MECHANISM OF DEATH 329 



Perhaps, with some organisms, the most sensitive gene is inactivated 

 before all enzyme in the cell is destroyed. 



The great resistance of spores to heat is accounted 

 for by some authors by the assumption of a very con- 

 centrated cell content. According to Almquist (1898), 

 spores have a specific gravity of 1.35-1.40; this would 

 indicate a very low moisture content. The data had 

 been obtained by centrifugalizing the organisms in salt 

 solutions of known density. 



In favor of a low moisture content speaks the com- 

 paratively low temperature coefficient of the death-rate 

 of spores, as contrasted with vegetative cells. Data 

 on the death-rates of vegetative cells and spores of the 

 same organism are not known to the author, but the 

 temperature coefficient of spores seems to be fairly 

 uniformly about 10, while that of non-sporulating 

 bacteria is usually higher. 



It is not necessary, however, to resort to this expla- 

 nation. Thermophilic bacteria are known to grow at 

 temperatures which kill most vegetative forms; they 

 multiply rapidly in milk during pasteurization. Actively 

 growing cells cannot possess concentrated cell contents. 

 Thermophilic bacteria seem to have a protoplasm which 

 is less sensitive to temperatures between 40 and 70°C. 

 The temperature coefficient of the death rate may not 

 differ from that of other bacteria, but the death-rate 

 itself may be much lower, and a higher temperature 

 would then be necessary to bring about inactivation of 

 the growth mechanism. 



The resistance of spores may be due to this same cause, 

 i.e. to a smaller death-rate. The evidence, so far, seems 

 to be in favor of the assumption of concentrated cell 

 contents. The experiments of Williams (1929) suggest 



