Effect of Temperatures 37 



been learned that, under the direct rays of the sun, the 

 temperature in such places may reach, for short periods 

 at least, the optimum or best for these bacteria. 

 All of the thermophile bacteria form spores. They can, 

 therefore, resist unfavorable conditions for their devel- 

 opment by remaining dormant for more or less consid- 

 erable periods. 



Effect of cold. — Temperatures below the minimum 

 suspend the bacterial activities, but are not very effec- 

 tive in destroying the organisms. Disease bacteria, 

 frozen in ice, are as active as ever after the temperature 

 is raised. It has been demonstrated experimentally 

 that extremely low temperatures injure the bacteria 

 only after prolonged exposure. Even immersion in 

 liquid air does not always succeed in destroying them. 



Temperatures beyond the maximum lead, on the 

 contrary, to a rapid destruction of the organisms. The 

 non-spore-forming bacteria perish when subjected to 

 a temperature of 130° to 140° Fahr., for ten minutes, 

 and in less time when subjected to still higher temper- 

 atures. Spore-forming bacteria are much more resistant 

 to heat because of the inert character of their spores. 

 They will withstand dry heat well above 250° Fahr., and, 

 while not as resistant to moist heat, will sometimes 

 withstand boiling for an hour or more. 



Attenuated cultures. — When bacteria, or their spores, 

 are subjected to heat insufficient to destroy them en- 

 tirely, yet considerably above their maximum, they 

 may become weakened to a very marked degree. The 

 injury to the organisms may appear then in their less 

 vigorous growth, or in their impaired power to produce 



