100 INFLUENCE OF TEMPERATURE ON BACTERIA 



presence or absence of spores. The thermal death point of bacteria 

 varies with the specific character of the organism (some organisms 

 being much more resistant to heat than are others) and the age of 

 the culture, young cultures being more resistant than older cultures 

 which have not formed spores, especially when heated in the prod- 

 ucts resulting from their metabolism. 



Cold. It has been shown that the criterion for death is the non- 

 reversibility of the change brought about by the agency in question. 

 Now, does the lowering of the temperature bring about irreversible 

 changes in the protoplasm as does the raising of the temperature? 

 It is known that even intense cold does not cause these irreversible 

 reactions in proteins. Where death does occur in cold-blooded 

 animals and in plants, it must be due to the formation of ice crystals 

 in the cells which may mechanically injure and kill them. This 

 seems to be the case in the freezing of plants. Another irreversible 

 change is connected with the thawing of the cells which have been 

 frozen. Barring these two secondary and mechanical complications, 

 the lowering of the temperatures does not seem to bring about 

 irreversible changes in the condition of the protoplasm which are 

 incompatible with life. 



When the temperature of the protoplasm becomes sufficiently 

 low, for example, approximately C., the velocity of the chemical 

 reaction becomes so small that the manifestations of life cease. The 

 same is the case where the water content is sufficiently decreased. 

 This is the reason why seeds of higher plants and spores of bacteria 

 can be kept alive so long. Lack of water may reduce the reaction 

 velocity of the hydrolytic processes in these at ordinary tempera- 

 ture to such an extent that it may become practically zero. So 

 resistant are bacteria to low temperature that they may be frozen 

 solid and kept in this condition for days and even weeks, and many 

 survive. Many bacteria, including the typhoid and colon bacilli, 

 will survive freezing for twenty-four hours in liquid hydrogen 

 (252 C.) and develop vigorously when brought into suitable 

 media at an optimum temperature. Bacteria do not lose their 

 virulence when exposed to low temperatures, as is the case when 

 exposed to comparatively high temperatures. There is, however, 

 a tendency for the number of organisms gradually to decrease as 

 they are kept in the frozen condition. When typhoid bacilli are 

 frozen in water, approximately 90 per cent, of them die during the 

 first week, !)"> per cent, succumb by the end of four weeks; but from 

 four to six months' continuous freezing is required to kill all of the 

 organisms. The speed with which bacteria disappear from a frozen 

 medium varies greatly with the nature of the medium. It is very 

 slow in colloidal substances and much faster in crystalloids. Alter- 

 nate freezing and thawing in colloids is much less disastrous to 

 bacteria than the same treatment in aqueous solutions. It is prob- 



