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114 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

sugar, wliich turns the plane of polarization to the leff, reduces
cupric to cuprous oxide, and is directly fermentable.

Cane-sugar. Water. Grape-sugar. Fruit-sugar.
The ferment which occasions this conversion of cane-sugar,
designated as a hydrolytic decomposition, seems to be produced
by many mouth-bacteria, since under their action cane-sugar very
soon acquires the properties of invert-sugar.
That this conversion is really due to the action of an enzym
formed by and separal)le from the bacterium, I have, as I think,
proved in one case by the following experiment. A number of
cultures in Florence flasks having been made at the same time,
I was able after forty-eight hours to obtain the bacteria which
had collected at the l)ottom of the vessels in considerable quan-
tity. In this way I ol)tained about 10 c.cm. of a rather thick
j»ap. The organisms were then devitalized by 90 per cent,
alcohol, dried in a porcelain bowl, rubbed with sand, thoroughly
soaked in water, and filtered. The filtrate, or watery extract,
which must be clear, was added to a cane-sugar solution at
room temperature ; this solution then showed in the long tube
of a ;Mitscherlich double-shadow polaristrobometer a rotation of
5.19 as the average of nine readings. The solution was left
standing in a damp chamber for four hours at 39° C, whereupon
it produced a rotation of 4.98. There was consequently a de-
crease of two-thirds of a degree, indicating a corresponding in-
version of the cane-sugar solution. The solution also caused a
slight reduction of an alkaline solution of sulpliate of copper.
Many ferment bacteria of the mouth do not seem to be depend-
ent upon the presence of free oxygen for their fermentative
action. I have made cultures in which (1) the air was excluded
by a thick layer of oil (2) in vessels from which the air had
;
been exhausted l)y a mercury air-pump (3) in vessels which
;
liad been freed from oxygen by an alkaline solution of pyrogallic
acicL It was ascertained that as much acid was generated in
these cases as when the air had fi-ee access. We must conclude
from the above that fcrmcntcf/'on itself is not a process which re-
quires oxygen, since the traces of oxygen still present in these
experiments were entirely out of proportion to the acids formed.