CULTIVATION OF MICROORGANISMS 37 



tight apparatus. Steam is generated therein, and the air 

 is allowed to escape so that the entire space will be filled 

 with steam. The chamber is then closed. The tempera- 

 ture obtained will depend on the steam-pressure. A 

 steam-pressure of 1"> pounds to a square inch yields a tem- 

 perature of 120 C. (248 F.). Exposure for fifteen min- 

 utes to this temperature is usually sufficient to sterilize the 

 media. Where any amount of organic matter is to be steril- 

 ized, the latter method is used, as in bacteriological labora- 

 tories and in fanning factories. The apparatus employed 

 is ordinarily termed an autoclave. 



The same method can be used for the sterilization of glass 

 and metal objects, and of clothing. 



The culture media rendered sterile can be kept for an in- 

 definite time if protected from contamination by micro- 

 organisms and from desiccation. 



Sterilization by dry heat. It is sometimes more con- 

 venient to use dry heat, such as that of an oven, for the 

 sterilization of objects that will not be injured thereby, as 

 glass and metals. Dry heat is far less destructive to life 

 than is moist heat. It is thus necessary to prolong the pe- 

 riod of exposure and to increase the temperature over that 

 used in the autoclave in order to insure sterilization. A 

 temperature of 150 to 170 C. (302-338 F.) must be 

 maintained for an hour. All organic matter will be in- 

 jured by such treatment. 



Small objects that will not be injured by heat may be 

 sterilized in the direct flame of a gas-burner. The needles 

 used for transferring bacteria from one culture-tube to 

 another in the laboratory are thus rendered germ-free. 



Sterilization by filtration. It is essential that sterilized 

 culture media and other organic materials be protected 

 from contamination after treatment. In the canning in- 

 dustry this is accomplished by hermetically sealing the con- 



