Pathogenic Bacteria

CULTURES, AND THEIR STUDY. 145

through the bottom of the inverted dish, will be more
than ever apparent.

The colonies should be viewed from time to time in
their growth, drawings being made of the appearances,
so as to form a series showing the developmental cycle.
Most colonies will be found to originate as spherical, cir-
cumscribed, slightly granular, yellowish, greenish, or
brownish dots, and later to send out offshoots or filaments
or to develop concentric rings or characteristic liquefac-
tions. A few appear from the very first as woolly clumps
of entangled threads.

Some of the most diverse forms of colonies are repre-
sented in the accompanying illustrations (Figs. 28-32).

FIGS. 28, 29, 30.—The various appearances of colonies of bacteria under the
microscope: a, colony of Bacillus liquefaciens parvus (Luderitz); b, colony
of Bacillus polypiformis (Liborius); <r, colony of Bacillus radiatus (Luderitz).

A pure culture, when obtained from colonies growing
upon a plate, must always be made from a single colony^
the transplantation being accomplished under a low power
of the microscope. The naked eye can rarely be depended!
upon to recognize the purity of a colony or its isolation.

Selecting as isolated, large, and characteristic a colony
as possible, it is brought to the centre of the field. A
platinum wire, securely fused into a glass handle about
8 inches long, is sterilized by being made incandescent
in a Bunsen flame, cooled, and then cautiously manipu-
lated until, while it is watched through the microscope,
10