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MOUTH-BACTERIA AS EXCITERS OF FERMENTATION. 105

may somewhat contribute to our knowledge of the different fer-
mentative processes in the hnman mouth and the diseases conse-
quent upon them.
The chief source of nourishment for micro-organisms in the
human mouth is furnished by two groups of substances, the
carbohydrates and the alliuminous substances. Both are ahnost
invariably present in the human mouth in depressions, in fissures
of the teeth, or in the spaces between them, or finally upon their
free surfaces.
The carbohydrates undergo fermentations which lead to an
acid reaction of the medium, while decompositions of albumin-
ous substances are accompanied by an alkaline reaction. As a
rule, mixtures of both produce an acid reaction. The reaction,
which may be observed in any hidden recess of the human
mouth, depends, consequently, partly upon the nature of the
food present at the time, partly upon the kind of bacterium.
In the case of one bacterium which I examined with particular
reference to this question, I found that it caused a neutral reac-
tion when cultivated in a 3 per cent, solution of beef extract in
the presence of ^'*?ttt. of sugar, while the reaction became
yV P^^'
acid on increasing the quantity of sugar and alkaline upon
diminishing it. I shall revert to these facts in Chapter YIII, in-
asmuch as they explain a number of phenomena accompanying
dental caries,
A. AcTiox OF Mouth-Bacteria upon Carbohydrates.
1. Lactic Acid Fermentation.
The action of bacteria upon carbohydrates is of the greatest
importance to the dentist in particular, to the medical practi-
tioner, and, in tact, to every one, inasmuch as the origin of decay,
with its evil consequences, depends upon it. Of the twenty-tAvo
kinds of mouth-bacteria already mentioned, sixteen soon brought
about an acid reaction when cultivated in beef-extract-peptone-
sugar solutions, four produced an alkaline reaction under the
same conditions, while in case of two only the reaction remained
neutral.
In a later series of experiments, twenty-five mouth-l)aeteria,
thirteen stomach-bacteria, and fourteen bacteria from the intes-