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BIOLOGICAL STUDIES OX THE BACTERIA OF THE MOUTH. 91

matters are the chief factors in bringing about the pigmentatinn
in question.
The following presentation of the characteristics of the color-
forming bacteria of the human mouth, as far as they have as yet
been revealed to us by actual research, is undertaken in the hope
of determining how far they are accountable for the work which
has been assigned to them.
Chromogenic bacteria are widely distributed in nature. A
plate of gelatine exposed to impure air for a short time will
almost invariably develop one or more colored colonies; green,
various shades ofyellow, brown, red, being sometimes represented
on the same plate. Such being the case, it must naturally
happen that these bacteria often find their way into the human
mouth with food, drink, and chiefly with the air. As a matter
of fact, they are by no means seldom met with in the oral cavity.
As a rule, the colorless bacteria predominate in the liuniaii
mouth to such an extent that the chromogenic bacteria, if
present, cannot be detected ; occasionally, however, they may be
easily recognized even by the naked eye. 1 have observed a
brick-red color more frequently than any other, on the lingual
surface of the lower front teeth and on the buccal surface of the
molar teeth.
When this organism has once become established in the
mouth, it does not readily allow itself to be expelled. I have in
my practice a number of patients in whose mouths I have
watched its growth for six to eight years. One case which par-
ticularly interested me was that of four children, brothers and
sisters, aged about ten to seventeen years, for whom I repeatedly
removed the brick-colored deposit on the lower incisors only to
see it faithfully return in the course of a few weeks. Having
recently had occasion to examine the mouth of the mother, I
found the same deposit, from which I conclude that there is
some constitutional peculiarity about these children, inherited
from the mother, which renders the secretions of their mouths
peculiarly adapted to the needs of this organism.
Attempts to cultivate the bacterium of lirick-colored deposit
have been unsuccessful, although I did succeed in isolating from
the mouth an organism which produces about the same color