8 GENERAL MICROBIOLOGY 



Apparatus must be absolutely clean and dry before 

 being sterilized. 



IV. Sterilization by Moist Heat. Sterilization by moist 

 heat may be effected in one of four ways: 



1. By continuous or discontinuous heating at low 

 temperatures (56-80 C.). 



2. By continuous or discontinuous heating in water 

 at 100 C. 



3. By continuous or discontinuous heating in flowing 

 steam at 100 C. 



4. By one heating in superheated steam (steam under 

 pressure) at temperatures above 100 C., generally 115 C. 

 (about 10 Ibs. pressure) or 120 C. (about 15 Ibs.). 



1. Sterilization by Continuous or Discontinuous Heating at 

 Low Temperatures. Some substances used as culture media, 

 being rich in volatile or otherwise chemically unstable 

 substances, cannot be heated to 100 C. without a marked 

 alteration (e.g., coagulation) and to some extent a destruc- 

 tion of their properties; blood serum, for example. 



Pasteur showed that such media can be better ster- 

 ilized by heating them at a low temperature (55-60 C.) 

 for a long time than at a high temperature (70 C. or 

 even 100 C.) for a short time. In this process, heat is 

 not applied 'directly, as a rule. Control of the temperature 

 is ordinarily accomplished by means of water heated to 

 the degree desired. 



Prolonged heating at a low temperature constitutes 

 pasteurization. In practice, however, it is found that in 

 order to kill all organisms pasteurization must be com- 

 bined with the method of discontinuous heating devised 

 by Tyndall. Albuminous media subjected to the Tyndall 

 method must be incubated finally at 37 C. for forty-eight 

 hours to eliminate all specimens showing contamination. 



2. Sterilization by Continuous or Discontinuous Heating 

 in Water at 100 C. (a) Continuous Heating. Water at 

 100 C. destroys the vegetative forms of bacteria almost 



