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14 THE MICRO-ORGANISMS OF THE HUMAN MOUTH. —

of bacteria into a common culture medium, it will 1)3 found that
they do not all develop with equal rapidity; one kind will
always attain the supremacy. If we infect a second nutrient
medium with this culture and continue the operation through a
series of generations, we may find that at last only one kind
remain?, the others having been crowded out. It depends upon
the composition of the solution, the temperature, and various
other conditions, as to which species will gain the victory. It
may also happen that at first one kind prevails, and that later,
throus'h a chansre in the character of the medium, another kind
gains the upper hand; or again, in one part of the medium
for example, on the surface—one kind may prevail, in another
part another. In the human mouth this struggle for existence
seems to play an important part ; otherwise we should invariably
expect to find a much larger number of different kinds of bac-
teria than are actually present. The attempt to make use of
this principle in therapeutics, by bringing large numbers of
harmless bacteria into the human body in order to drive out
others of a pathogenic nature (Cantani), has hitherto not been
accompanied by the results which we had been led to hope for.

8. Self-Destructiox of Bacteria.

The growth and ferment activity of Ijacteria are always more
or less influenced by their own waste products. Lactic acid fer-
mentation not only ceases w4ien the acidity of the solution has
reached 0.75 to 0.80 per cent., but the bacteria themselves are
often destroyed by the action of the acid which they have pro-
duced. The vital action of yeast cells is also hemmed by the
alcohol which accumulates in the solution, and for the same
reason the ammoniac fermentation of urine ceases when the
amount of ammonium carbonate produced amounts to 13 per
cent. (Fliigge).
Some products of putrefaction appear to exert a similar action
upon the life of bacteria. Up to the present, however, it has
not been possible to determine with certainty to which of the
many products of putrefaction this action is to be ascribed.