37°	PA THOGENIC BA CTERIA.
culture. To each of several tubes of melted gelatin 0.05
per cent, of carbolic acid is added. This addition is most
easily calculated by supposing the average amount of
gelatin contained in a tube to be 10 c.cm. To the aver-
age tube j1-^ c. cm. of a 5 per cent, solution of carbolic acid
is added, and gives very nearly the desired quantity. A
minute portion of the feces is broken up with a platinum
loop and stirred in the tube of melted gelatin; a drop
from this dilution is transferred to the second tube, a
drop from it to a third, and then the contents of each
tube are poured upon a sterile plate or into a Petri dish,

fig. 107.—Bacillus typhi abdominalis:  superficial colony two days old, as
seen upon the surface of a gelatin plate;  x 20 (^Heim).
or rolled, according to Esmarch's plan, in the manner
already described. The carbolic acid present in these
cases prevents the great mass of saprophytes from de-
veloping, but allows the perfect development of the
typhoid bacillus (Fig. 107) and its near congener, the
Bacillus coli communis (Fig. no).
The colonies that develop upon such gelatin plate-
cultures are seen under the microscope to be brownish-
yellow in color, spindle-shaped, and sharply circum-
scribed. When superficial they are larger and form a
bluish iridescent layer with notched edges. These colo-
nies are often described as resembling grape-vine leaves.