50 PA THOGENIC BA CTERIA.

Zymogenic, or bacteria of fermentation.
Saprogenic, or bacteria of putrefaction.
Chromogenic, or color producers.
Photogenic, or phosphorescent bacteria.
Aerogenic, or gas producers.
Pathogenic, or disease producers.

The parasitic organisms alone possess much interest to
the physician, but as in their growth the saprophytes ex-
hibit many interesting vital manifestations, it is not well
to exclude them or their products from the following
consideration of the

Results of Vital Activity in Bacteria.—i. Fermenta-
tiOHf—The alcoholic fermentation, which is a familiar phe-
nomenon to the layman as well as to the brewer and the
chemist, is not the work of a bacterium, but of a yeast-
plant, one of the saccharomyces fungi. The acetic-acid,
lactic-acid, and butyric-acid fermentations are, however,
caused by bacilli. A considerable number of bacilli seem
capable of converting milk-sugar into lactic acid, some-
times associating this with coagulation of milk, some-
times not. The production of coagulation in milk is not
always associated with acid-production, but with the pro-
duction of a curdling ferment similar to that belonging
to the gastric juice. There seems to be no real specific
micro-organism for the lactic-acid fermentation, although
the Bacillus acidi lactici seems to be the most powerful
generator of the acid. There may also be several bac-
teria which produce the acetic fermentation, though it is
generally attributed to a special common form, the Myco-
denna aceti or Bacillus aceticus. The butyric fermenta-
tion is generally due to the Bacillus butyricus, though it
also may be caused by other bacilli, the one named sim-
ply being the most common. (For .an exact description
of the chemistry of the fermentations reference must be
made to text-books upon that subject, as their considera-
tion here would occupy too much space.)

a. Putrefaction.-~$pM& process is in many respects sim-