Pathogenic Bacteria

BACILLUS COLI COMMUNIS. 391

media. Upon gelatin plates the colonies develop in
twenty-four hours. Those situated below the surface
appear round, yellow-brown, and homogeneous. As they
grow older they increase in size and become opaque. The
superficial colonies are larger and spread out upon the
surface. Their edges are dentate and resemble grape-
leaves, often showing radiating ridges suggestive of the
veins of a leaf. They may have a slightly concentric
appearance. The colonies rapidly increase in size and
become more and more opaque. The gelatin is not
liquefied.

In gelatin punctures the culture, developing rapidly
upon the surface, and also in the needle's track, causes
the formation of a nail-like growth. The head of the
nail may reach the walls of the test-tube. Not infre-
quently gas is formed in ordinary gelatin, and when i
per cent, of glucose is dissolved in the medium the gas-
production is often so copious and rapid as to form large
bubbles, which by their distentioii subsequently break it
up into irregular pieces. Sometimes the gelatin becomes
slightly clouded as the bacilli grow.

Upon agar-agar along the line of the inoculation a
grayish-white, translucent, smeary growth takes place.
It is devoid of any characteristics. The entire surface
of the culture-medium is never covered, the growth re-
maining confined to the inoculation-line, except where
the moisture of the condensation-fluid allows it to spread
out at the bottom. Kruse says that in old cultures crys-
tals may form. I have never seen them.

Bouillon is soon evenly clouded by the development
of the bacteria. Sometimes a delicate pellicle forms upon
the surface. There is rarely much sediment in the cul-
ture.

Wiirtz found that the bacillus produced ammonia in
culture-media free from sugar, and thus caused an intense
alkaline reaction in the culture-media. The cultures
usually give off an odor that varies somewhat, but is, as
a rule, unpleasant.