Page 132 - My FlipBook

106 lUE MICRO-ORGAMSMS OF THE HUMAN MOUTH.
tines were examined in regard to the reaetion which they pro-
duce in saccharine solutions. Of the mouth-bacteria, sixteen
occasioned acid reaction, four alkaline, while five yielded incon-
stant results ; the corresponding numl)ers for the stomach-bac-
teria were nine, two, and two; for the intestine-bacteria, six,
five, and three. The number of acidifying bacteria is therefore
comparatively larger in the stomach and mouth than in the in-
testines. Whether these relations are constant could, of course,
be established only l)y a larger numl)er of experiments than I am
as yet able to record.
Cultures on sterilized milk give slightly difi'erent results. It
was furthermore found impossible to draw a sharp line between
those bacteria which produce an acid reaction and such as cause
an alkaline reaction, because in some cases the reaction was very
weak and changed during the course of the experiment, while
in others it was materially influenced by change in the amount
of sugar.
In consequence of the large number of l)acteria experimented
upon, it was naturally not possible to undertake for each one a
qualitative determination of the acid which it produced. In
the case of eighteen difterent bacteria, however, the examination
was made, and in ten cases out of these eighteen lactic acid was
found. It was formerly believed that the lactic acid fermenta-
tion could be brought about by only one specific micro-organism,
which was designated as the lactic-acid fungus (Bacterium acidi
lactici). Some years ago I called attention to the fact that many
difterent bacteria are found in the human mouth which are
capal)le of forming lactic acid out of sugar, and it has now been
established by various investigators that this property, as well as
the inverting and peptonizing property, is very widely distributed
among bacteria. This fact ofters an easy explanation for the
long-known constant presence of lactic acid in the stomach and
intestines.
The determination ofthe acid was made partly by analysis, partly
by crystallization tests, and partly l>y the color test of Ewald.*
* Five to ten cc. of water plus one drop of chloride of iron plus two drops cai-
.
^^ bolic acid gives a violet color, which becomes yellow on addition of lactic acid
even in very minute quantities.