MECHANISM OF DEATH 335 



in dry condition; dry bacteria at high temperatures do 

 not die from coagulation, but from oxidation. 



The greater resistance to heat of thermophilic bacteria 

 may be due to a protoplasm of a higher average coagula- 

 tion temperature. The rate of coagulation must be 

 considerably lower than with most cell proteins, while 

 the general rule of the high temperature coefficient 

 probably holds true. 



The greater resistance of spores may be due to the 

 same cause, or it may be due to a high concentration of 

 the spore contents; this would necessitate a low concen- 

 tration of water which seems essential in the coagulation 

 process; it may also be due to a very low ash content. 



The slower death rates of bacteria in concentrated 

 solutions of sugar, or of salt, coincide well with the 

 assumption of death as a coagulation process, and with 

 the necessity of water for this reaction. 



There is no real thermal death-point, as there is no 

 real coagulation temperature. The thermal death-point 

 varies with the initial number of cells. But owing to 

 the high temperature coefficients usually found in death 

 by heat, the variation is so slight as to be within the 

 very wide limits of error of this method. A much greater 

 error enters from the medium in which the bacteria are 

 tested. Water has a greatly varying effect, and a 

 nutrient medium would be a much better test medium 

 than water for the thermal death point. 



Vn. DEATH BY POISONS 



(a) METfiODS 



The Medium for Testing. — In the Chapter on Death 

 by Heat, it has been shown that accurate results can be 

 expected only if the bacteria are exposed to the harmful 



