUES 87
even where most abundant, is in such minute quantities as to be quite
without influence.
The fact that thousands of bacteria pass through our digestive organs
every day suggests the question whether any of the digestive secretions
function as natural disinfectants. The saliva and the pancreatic juice are
alkaline in reaction and unable to injure bacteria, The pancreatic juice, by
reason of the proteids it contains, is even nutritious. The bile acids
inhibit the growth of bacteria, but of all the digestive secretions the free
acid of the gastric juice (2-3 per thousand H Cl) alone is able to kill them.
The action of this even is in any case uncertain, and can only affect sporeless
cells. Normal gastric juice ina test tube destroys in half an hour the
bacteria of cholera, typhoid and glanders, pus cocci, and the sporeless cells
of anthrax and tetanus (57), but spores are not injured by passing through the
stomach. To destroy anthrax spores, they must lie for six hours in 2 per
cent. H Cl, so that it is evident the gastric secretion with only from -2 to
*3 per cent. HCl is quite inadequate, even if allowed to act for days.
Even against sporeless bacteria it is of little effect, as the examples quoted
show. Bacteria given to animals in their food (B. pyocyaneus, blood in-
fected with anthrax, tuberculous tissues) are not entirely destroyed even after
six or eight hours (58).
Chemical disinfection of the diseased body is not possible, for the tissue
cells would be just as much injured by the disinfectant as the bacteria.
Nor is it possible to disinfect by chemical means wounds in which bacteria
have taken up their abode ; an aztisepsis of wounds is not possible. When
a wound cannot be purified by mechanical means the body must be helped
to fight against the invaders, and the only way to do this is to secure clean-
liness, asepsis. sepsis confines itself to the treatment of wounds with
germ-free instruments and dressings without using chemical disinfectants,
and is sufficient to prevent clean fresh wounds, such as are made at
operations, from becoming infected.
As to the cause of the fatal action of disinfectants upon the bacteria we
know but little. We know that the salts of the heavy metals, corrosive
sublimate or nitrate of silver for instance, coagulate protoplasm. They
destroy life probably by precipitating certain substances from the extremely
complex protoplasm of the cells. Other compounds, such as alkalies and
acids, may perhaps act by the separation and solution of proteid bodies,
resulting in a destruction of the protoplasmic structure. Even a change
in reaction might cause the precipitation of certain constituents. In most
cases, however, the matter is beyond any explanation, because we have no
knowledge of the peculiarities of protoplasmic structure on which the
phenomena of life depend.
CHAPTER. X
BACTERIA AND THE CIRCULATION OF NITROGEN
IN NATURE
1. Introduction: The Assimilation of Free Nitrogen by the Bacteria
of the Root Nodules of Leguminous Plants, and by Bacteria in
the Soil.
APART from the activity of organisms like the pigment and phos-
phorescent bacteria, and the remarkable metabolism of the sulphur- and
iron-bacteria, the work of bacteria in nature embraces three great processes :
1. The circulation of nitrogen: effected by putrefaction, the formation of nitrates,
and the assimilation of atmospheric nitrogen.
2. The circulation of carbon by the ‘fermentation ’ of carbohydrates and other non-
nitrogenous products of animals and plants.
3. The causation of disease in other organisms, particularly in man and the higher
animals,
There are in nature five sources of nitrogen open to plants and animals:
1. The atmosphere (79 per cent. by volume of free nitrogen),
2. The nitrates of the soil and the traces of nitrous acid formed in the air during
thunderstorms.
3. Ammonia, which occurs in minute quantities in the air, and is set free abundantly
by the putrefaction and decay of dead organisms.
4. Animal excreta, which contain nitrogen compounds of many kinds, even down to
ammonia; and
5. The tissues of plants and animals.
The first three of the above-named sources are useless to animals, for
these obtain their nitrogen from plants only, either directly (herbivora), or
indirectly through other animals (carnivora). To plants, on the other hand,
nitrogen seemed until comparatively recently to be available only in one
of these three forms. Vegetable physiologists had come to the conclusion
that in nature the nitrates of the soil were the one and only form in which
nitrogen was taken up by plants. For, although it was known that in
experiments plants could be got to take up ammonia salts and even gaseous
ammonia, it was evident that ammonia was not a common source of nitrogen
in nature. And, finally, the atmosphere, the greatest storehouse of nitrogen in
nature, seemed closed to plants. These views, however, were overturned by
ASSIMILATION OF FREE NITROGEN 89
detailed investigations into the nutrition of the Leguminosae. That the Legu-
minosae could grow in soil poor in nitrogen and thrive thereon, even without
nitrogenous manuring, had long been known, and it has now been demon-
strated that they take up nitrogen from the air (59) and convey it to the soil.
This enrichment is particularly evident when the plants are ploughed in.
All other plants, all our cereals and food crops, are, as regards the soil,
merely consumers of nitrogen, since they are unable to take it up in any
form but that of the nitrates of the soil.
The difference is seen even in the amount of nitrogen present in the
tissues of the two groups of plants. Lupine seeds contain 5-7 per cent. N,
wheat grains only 2-1 per cent., lupine haulms 0-94 per cent., wheat straw only
o-5 per cent. An experiment with peas showed that a quantity containing
16 mgr. of nitrogen gave rise to a crop containing 499 mgr., whilst the
quantity of nitrogen in the 4 kilograms of mould they grew in had increased
from 22 to 57 milligrams ; a total gain of 518 mgr. of nitrogen. Calculated
for larger quantities of plants it will be at once seen that these figures
represent an enormous profit. One hectare (=2-4 acres) of lupines is cal-
culated to enrich the earth by 227 kilograms of nitrogen annually. The
amount gained by the meteoric fixation of nitrogen (production of HNO,
and HNO, during thunderstorms) would for the same area be only 0-09 to
1-8 kilogram per annum, so that there is no doubt that it is the atmosphere,
and the atmosphere alone, from which these plants extract the gas.
Seeing that no other cultivated plants* (not even Brassica alba, the
White Mustard) are able to act in this way, we apparently have in the Legu-
minosae a remarkable and unique group of organisms. But we shall be
wrong if we attribute the power of fixing free nitrogen to the tissues of the
plant itself. They are powerless to do this, and behave towards nitrogen in
‘no way differently from the tissues of non-leguminous plants. Not until
they enter into partnership with certain bacteria, the dacteria of the root-
nodules, do the plants act as usurers of nitrogen, gathering and storing it
and growing ever richer. The nodules (60) which arise upon the roots
of the seedling plants when they are a few weeks old are minute white or
pinkish excrescences that soon increase in size and sometimes give the roots
a distorted appearance, as though they were attacked by galls (Fig. 19,
aand 6). At first hard and smooth, they become wrinkled as the foliage
of the plant grows and, by the time the pods are ripe, are shrivelled and
cracked. Their dried remains rot in the ground with the rest of the root.
The nodules are either separate excrescences on the surface of the root,
with whose vessels they are connected by a tiny strand of vascular fibres, or
the root itself swells up in places. In both cases there is always'a close
connexion between the cells of the nodule and the conductive tissue of the
* The nodule-bearing alder and Elaeagnus are perhaps exceptions.
go BACTERIA AND THE NITROGEN CYCLE
plant (Fig. 19,0). Ifa transverse section of a young and firm nodule, which
exudes a turbid milky juice on crushing, be examined, masses of large cells
are seen, distinguished from those that surround them by being filled with
a finely striate or punctate mass. Sometimes there are numerous small
. nests of cells, sometimes they run together to form larger masses. These
cells, which constitute what was formerly known as the ‘bacteroidal tissue’
(Fig. 19,4 and c), are nothing more or less than the enlarged cells of the root
itself crammed full of fine, slender, rod-shaped bodies (Fig. 19d). The nature
of these has been variously inter-
preted. Woronin, the first observer
(1866), took them to be bacteria-like
parasites; other investigators since
then have looked upon them as pro-
teid crystals. If this view were correct
we should have to regard the root-
nodules not as pathological structures,
but rather as a kind of nitrogenous
potato with the albumen in the form
of bacteroids. Recent investigations
have, however, placed it beyond dis-
pute that these rod-shaped bodies are
bacteria, living and growing in the
cells of the root (Fig. 19, c-f). The
normal straight, rod-shaped bacteria,
well tingible with anilin colours, are
only found in the young nodules. As
these grow older the bacteria take
Fic. 19. Root-nodules of Leguminosae. a, root on all manner of distorted shapes ;
nodule of the lupine, nat. size (from Woronin); 4,
longitudinal section through a lupine root and nodale ; indle-shaped, branched, bifurcated
£, fibro-vascular bundle of root ans off fine branches =e ped, 4
to every part of the nodule, and its bacteroidal tissue or inflated forms are common. It is to
(w), low ae, from Woronin); c, a cell from a lupine
odule filled with bacteria (black) between which the i i
finer mime of the pretorlasm is ae. at the an les these deformed bacteria only (Fig. 19,
of the cells intercellular ; from a section (fixed i i i
ot cieels ert Inlae meies rues 2 se foaled 52 and /) that the term bacteroids is still
degrcuieiorm: Cinsvriatetdsheniecvie @PPlied. They are so-called ‘involu-
a pre niles ro OEE, Meee ON, ee ion forms,’ similar tothose found among
many other species of bacteria when
the conditions of growth are unfavourable (the acetic ferment, for instance,
or the bacilli in old tubercle or diphtheria cultures). The cell contents as
well as the shape are affected. They stain badly, and large numbers of the
bacteroids seem to be mere empty shells. The conversion of the bacteria
into bacteroids is, in short, nothing but a sign that the micro-organisms are
dying and yielding up their protoplasmic contents to the plant, which begins
to grow more vigorously as soon as the bacteroids appear. By the time the
seed of the plant is ripe the shrunken empty nodules contain only fragments
ROOT-NODULES OF LEGUMINOSAE gI
of the bacteroids, together with a few intact healthy rods that remain in the
soil and serve as seed material for next year’s nodules.
Root-nodules have been found in all the sub-orders of the Leguminosae
(Papilionaceae, Mimoseae, Caesalpineae), and they always contain bacteria
and bacteroids. Root-nodules without bacteria do not exist, and since Legu-
minosae destitute of nodules behave with regard to nitrogen just as other
plants do, it seems evident that the bacteria are the nitrogen collectors. As
an unproved hypothesis this idea was floating in the air for a long time, until
the classical researches of Hellriegel and Wilfahrt (59) placed the matter
beyond the reach of controversy. Their experiments proved that it was
possible by planting sterilized seeds in sterilized earth to grow leguminous
plants for some months without root-nodules, if they were well protected from
subsequent infection. They showed further that such plants had lost the
power of storing nitrogen, but regained it if the earth they grew in were inocu-
lated with an infusion of soil whereon Leguminosae had previously flourished.
Finally their researches brought to light the marked contrast between
nitrogen-storers like Leguminosae, and nitrogen-consumers such as wheat or
oats. The following table gives a few of the chief results of their investigations.
Amount of Amount of | Gain or Loss
Nitrogen in Nitrogen of Nitrogen
seed and soil. in crop. in crop.
I. NOT STERILIZED AND NOT INOCULATED:
(a) Without nitrogenous manure Grammes. {| Grammes. | Grammes.
Oats. é - is 7 . 0-027 0-007 ~ 0-020
Peas . : ‘ « 5 ‘ 0-041 1-283 + 1-242
(4) Manured with nitrate of calcium
(N =o-112 grammes)
Oats . 7 . . . . 0-139 0-09 - 0-049
Peas . 7 ‘ . F : 0-153 0-700 +0°547
II], INOCULATED WITH SOIL IN WHICH LEGU-
MINOSAE HAD BEEN GROWN, NOT
STERILIZED:
(a) Without nitrogenous manure
Oats . ; i . . ; 0-027 0-007 - 0:020
Peas 4. 6 “Ss oe oa 0-038 0-459 +0-421
(4) With nitrate of calcium
(N =o0-112 grammes)
Oats . és % ‘ é ‘ 0-139 0-088 ~ 0-051
Peas . ‘ * 3 é ‘ O-150 0-220 +0-070
III, INOCULATED AND STERILIZED:
(a) Without nitrogenous manure
Peas . ‘ ‘ - 5 . 0:038 O-O15 — 0:023
(6) With nitrate of calcium
(N =o.112 grammes)
Peas . . . . . . 0-045 O-014 - 0-031
92 BACTERIA AND THE NITROGEN CYCLE
These tables explain themselves. It is clear that the pea nourishes
itself in a similar manner to the oat when cultivated in sterilized soil and
unable to form nodules (III), whereas under natural conditions it bears these
prolifically and stores up nitrogen in large quantities. Furthermore, the
tables show that, whilst oats are rendered much richer in nitrogen by suitable
manuring (Id and II), peas are not affected (III); they cannot utilize
nitrogen offered to them in this form. For oats, on the other hand, it is
a matter of indifference whether the soil is sterilized or not, since they
form no nodules. The advantage gained from non-nitrogenous manure,
such as phosphate of potash, is shared equally by oats and peas.
The next task was to obtain pure cultures of the bacteria of the root-
nodules, and ascertain their behaviour towards atmospheric nitrogen. They
are easy to cultivate on a decoction of leguminous plants to which } per cent.
asparagin and 2 per cent. sugar has been added. In such a medium the
bacteria grow well in the form of slender rods, aerobic and mobile, with
a tendency to produce involution forms or bacteroids. They at first get
their nitrogen from the asparagin, but after about two months’ growth
every litre of the culture has gained from 9 to 18 milligrams of nitrogen
which must necessarily have come from the atmosphere (Beyerinck). Mazé
in similar experiments obtained an increase of 47-5 mgr. nitrogen in fifteen
days, and in another case 23-4 mgr. in eighteen days, so that although further
investigations are desirable there can be no doubt that pure cultures of the
bacteria from root-nodules do actually assimilate and fix the nitrogen of
the air (61).
Morphologically the bacteria from different kinds of Leguminosae,
whether in the nodules themselves or in pure culture, are very similar in
appearance, and the growths on gelatine (which is not liquefied) are all of the
same character. The ‘bacteroid’ forms are likewise of the same kind in
all cases, so that it would seem as if all Leguminosae were inhabited by the
same species of bacterium, to which the name Bacillus radicicola was given by
Beyerinck and Rhizobium leguminosarum by Frank. But as soon as we
investigate the action of these bacteria upon plants we find differences
between them.
If plants of clover, pea, and vetch growing in sterilized earth and
destitute of root-nodules be watered with an infusion of bacteria derived
from the root-nodules of peas, we find that numerous root-nodules arise
both on the pea and on the vetch, whilst the clover forms very few or none
at all. Conversely, clover bacteria are almost useless to the pea and the
vetch. Nobbe and Hiltner (62) as a result of experiments, which although
successful are not beyond criticism, have come to the conclusion that it is
only between members of the same group of Papilionaceae that the bacteria
of the root-nodules are exchangeable, that, for instance, clover bacteria can
be used by other Trifolieae such as lucerne, but not by Phaseolus \upines
SYMBIOSIS: PARASITISM 93
or other Phaseoleae or by Viciae such as Vicia, Ervum, or Pisum, the
bacteria of these again being useless to Trifolieae. We should have, in fact,
if these views be correct, different breeds of one and the same species of
bacterium (B. radicicola), comparable to the different races of brewers’
yeasts or to those described by Eriksson in the wheat rust-fungus Puccinia.
Nobbe and Hiltner have attempted to turn their improved theories to
practical account by the introduction of ‘ Nitragin, a preparation manufac-
tured at the Héchst Chemical Works. Nitragin is a pure culture on gelatine
of bacteria from root-nodules—a kind of living manure to be mixed with the
seed material or soil and strewn over the fields. There are eight different
sorts of nitragin on the market, suitable for peas, lupines, beans, &c., and it
is said to be very advantageous when first planting Leguminosae in virgin
soil, or in soil of poor quality, moors for instance, or where for many years
no Leguminosae have grown and the land is presumably destitute of nodule
organisms. The results are extremely inconstant and frequently very
difficult to judge, so that it is not surprising
that opinions are divided as to its efficacy *.
Instead of nitragin, ‘leguminous earth, i.e.
soil in which Leguminosae thrive well, is
sometimes used with advantage to ‘ inocu-
late’ new ground and bad earth such as
moorland.
The remarkable association of bacteria
with leguminous plants is generally re-
garded as an instance of symbiosis, a :
connexion from which both parties reap Fortis Maire teage a este aad pete
advantage similar to the ‘ mutual’ associa- by in winch Ge ee
tion of alga and fungus in lichens. Here, as icotepaen ee ee ee ee
is well known, the green or brown cells of
the alga are enclosed by the thickly matted filaments of the fungus (Fig. 20).
The alga supplies the metatrophic fungus with organic compounds and the
fungus repays the debt by providing the alga with the necessary moisture
and mineral salts, not to speak of the protection that its embrace involves.
This at least is the opinion of those who, following the ‘symbiotic’ tendency
of the day, look upon a lichen as a co-operative concern. But it is more
than doubtful whether this view is correct. All the algae that live in lichens
are known to be able to live in a free state. They certainly do not need to
be supplied by the fungus with water and mineral salts, and as regards
protection it is difficult to see how the algal cells can be ‘ protected’ by
hyphae which cling round them tightly and send ‘ suckers’ into their interior
(Fig. 20, 6). The fungus behaves in every way as a parasite which lives upon
* (See Dawson, FAi/. Trans. 1899, Vol. 192, pp. 1-28.]
94 BACTERIA AND THE NITROGEN CYCLE
the alga. As a parasite it must establish a close connexion between its
tissues and those of its host, and, since it cannot crawl into the alga as the
tape-worm does into man, it winds its hyphae around the cells, Enclosed
thus in the light, well-ventilated thallus of the fungus, the algae continue to
live, but they draw no nourishment from it. The advantage is entirely on
the side of the fungus, which is a true parasite.
The association between the leguminous plants and the bacteria of
their root-nodules, paradoxical as it appears at first sight, is of a precisely
similar character; the plant is parasitic upon the micro-organism. To
render this view more comprehensible, we will consider in detail the
development of the nodules. The fine root-hairs of a young leguminous
plant, destitute as yet of nodules, push themselves everywhere into the
crannies between the particles of soil, taking up water and mineral salts and
secreting fluids which dissolve and
render assimilable the substances with
which the cell-wall comes into contact.
There can be no doubt that some of
the secretions of the root-hairs act
chemotactically upon many of the
bacteria that swim in the water-filled
crevices of the soil ; and these would be
attracted furthermore by organic sub-
stances issuing from wounds or scratches
upon the root-hairs or upon the epi-
dermis of the root. Asparagin, one of
the most powerfully chemotactic com-
: pounds known, is present in large
Fic. 21. Invasion of leguminous roots by bacteria. quantities in the tissues of germinating
a, Cell from the integument of root of the pea with Bs
steam of tauisie sooges tha passes ea'aey Prants, and must necessarily escape
sroottaroftepea;atnnginpartctectsann, Wherever the cell-walls are injured.
Insidethe halt protoplasm mixed withbacteia whens ¢ #8 quite possible that this substance
rot ay Oth & thin aca upward. ~ acts: asia ‘bait’ to attract the bacteria
which would swarm into a torn root-
hair just as they do into a capillary filled with asparagin (Fig. 21,4). There
are reasons, too, for thinking that the plant facilitates the entrance of the
micro-organisms by a softening of the cell-membrane of the root-hairs.
This much is certain, that the bacteria are chemotactically attracted to the
plant, and that, nourished by the organic substances offered to them, they
multiply rapidly and force their way from the surface of the root inwards.
Here again by softening its cell-walls the plant seems to smooth their path,
and in thick columns the bacteria zoogloea* pushes on from cell to cell
* The so-called ‘ infection-thread,’ first observed penetrating the root-hairs by Marshall Ward
(see Phil. Trans. 1887, Vol. 178, p. 545).
BIOLOGY OF THE ROOT-NODULES 95
into the inner tissues of the root (Fig. 21,¢ & 4). And now the plant begins
visibly to react to the stimulus. Many of the cells of the root become
larger, and carbohydrates and asparagin are brought down from the upper
parts of the plant to be given unstintingly to the micro-organisms, whose
accelerated growth and multiplication soon become manifest in the fast
developing nodules. The root-nodules are nothing else than pure cultures
of bacteria maintained by the plant for its own ends. The microbes, which
grow at first at the expense of the tissues they live in, become before long
more independent and draw their nitrogen from the atmosphere. Carbon
they most likely obtain the whole summer through from the plant, which
probably also secretes the ferment by which the starch is changed into
assimilable sugar.
The root-nodules are now in full activity, the bacteria in the cells
absorbing nitrogen from the air in the intercellular spaces (63), and storing it
up in their own bodies. But this state of exuberant vitality does not last.
From overcrowding and other causes the multiplication of the bacteria
becomes less rapid and bacteroids (involution forms) appear, a sign that the
conditions of life are becoming less favourable. The plant asserts itself, and
begins to absorb the substance of the bacterial cells, conveying the nitrogen
from the root-nodules into the growing seeds. In the lupine at the time of
flowering the nodules contain 5-2 per cent. N, which then becomes gradually
less until, by the time the pods are ripe, only 1-7 per cent. is present. It is the
nodules alone which yield up their nitrogen in this way, the amount contained
in the other parts of the root remaining from first to last unchanged, namely
about 1-6 per cent. The precise way in which the leguminous plant absorbs
the bacterial cells is not clearly understood, but in all probability it dissolves
them by means of a peptonizing enzyme. Only a remnant of the bacteria
return to the soil when the root decays, the greater number are literally
eaten up by the plant. To call this symbiosis is certainly a misapplication
of the term*. The carbohydrates and asparagin that the plant offers the
bacteria in the beginning are nothing but a usurer’s loan, for in the shape of
valuable nitrogen they are demanded back again later on with heavy
interest. Looked at from this standpoint, the view that the leguminous plant
is parasitic on the bacteria no longer appears absurd. It is obliged to ‘ put
itself outside ’ its hosts, just as the fungus is obliged to enclose the algae in
the lichen thallus. But, whilst the lichen fungus is completely parasitic, the
leguminous plant is only partly so. It only derives its nitrogen from its host,
supplying its other needs (carbohydrates, mineral salts, &c.) in the same
way as all ordinary plants, from which it differs indeed only in being unable
* [It should be pointed out that the author's views of the relations between the organisms here
and in the Lichens are not those accepted by botanists generally. See, for example, Pfeffer,
Physiology of Plants, Vol. 1, Engl. ed., pp. 364 and 371.]
96 BACTERIA AND' THE NITROGEN. CYCLE
to assimilate nitrogen from the nitrates in the soil. The table (III) on
p. 91 shows this very clearly. In this semi-parasitic mode of life the
Leguminosae resemble the Rhinanthaceae, Thesium, and some other green
plants, of which the roots graft themselves on to the roots of adjacent
plants of other species. These function as hosts, but it is not known what
substances they yield up to their parasites.
Since it is evident that the soil of every field in which Leguminosae
grow, and probably indeed soil of all kinds, contains the bacteria of root-
nodules, attempts have naturally been made to obtain them direct from
the soil. Up till now these experiments have not been attended with
success, nor is it known whether the bacteria live and multiply in the soil, or
whether they exist merely in the form of spores (as yet unknown) which
are quickened into life by contact with the roots of leguminous plants.
But although it has been hitherto impossible to isolate the bacteria of
root-nodules from soil, Winogradsky has discovered another soil bacterium
which fixes atmospheric nitrogen (64). It has been named Clostridium
Pasteurianum, and belongs to the group of butyric ferments. It can be
isolated in a nutritive solution containing sugar (as a source of carbon) and
mineral salts, nitrates or other nitrogen compounds being of course rigidly
excluded. The sugar is broken up into butyric, acetic, and carbonic acid,
and hydrogen, besides some by-products of uncertain composition, and at
the same time nitrogen is taken from the atmosphere and stored up in the
body of the cells. The more sugar there is present (i.e. the more energetic
the fermentation), the greater is the amount of nitrogen taken up, as the
following figures show:
Dextrose in nutritive N. in nutritive N. in the crop
solution. solution. after fermentation.
Grammes. Milligrammes.
I fo) 3:0
2 {o) 2-9
3 ° 8-1
6 fo) 12-8
Possibly the nitrogen is fixed by nascent hydrogen, and ammonia
formed as the first product. The bacterium in pure culture is an anaerobic
actively motile bacillus which grows in slimy masses. It is considerably
different in appearance from the bacteria of root-nodules, becoming spindle-
shaped during sporulation (hence the name Clostridium) and, like other
butyric bacteria, it stains blue with iodine.
What the conditions are under which Clostridium Pasteurianum exists
in nature, and whence it obtains its carbon, is unknown. Should sugar be
a necessity it could not grow in unmanured soil, but might occur in situa-
tions where the processes of fermentation and putrefaction give rise to a
variety of organic compounds.
NITROGEN-FIXING BACTERIA 97
It is not known whether other butyric organisms are also able to fix
atmospheric nitrogen, but presumably other, perhaps prototrophic, bacteria
of the soil do so. It is highly probable that we shall find such in forests
where the earth is never ‘manured’ and where, notwithstanding, enormous
quantities of nitrogen are fixed and worked up into the living substance of
plants. But all researches in this direction must be received very critically,
for there can be no doubt that the idea now very common in agricultural
circles that a// soil bacteria can assimilate free nitrogen is quite incorrect.
Green and blue-green algae were formerly supposed to be able to do so,
but this opinion is now held to be incorrect (65), and although it is stated
that mould-fungi can fix free nitrogen the experiments on which the state-
ment is based did not take into consideration all the possible sources of
error. The time will perhaps come when ‘Nitragin’ will be replaced by
pure cultures of free-living, nitrogen-fixing bacteria, which will be ‘sown’ so
as to alternate with nitrogen-consuming crops.
FISCHER H
CHAPTER XI
BACTERIA AND THE CIRCULATION OF NITROGEN
(Continued)
2. The Liberation of Organic Nitrogen by Putrefaction, and its
Mineralization by the Nitrifying Bacteria.
ONE of the most conspicuous physiological differences between plants
and animals is the absence in plants of any xitrogen-containing excretions.
Nitrogen, once assimilated by the plant, is built up into proteids,
alkaloids, colouring-matters (chlorophyll, indigo), and other compounds, but
as long as the plant lives is not excreted. It is set into circulation again
only by the death and decay of the tissues, for the living plant can be used
as a source of nitrogen only by parasites and by plant-eating animals,
In the animal body nitrogen is present chiefly in the form of proteids
and their derivatives the albuminoids, e.g. mucin, glutin, keratin, elastin, as
well as other highly complex bodies such as lecithin, haemoglobin, chilin,
and nuclein. In the animal, as in the plant, the nitrogen of these substances
is set free at death, but, in addition, the animal, unlike the plant, gives off in
its secretions (milk) and excretions (dung, urine) a number of nitrogenous
compounds. It is to these substances, enriched still more by the nitrogen
contained in straw, that stable-manure owes its high value as a fertilizer for
plants.
But the nitrogen in fresh stable-manure is in a form that is useless to
plants; they cannot assimilate it. In the urine of herbivora nitrogen is
present mainly as hippuric acid, in human urine mainly as urea and uric
acid. In the excrements it is contained in the remnants of undigested
nitrogenous food, and the products of decomposition set up by the bacteria
of the intestine. These products include indol, skatol, leucin, tyrosin, and
many simpler nitrogen compounds even down to ammonia. But none of
these, not even ammonia, are directly available as nutritive materials for
green plants. Not until the nitrogen has been removed from the organic
molecule by the process of putrefaction, and has been united to a mineral
base by the process of nitrification, can it be taken up by the green plant.
PUTREFACTION 99
Through the tissues of green plants all organic nitrogen must, however,
sooner or later, circulate.
Putrefaction is a purely biochemical process, and can only take place
when the fundamental conditions for all vital action are fulfilled. Ifthe
temperature sink below a certain point, organic substances cannot putrefy,
as was well shown by the frozen Siberian mammoths. When discovered,
their flesh was so little changed that it was eaten by the hunters’ dogs ; yet
it must have lain in Nature’s refrigators for countless centuries. Absence
of moisture acts in the same way as absence of warmth. Dry meat does
not putrefy. Dryness and a low temperature together will preserve organic
matter for any length of time, as is well illustrated by the undecayed
yet unembalmed bodies that have been kept for centuries in the vaults of
certain churches. Other means of preservation by chemical and physical
agencies have been already referred to, but in all methods the funda-
mental principle is the same, namely, to create such conditions that bacteria
cannot live; for putrefaction—the splitting-up of the nitrogenous con-
stituents of organic matter—is the work of bacteria, and of bacteria alone.
The flesh of fruits, being poor in nitrogen and rich in organic acids, is
not attacked by bacteria. The rotting of pears, grapes, oranges, and other
fruits of a like character, is brought about principally by mould-fungi (66)
(Penicillium, Mucor, Botrytis).
The decomposition of dead animal bodies, of vegetable tissues, or of
substances like stable-manure, is far from being a simple putrefactive process.
Side by side with the disintegration of nitrogenous bodies there are going
on a number of fermentative changes by which oz-xitrogenous compounds
are being broken up, besides nitrification and other biochemical processes.
For this reason it is always difficult, and often impossible, to determine the
respective parts played by the different species of bacteria. Putrefactive
processes are going on in all places where nitrogenous organic matter is
under suitable conditions of temperature and moisture, manure heaps for
instance, cesspools, the muddy bottoms of lakes and rivers, and the ocean.
floor.
Proteids are split up by putrefaction into a large number of simpler
compounds both nitrogenous and non-nitrogenous. The substances thus
produced are precisely similar to those resulting from the artificial decom-
position of proteids by fusion with caustic potash, or boiling with hydro-
chloric acid or barium hydrate. Five groups may be distinguished :
1. Albumoses and pepitones: soluble diffusible bodies closely resembling albumen.
They are produced by the action on albumen of bacterial enzymes similar to the enzymes
(pepsin and pancreatin) which give rise to peptones in the digestive tract of man.
2. Aromatic compounds: among others indol and skatol which give the characteristic
odour to human excrement; also some non-nitrogenous substances such as phenol,
phenylacetic acid, and phenylpropionic acid.
3. Amido compounds, all nitrogenous ; leucin, tyrosin, aspartic acid, glycocol.
Hg
100 BACTERIA AND THE NITROGEN CYCLE
4. Fatty and aromatic acids, all non-nitrogenous and therefore having no part in the
circulation of nitrogen ; acetic, butyric, succinic, and valerianic acids.
5. Jnorganic end-products of putrefaction: free nitrogen, ammonia, free hydrogen,
methane, carbonic acid, methylmercaptan, sulphuretted hydrogen. It is probable also,
but*not certain, that phosphuretted hydrogen is formed and is oxidized at once by the
free oxygen of the atmosphere.
Most of these substances are formed also by the chemical decomposition of
proteids, but there is a sixth group which may be termed specific putrefac-
tive products. These are the so-called ptomaines or putrefactive alkaloids
(67). They are nitrogenous compounds (amine-bases), but although a large
number have been described they are, in consequence of the difficulty of
preparing them, still only imperfectly known. From putrid flesh (mammalian,
including human), fish, and gelatine Brieger isolated neuridine (C, Hy, No),
trimethylamine (C,H,N), cadaverine (pentamethylendiamine, C; H,, N,), and
putrescine, a diamine of the methylene series (C,H,,N,). These are all of
them either not poisonous or only poisonous in large doses. Other ptomaines,
derived from putrid foods of various kinds (sausages, cheese), are highly
toxic (Plomatropine, Tyrotoxine). The name toxine was formerly used for
the amine bases only, but it is now applied indiscriminately to all bacterial
poisons irrespectively of their chemical constitution. (See also Ch. XVII.)
As far as the circulation of nitrogen is concerned, we have to consider
only the end-products of putrefaction, free nitrogen and ammonia. All
the more complex intermediate bodies are broken down finally to these.
Leucin, for instance, splits up into valeric acid, ammonia, carbonic acid, and
free hydrogen. Tyrosin yields by aerobic putrefaction hydroparacumaric acid,
paraoxyphenylacetic acid, paracresol, phenol, ammonia, and carbonic acid.
By anaerobic putrefaction it gives rise to indol, carbonic acid, and hydrogen.
The phenomena of putrefaction are so complicated that we do not
know all of the compounds that arise during the process. The course it
runs is, however, greatly modified by the presence of oxygen (68).
This list of putrefactive products is far from being complete, for even
the qualitative investigation of the processes is still unfinished ; quantitative
analyses are at present impossible. We do not know for instance what
determines the predominance of one or the other intermediate product.
The effects of the presence of oxygen are somewhat better understood (68).
If air have free access, putrefaction may go on without any odour at
all, the evil-smelling gases (NH, and SH, for example) being oxidized at
once to form nitrates and sulphates. Aerobic bacteria too, such as the
nitre and sulphur bacteria, bring about this mineralization of organic
nitrogen. Moreover, when air is circulating freely, there is no accumulation
of intermediate products such as skatol or indol. To this kind of aerobic
decomposition, proceeding without offensive smell, the term ‘decay’ may
be applied as distinguished from ‘ putrefaction.’ It occurs on the surface of
manure-heaps, on the outer surface of carcases, and in well-ventilated soil.
PUTREFACTION IOI
In anaerobic decomposition (putrefaction proper), as in anaerobic fer-
mentation, the organic molecules are at first only partly disintegrated, inter-
mediate products such as leucine, tyrosine, skatol, and indol being formed.
In the absence of air these accumulate, and hence it is that the putrefaction
going on in the mud of ponds or ditches, or inside carcases, is accompanied
by such evil odours.
Although the details of the process vary considerably according to the
presence or absence of air, the ultimate products of putrefaction are in both
cases the same: namely, free nitrogen, free hydrogen, ammonia, methane,
carbonic acid, and sulphuretted hydrogen. These also are the end-results of
the disintegration of the human body.
The decomposition of plant tissues, poor in nitrogen but containing
large quantities of cellulose, gives rise to humin compounds. There can be
no doubt that it is a biochemical process, but the nature and functions of
the bacteria concerned in it have not yet been carefully studied (69).
One of the bacteria which was formerly looked upon as a putrefactive
organism par excellence was the Bacterium termo. Cohn described under this
name a short, oval, actively motile bacillus, slightly fluorescent in cultures
(Fig. 22, a). The species is, however, no longer to be identified, for his
description would apply to several common forms. Asa matter of fact there
are many bacteria going by this name which is properly only applicable as
a collective designation for motile bacteria in putrescible substrata.
In the rich bacterial flora of a putrescent fluid (70) two distinct biological
groups must be distinguished, the saprogenic and the saprophile. To desig-
nate all bacteria living upon dead organic matter as saprophytes is insuffi-
cient. As we have already seen, the saprogenic bacteria are able, by the
chemical ferments they secrete, to break up the molecules of proteids. The
saprophile organisms cannot do this, but they find a rich supply of nutri-
ment in the products thus formed.