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_2                  PATHOGENIC BACTERIA.


due the growth of various bacteria in stale meat bring-
ino-about in its proteid substances the development of
toxic ptomaines. Kaensche1 carefully investigated the
subject, and gives a synoptical table containing all the
bacteria of this class described. His researches show
that there are at least three different bacilli whose growth
causes the development of poisonous ptomaines in meat.

Toxins and toxalbumins are also very common.

3. Chromogenesis.—Those, bacteria which produce col-
ored colonies or impart color to the medium in which
they GTTOW are called chromogenic ; those with which no
color is associated, non-chromogenic. Most chromogenic
bacteria are saprophytic and non-pathogenic. Some of
the pathogenic forms, as the Staphylococcus pyogenes
aureus and citreus, are, however, color-producers. It
seems likely that the bacteria do not form the actual
pigments, but certain chromogenetic substances which,
uniting with substances in the culture-medium, pro-
duce the colors.

Galleotti has described two kinds of pigment, one of
which, being soluble, readily penetrates all neighboring
portions of the culture-medium, like the colors of Bacillus
pyocyaneus, and an insoluble pigment which does not
tinge the solid culture-media at all, but is constantly
found associated with the colonies, like the pigment of
Bacillus prodigiosus. The pigments are found in their
greatest intensity near the surface of the colony. The
coloring matter never occupies the protoplasm of the
bacteria (except the Bacillus prodigiosus, in whose cells
occasional pigment-granules may be seen), but occurs in
an intercellular excrementitious substance.

The pigments are so varied as to give almost every
known color. It sometimes happens that a bacterium
will elaborate two or more colors. The Bacillus pvo~
cyaneus thus produces pyocyanin and fluorescin, both
being soluble pigments-one blue, the other green
Gessatd has shown that when the Bacillus pyocyaueus

5 Zeitokriftfir Hygiene, etc., Bd. xxii., Heft I, June 25/1896