48 General Bacteriology. 



EXERCISE XLH. COHPARATIVE 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. 



b. 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. 



REFERENCES. L. 101; S. 146. 



SPECIAL DIRECTIONS. Use an old (spore-bearing) culture of B. subtilis. Arrange 

 data in the form of a table. 



EXERCISE XLIII. EFFECT OF DESICCATION. 



GENERAL DIRECTIONS. 



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

 ture, as directed in XLII. 



b. Place them in a sterile Petri dish, 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. 93; McF. 46; S. 151. 



SPECIAL DIRECTIONS. Use a young culture of B. coli and an old (spore-bearing) cul- 

 ture of B. subtilis. Tabulate results. 



EXERCISE XLIV. EFFECT OF CHEMICALS ON BACTERIA. 



GENERAL DIRECTIONS. 



a. Inoculate three tubes containing 10 cc. of sterile bouillon, with three loopfuls of 

 a 24-hour old broth culture of organism to be studied. 



b. Add 0.1 cc. of a 5% solution of carbolic acid to one tube (No. 1); 0.6 cc. to an- 

 other (No. 2) ; and 2 cc. to the third (No. 3) . 



c. Two hours later transfer three loopfuls from each tube to sterile bouillon and in- 

 cubate all of the tubes at 38 C. 



d. The carbolic acid in No. 1 and its sub-culture does not prevent growth. In No. 

 2 no growth, but abundant in its sub-culture (acts as an antiseptic). In both No. 3 and 

 its sub-culture no growth (acts as a disinfectant) . 



REFERENCES. F. 81; L. & N. 90; L. 107; McF. 46. 

 SPECIAL DIRECTIONS. Use B. coli. 



