174 



Sterilization ,and Disinfection 



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 

 to ioo°C. It is appropriate only when the organisms to be killed 

 are without spores and without marked resisting powers. 



Fig. 32. — Arnold's steam sterilizer (Boston Board of Health form). ■ 



Sterilization in the Autoclave. — If it should be desirable to sterilize 

 a medium at once, not waiting the three days required by the inter- 

 mi tteiit method, it may be done by superheated steam under pres- 

 sure, 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 

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

 and the vacuum be slowly relieved. If the valve be opened suddenly the fluids 

 boil rapidly and the cotton plugs may be forced into the tubes or flasks by the 

 air pressure. The chief objection to the use of the autoclave is that the high 

 temperature sometimes brings about chemic changes in the media by which the 

 reaction is altered. 



