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54                  PATHOGENIC BACTERIA.

These products are described as c 'tryptic enzymes " by
Fermi, who found that heat destroyed them. Mineral
acids seem to check their power to act upon gelatin.
Formalin renders the gelatin insoluble. As some of
the bacteria not only liquefy the gelatin, but do so in a
peculiar and constantly similar manner, the presence or
absence of the change becomes extremely useful for the
separation of different species.

5.  Production of Acids and Alkalies.—Under the head
of "Fermentation" the formation of acetic, lactic, and
butyric acids has been discussed.    These, however, are
by no means all the acids resulting from microbic me-
tabolism.   Ziegler mentions formic, propionic, baldrianic,
palmitic, and margaric as being among those produced,
and even this list may not comprehend them all.    As
the acidity due to the microbic metabolism progresses, it
impedes, and ultimately completely inhibits, the develop-
ment of the bacteria.    The addition of phenolphthalein
and  litmus to the  culture-medium is one of the best
methods for detecting the acids.    Milk, to which litmus
is added, is particularly convenient.    Rosalie acid may
also be used, the acid converting its red into an orange
color.    The same tests will also determine the alkali-
production, which occurs rather less frequently than acid-
formation and depends chiefly upon the salts of ammo-

6.  Production of Gases.—This seems, in reality, to be
a part of the process of decomposition and fermentation.
Among the gases due to bacterial action, CO2, H2S, NH4,
CH4, and others have been described.    If the bacterium
be anaerobic and develop at the lower part of a tube of
gelatin, not infrequently a bubble of gas will be formed
about the colonies.    This is almost constant in tetanus
and malignant  edema.    Ordinarily,   the production  or
liberation of gases passes undetected, the vapors escaping
from the surface of the culture-medium.

To determine the gas production where it is suspected
but not apparent, the ordinary fermentation-tubes can be