RELATION TO ENVIRONMENT 139 



as possible to various food substances so as not to injure them, 

 and at the same time the temperature must be sufficiently 

 high to destroy all germs capable of producing disease. The 

 temperature necessary to kill bacteria in a general way is not 

 far from 60 C. when this is applied for ten minutes in a moist 

 condition. When spores are present, a temperature of 90 to 

 1 00 C. is necessary, and sometimes this, must be applied by 

 the intermittent method in order to produce complete sterility. 

 The thermal death point of different organisms vary, depending 

 upon inherent characteristics, as seen in the following table : 



Micrcspira comma 52 C. 



Streptococcus pyogenes 54 C. 



Bacillus typhosus 56 C. 



Bacterium diphtherias 58 C. 



Micrococcus (species unknown) 76 C. 



The thermal death point varies with the reaction of the 

 medium upon which the germ is grown and perhaps also with 

 that of the medium in which the test is made. The same vari- 

 ation among individuals that was noted above in considering 

 the effect of cold upon bacteria is noted here. If a quantita- 

 tive determination is made of the number of bacteria killed at 

 various temperatures just below the thermal death point, it 

 will be found that within a few degrees the great majority 

 of the individuals in a single culture are killed, and this we 

 may speak of as the normal thermal death point. Certain in- 

 dividuals possess a high power of resistance and require a 

 higher degree of heat to kill them. The necessary heat re- 



