24 GENERAL BACTERIOLOGY 



h. Streak Cultures are cultures made by drawing the needle, or 

 better, the 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. Liquid Cultures (bouillon, milk, etc.) are inoculated by trans- 

 ferring the desired material to them on either the needle or loop. 



Kbpbeences. a. 152 ; H. 58 ; M. & R. 51 ; McF. 198. 



Special Directions. 



a. Make a gelatin stab, an agar streak, a potato streak, and a 

 'bouillon culture oi Bacillus subtilis (Ehrenb.) Cohn (hay bacillus) 

 and Bacillus coli (Esch.) Mig. (colon bacillus) from agar cultures 

 supplied. Rule VIII. 



b. Label each tube, writing the name of the organism, the date of 

 inoculation and your own name. Rule VII. 



c. Place the gelatin in the cool chamber, and the other cultures in 

 the incubator at 28° C. See next Exercise. 



EXERCISE 14. INCUBATION OF CULTURES. 



Explanatory. Most bacteria grow at ordinary temperatures 

 (22° C), but their growth is usually hastened by a higher tempera- 

 ture (e. g. 28°-30° C.) The pathogenic, or disease-producing bac- 

 teria grow best at the temperature of the human body (38° C). 

 All bacteriological laboratories are, therefore, supplied with appara- 

 tus 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, how- 

 ever, must he kept at a temperature several degrees heloiv the mclt- 

 mg point of gelatin, i. e., not above 22° C. Ordinarily the tempera- 

 ture of the locker, especially near the floor, will be found satisfac- 

 tory. In a very warm room, particularly in the summer, an artifi- 

 cially 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. Ch. VIII ; H. 55 ; M. & R. 82 ; P. 231. 



Special Directions. 



a. Incubate all cultures of the non-pathogenic bacteria at 28° C, 



