BIOLOGY OF BACTERIA. 57

Milk usually contains bacteria, entering it from the
dust of the dairy, possessing this power. In the process
of peptonization the milk may become bitter, but need
not change its original reaction. As the peptonization
progresses the milk very often becomes poisonous, espe-
cially to individuals under two years of age, and may
bring about a fatal enterocolitis or "summer complaint"
The disease does not only occur in consequence of toxic
substances formed from the split-up albumins, or from
the presence of metabolic products of the bacteria, but,
as Liibbert has shown,1 from the presence of the bacteria
themselves. One reason that the enterocolitis caused in
this way comes on in summer is that it is only in un-
usually warm weather that these bacteria are able to
grow luxuriantly.

Sometimes the properties of coagulation and digestion
of milk are valuable aids in the separation of different
species of bacteria.

12. Production of Disease.—Bacteria which produce
diseases are known as pathogenic; those which do not,
as non-pathogenic. Between the two groups there is no
sharp line of separation, for true pathogens may be culti-
vated under such adverse conditions that their virulence
will be entirely lost, while at times bacteria ordinarily
harmless may be made toxic by certain manipulations or
by introducing them into animals in certain combina-
tions. The diseases produced are the result of the sum
of numerous activities exhibited by the bacteria. For
example, it may be that a microbe, having effected its
entrance into an animal, grows with great rapidity,
completely blocking up the blood- and lymph-channels,
so that the proper circulation of these fluids is stopped
and disease and death must result. Perhaps more com-
mon than this is a local establishment of the organisms,
with a resulting inflammation, due partly to the presence
of the foreign organisms, and partly to their toxic me-
tabolic products. More often, however, the pathogenic

1 Zeitschriftfur Hygiene, xxii., Heft 2, 1896, p. I.