HEAT PRODUCTION 43 



numbers at least, in liquid air ( 176 C.) or even liquid hydrogen 

 (252 C.) for several hours, and develop vigorously when they are 

 again placed in a suitable environment at the optimum temperature. 



3. Heat. Bacteria are distinctly injured by exposure to even slight 

 increases of temperature above that optimum for their growth, although 

 there are considerable differences met with among different kinds of 

 organisms in this respect. Generally speaking, the saprophytic bac- 

 teria exhibit greater latitude than the pathogenic bacteria. If the 

 maximum temperature of growth be exceeded by even a very few 

 degrees, the death of the organisms follows rather promptly. The 

 greater the degree of heat, the shorter the time required to kill them. 

 Therefore, the thermal death point of bacteria, that temperature at 

 which specific organisms die, is dependent not only upon the actual 

 temperature to which they are exposed, but also to the length of time 

 of exposure. A standard exposure of ten minutes has been proposed, 

 so that the thermal death point of the bacterium may be defined as 

 the lowest temperature to which it must be exposed for ten minutes 

 under constant conditions to ensure the sterility of the culture. The 

 determination of the thermal death point is influenced by many factors 

 besides the kind of organism under observation and the temperature. 

 Older cultures are usually less resistant than younger cultures of the 

 same kind. The reaction of the medium (acids particularly decrease 

 thermal resistance), the presence of extraneous substances as mucin 

 and other non-conductors of heat, all play a part. Certain modifica- 

 tions in the characteristics of bacteria are observed when they are 

 exposed for several hours at the maximum temperature of growth 

 or a degree or ,two above this point. For example, anthrax bacilli, 

 which habituallyjorm spores, lose this property when they are exposed 

 to 44 C. for several hours. 



Dry Heat, Moist Heat Dry heat is less effective in killing bacteria 

 than moist heat. This is shown by the high temperature to which 

 glassware and other apparatus must be exposed in order to kill spores, 

 a temperature of 160 C. for one and one-half hours being required 

 to ensure sterility. Moist heat, which is best obtained by dry steam 

 under pressure, will kill even the most resistant spores in fifteen 

 minutes at fifteen pounds pressure. 



H. HEAT PRODUCTION. . 



The energy liberated by bacteria during the decomposition of organic 

 substances by bacterial growth is partly utilized by them for their 



