202 GENERAL MICROBIOLOGY 



REFERENCES 



ROSENAU: Preventive Medicine and Hygiene, pp. 780-781. 

 JORDAN: General Bacteriology, 4th Ed., pp. 36-37, 72. 

 MARSHALL: Microbiology, pp. 159-161. 

 NOVY: Laboratory Work in Bacteriology, pp. 513-518. 



EXERCISE 24. TO DETERMINE THE RELATIVE EFFECT 

 OF MOIST AND DRY HEAT ON BACTERIA 



Apparatus. Ten tubes of nutrient broth (large tubes); 

 ten sterile Esmarch dishes; ten sterile (flamed) cover-glasses; 

 autoclav; steam sterilizer; hot-air sterilizer. 



Cultures. Agar culture of a spore-forming organism 

 (having spores at the time). 



Milk culture of slimy milk organism, non-spore-forming. 



Method. 1. Make thick smears of each organism on five 

 cover-glasses. 



2. Place each cover-glass of the separate cultures in a 

 sterile Esmarch dish and mark. 



3. Place two Esmarch dishes of each culture in the hot- 

 air sterilizer; heat to 120 C. and remove one dish of each 

 culture after ten minutes at 120 C., the other two after 

 thirty minutes. 



4. Place two smears of each organism in the steam 

 sterilizer;, remove one of each after ten minutes, the two 

 remaining after thirty minutes. 



6. Place the two remaining Esmarch dishes in the auto- 

 clav and heat for ten minutes at 120 C. 



6. When cool, transfer each of the cover-glasses to a 

 tube of sterile broth; mark carefully. 



7. Note in which tubes growth appears. 



8. What is one of the most necessary factors for the 

 prompt destruction of microorganisms by heat? Why? 



Not considering moisture, what various conditions in- 

 fluence the destruction of microorganisms by heat? How 

 are molds and yeasts influenced by moist and dry heat? 



To what factors are the greater destructive powers of 

 the autoclav due? 



