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116 THE MICRO-ORGANISMS OF THE HUMAN MOUTH. ;

cultivated in a 3 per cent, beef-extract solution. Increase of sugar
caused acid reaction, while a diminution rendered it alkaline.
The products arising from the process of assimilation and
growth of the living cells, or, in other words, their waste pro-
ducts, are such as impart an alkaline reaction to the medium
;
those arising from the fermentation of sugar are acid. The
resulting reaction depends upon the predominance of the former
or the latter. In my opinion, no sharp distinction can be drawn
between fermentative and putrefactive bacteria, since many of
the former possess also a ferment whicli decomposes all)umen
under formation of the characteristic products of putrefaction,
while, on tlie other hand, there are fnn as })utrefactive, when cultivated in sacchaiine solutions cause
them to ferment without displaying the slightest signs of putre-
faction.
One of the mouth-bactirial examined ra})idly dissolved boiled
white of i^gg, forming l)ad-smelling products, among which sul-
phuretted hydrogen and ammonia w^re easily detectable. It
also showed an inverting action, transforming cane-sugar into
dextrose and levulose ; in the third place it decomposed fer-
mentable sugars into lactic acid with generation of carbonic acid
it, fourthly, caused an acid reaction in an amylaceous beef-extract
solution, investing this solution at the same time ^\ith a slight
power of reducing cupric oxide ; in other words, it exerts finally
a diastatie action also.
By-products presumal)ly also arise in the last stages of these
various fermentations, making the transformations caused by one
and the same organism and the substances produced l)y it very
numerous. This fact explains the profusion of products which
may arise in putrefying mixtures, and makes the supposition
somewhat doul>tful that the number of kinds of bacteiia in a
fermenting mixture coincides with that of the products of fermen-
tation. The twenty to thirty different substances which may be
formed in an ex[)osed putrefying mixture are not produced hy
one organism ak)ne ; it is, however, not in accordance with the
facts to assume that each product or each stage requires a differ-
ent organism.
Pure putrefactive processes are found in those localities of the