198

PATHOGENIC BACTERIA.

The superficial colonies upon gelatin plates form small,
irregular, ill-defined collections, which produce a fluores-
cence of the neighboring
gelatin. The gelatin soft-
ens gradually, and about
five days elapse before
liquefaction is complete.

The microscope shows
the colonies to be round,
coarsely-granulated masses

FIG. 57.—Bacillus pyocyaneus : colonies upon gelatin (Abbott).

with notched or filamentous borders. They have a yel-
low-green color. Upon the surface they form a delicate
clump with a smooth surface, finely granular, distinctly
green in the middle and pale at the edges. The colonies
sink into the gelatin as the liquefaction progresses.

In gelatin puncture-cultures most of the development
occurs at the upper part of the tube, where a deep saucer
of liquefaction forms. The growth slowly descends into
the medium, and is the point of origin of a beautiful
fluorescence. The bacterial growth sinks to the bottom
as it ages. At times a delicate mycoderma forms on the
surface.

Upon agar-agar the growth is at first bright green,
developing all along the line of inoculation. The green
pigment (fluorescin) is soluble, and soon saturates the cul-
ture-medium and makes it very characteristic. As the
culture ages, or if" the medium upon which it grows
contains much peptone, a second pigment (pyocyanin) is
developed, and the bright green fades to a deep blue-
green, dark-blue, or in some few cases to a deep reddish-
brown.

A well-known feature of the growth upon fresh agar-
agar, upon which much stress has recently been laid by