16 



General Bacteriolv;/ >/ . 



FIG. 7. Method of holding test-tubes. 



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 to be 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 test-tube and withdrawn.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- 

 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, etc.), are inoculated by transferring the desired 

 material to them on either the needle or loop. 



REFERENCES. A. 146; H. 51; M. & E. 60; McF. 146. 

 SPECIAL 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 (EsCH.) MIG. (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. 



EXPLANATORY. 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 0.). 

 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 a very 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 & 243; P. 231; P. & M. 37. 



