70 GENERAL BACTERIOLOGY 



(In more accurate work the temperature should be determined 

 within 2° C.) 



Eepeeences. M. & R. 70 ; McF. 246 ; P. 146 ; P. B. C. 32. 



Special Directions. Use B. coli or B. typhosus. 



EXERCISE 42. COMPARATIVE EFFICIENCY OF DRY AND MOIST 



HEAT. 



General Directions. 



a. Charge a water blank with culture of a spore-bearing bacillus, 

 shaking it well to break up the clumps. 



h. Sterilize eight cover-glasses by passing them several times 

 through the flame, and place four in each of two sterile Petri dishes. 



c. With a sterile loop place an equal quantity of the bacterial 

 suspension (a.) on each cover-glass, and dry by placing Petri dishes 

 in the incubator with the covers slightly raised. 



d. When dry place one Petri dish in the dry sterilizer (near the 

 thermometer), and the other in the steamer. 



e. Keep both sterilizers at a temperature of 100° C, and at the 

 end of 5, 10, 20 and 40 minutes respectively, remove one cover-glass 

 from each Petri, place it in a sterile Petri dish and pour a tube of 

 liquefied gelatin or agar over it. Tip the dish from side to side to 

 dislodge as many of the bacteria as possible from the cover-glass, 

 solidify the medium and incubate. 



Special Directions. Use an old (spore-bearing) culture of B. 

 suhtilis. Arrange data in the form of a table. 



exercise 43. effect of desiccation. 



General Directions. 



a. Prepare five cover-glasses each of a spore-bearing and a non- 

 spore-bearing culture, as directed in 42. 



b. Place them in sterile Petri dishes, and dry in the incubator. 



c. Next morning and every twenty-four hours later plate one of 

 the cover glasses. , 



d. In this way determine the length of time the organism in 

 question can withstand desiccation. 



References. F. 77 ; L. & N. 40. 



Special Directions. Use a young culture of B. coli and an old 

 (spore-bearing) culture of B. subtilis. Tabulate results. 



