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148                PATHOGENIC BACTERIA.

agar tube, without touching the surface. The tube is
then inclined so that the water flows over the agar,
after which it is stood away in the vertical position.
Colonies will grow where bacteria have been floated upon
the agar-agar, and may be picked up later in the same
manner as from a plate.

In other cases pure cultures may best be secured by
animal inoculation. For example, when the tubercle
bacillus is to be isolated from milk or urine which con-
tains rapidly growing bacteria that would outgrow the
slow-developing tubercle bacillus, it is better to inject
some of the fluid into the abdominal cavity of a guinea-
pig and await the development of tuberculosis, and then
seek to secure the bacillus from the unmixed material in
the softened lymphatic glands. Anthrax bacilli are also
more easily secured in pure culture by inoculating a
mouse and recovering the bacilli from a spleen or the
heart's blood after death, than by going to the trouble of
making plates and picking out the colonies.

In many cases when it is desired to isolate the micro-
coccus tetragenus, the pneumococcus, and others, it is
easier to inoculate the most susceptible animal and
recover the germ from the organs than to plate it out and
search for the colony among many others which may be
similar to it.

The development of bacteria in liquids is of less in-
terest than that upon solid media. The growth generally
manifests itself by a diffused turbidity. Sometimes flocculi
float in the otherwise clear medium. Some forms grow
most rapidly at the surface of the liquid, and produce a
distinct membranous pellicle called a mycoderma. In
such a growth multitudes of degenerated bacteria and
large numbers of spores are to be observed. On the
other hand, it occasionally happens that the growth
occurs chiefly below the surface, and may produce gelat-
inous masses which are known as zobglea.

In gelatin the bacteria exhibit a great variety of ap-
pearances, many of which are beautiful and interesting.