316 



PHYSIOLOGY OF BACTERIA 



plate count made. Parallel sets in physiological salt 

 solution and in broth were made of all organisms. 



The survivor curves are quite irregular, indicating 

 the possibility of more than one cause of death. The 

 indication of a period of no death at the start is noticeable 

 in most experiments. This may be due either to cluster- 

 ing of the cells, (see p. 296) or to the slowness of action, 

 for in death by very slowly acting agents, such curves 

 have been observed. 



The differences in resistance of the different species 

 have been summarized in Table 94 by giving the time 

 required to kill 99.9% of all cells. In salt solution, 

 almost all species can tolerate the freezing better than 

 in broth. 



Table 94. — Time Required to Kill 99.9% op all Cells by a 

 Temperature op —13 to — 15°C. 



This very slow rate of death is verified by the obser- 

 vation of Prucha and Brannon (1926) who found that 



