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34 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

The following well-known species also showed the power of ox-
idation : Micrococcus (Bacillus) prodigiosus, typhus and anthrax
bacilli, Finkler-Prior's and Denecke's spirilla, Staphylococcus
citreus. Brieger's and Miller's bacteria yielded inconstant
results. Contrary to Schlosing and Miintz, Heraeus found these
organisms in the atmosphere also. The latter declares Pasteur's
"anaerobium" to be untenable; there are, on the contrary, bac-
teria which call forth reductions under all circumstances, as well
as those which occasion oxidations.
Wherever bacteria find a favorable nutrient medium, in con-
centrated urine, saccharine solutions, meat-juices,—that is,
wherever there are large quantities of organic matter,—the
reducing bacteria will preponderate ; but wherever the condition
of the nutrient medium prevents rapid proliferation, the oxidiz-
ing bacteria will gain the ascendency.
Warrington ^^ was able to note only a reduction of nitrates to
nitrites ; gaseous products did not appear. His experiments with
artificial mixtures showed but minute traces of nitrification.
Fresh corpses, imbedded in powdered charcoal, are said not to
putrefy, but to decay in such a way that after several months
only bones and fat remain, while the charcoal contains a quantity
of nitric acid.**^ We probably have here to do with a process sim-
ilar to or identical with nitrification, the oxygen condensed in
the charcoal taking the place of the atmospheric air. The ques-
tion whether these processes occur in the human mouth will be
discussed in Chapter V. It appears to me not at all improbable
that the appearance of nitrous acid in the oral cavity is due to
the reduction of nitrates.
2. Chromogenic Bacteria.
Many kinds of bacteria occasion so-called pigment fermen-
tations in which coloring-matter is produced ; green, red, and
yellow colors predominate, brown, blue, and violet being of
less frequent occurrence. The colors sometimes appear in the
membrane or protoplasm of the cells, which is usually the case
with bacteria that form yellow coloring-matter. More frequently,
only the culture medium becomes colored, the cells remaining
colorless. This is the case Avith all bacteria that form orreen color-