206 



Sterilization and Disinfection 



streaming steam or to some temperature judged to be 

 sufficiently high, so that the adult micro-organisms con- 

 tained in it are killed. As the spores remain uninjured, 

 the medium is stood aside in a cool place for twenty -four 

 hours, and the spores allowed slowly to develop into adult 

 organisms. 



When the twenty-four hours have passed, the medium is 

 again exposed to the same temperature until these newly 

 developed bacteria are also killed. Eventually, the process 



is repeated a third time, lest a few 

 spores remain alive. When prop- 

 erly sterilized in this way culture- 

 media will remain free from con- 

 tamination indefinitely. 



In popular parlance, the inter- 

 mittent exposure of the culture- 

 media to steam is spoken of as 

 sterilization. 



A prolonged single exposure to 

 lower temperatures (6o -yo C.), 

 known as pasteurization, is em- 

 ployed for the destruction of bac- 

 teria in milk and other fluids that 

 are injured or coagulated by ex- 

 posure to 100 C. It is appro- 

 priate only when the organisms to 

 be killed are without spores and 

 without marked resisting powers. 



Sterilization in the Autoclave. 

 If it should be desirable to sterilize 

 a medium at once, not waiting the 

 three days required by the inter- 

 mittent method, it may be done 

 by superheated steam under a 

 pressure of two or three atmo- 

 spheres, sufficient heat being generated to immediately de- 

 stroy the spores. 



Because of its convenience many laboratory workers 

 habitually use the autoclave for the sterilization of all 

 media not injured by the high temperature. The steriliza- 

 tion, to be complete, requires that the exposure shall be 

 for fifteen minutes at 110 C. (six pounds' pressure). 



Fig. 40. Modern auto- 

 clave. 



