16 General Bacteriology. 
their diagnosis. The tube-cultures are serviceable in giving opportunity for a further 
study of the characters as well as to furnish the most convenient method of maintain- 
ing the cultures. 
GENERAL DIRECTIONS. Bacteria when ob- 
tained in ‘‘pure culture’’ are usually grown in 
test-tube cultures. To make these a small portion 
of a previous culture is transferred to fresh cul- 
ture media by means of the platinum needles. 
a. Stab Cultures are made in test-tubes con- 
taining solid, transparent media, such as gelatin 
and agar. The end of a sterile needle is infected 
with the material tobe transferred. The needle is 
then thrust into the medium to the bottom of the 
test-tube and withdrawn. In this way the bacteria 
are left along the entire length of the needle track. 
' For method of holding tubes see fig. 7. They are 
held in an inclined position to prevent the possi- Fic. 7. Method of holding test-tubes. 
bility of infection. 
b. Streak Cultures are cultures made by drawing the needle or loop over the surface 
of the medium (test-tubes with media having sloped surfaces or plate cultures). Agar, 
potato and blood serum are frequently used in this way, and occasionally gelatin. 
c. Fluid Cultures (bouillon, milk, ete.), are inoculated by transferring the desired 
material to them on either the needle or loop. 
REFERENCES. A. 146; H. 51; M. & R. 60; McF. 146. 
SprEcIAL DIRECTIONS. 
a. Make a gelatin stab, an agar streak, a potato streak, and a bouillon culture of 
Bacillus subtilis (EHRENB.) COHN (hay bacillus) and Bacillus coli (Escu.) Mic. (colon 
bacillus) from agar cultures supplied. 
b. Label each tube, writing the name of the organism, the date of inoculation and 
your own name. 
c. Place the gelatin in the cool chamber, and the other cultures in the incubator at 
28° C. (XIV). 
EXERCISE XIV. INCUBATION OF CULTURES. 
ExeLaNnatory. Most bacteria grow at ordinary temperatures (22° C.), but their . 
growth is usually hastened by a higher temperature (e. g. 28°-30° C.). The pathogenic, 
or disease-producing bacteria grow best at the temperature of the human body (38° C.). 
All bacteriological laboratories are, therefore, supplied with apparatus arranged for 
maintaining constant temperatures, known as thermostats or incubators. 
The non-pathogenic cultures are usually kept at 28° C., while the pathogenic ones 
are kept at 38° C. All gelatin cultures, however, must be kept at a temperature several 
degrees below the melting point of gelatin, i. e., not above 22° C. Ordinarily the temper- 
ature of the locker, especially near the floor, will be found satisfactory. In avery warm 
room, particularly in the summer, an artificially cooled chamber will be necessary. 
Test-tube cultures are stored in the various incubators in tin cans or glass tumblers 
with a layer of cotton in the bottom, while the Petri dishes are stacked in low piles. 
REFERENCES. A. 136; H. 48; M. & R. 88; N. 178 & 248; P. 231; P. & M. 37. 
