Sterilization in the Autoclave 



171 



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 properly sterilized 

 in this way culture-media will remain free from contamination 

 indefinitely. 



A prolonged single exposure to lower temperatures (6o°-7o°C.), 

 known as pasteurization, is employed for the destruction of bacteria 

 in milk and other fluids that are injured or coagulated by exposure 



Fig. 38. — Arnold's steam sterilizer (Boston Board of Health form). 



to ioo°C. It is appropriate 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 

 pressure, sufficient heat being generated to immediately destroy 

 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 sterilization, to be complete, requires 

 that the exposure shall be for fifteen minutes at iio°C. (six pounds' 

 pressure). 



The media to be sterilized should be placed in the autoclave, the top firmly- 

 screwed down, but the escape-valve allowed to remain open until steam is freely 

 generated within and replaces the hot air. The valve is then closed, and the 

 temperature maintained for fifteen minutes or longer if the media be in bulk in 

 The apparatus should be permitted to cool before the valve is opened, 



