64 GENERAL CONDITIONS OF GROWTH 



most readily killed by drying. The typhoid and cholera 

 organisms die in a few hours when dried, while pus cocci 

 and tubercle bacilli live much longer. 



TEMPERATURE. 



The temperature conditions for bacterial existence and 

 growth have been determined more accurately than any of 

 the other general conditions. The maximum for existence 

 must be placed at or near 100° since it is known that all 

 bacteria including spores may be killed by boiling in time. 

 Nevertheless, certain forms have been reported as thriving 

 in hot springs where the water temperature was 93°. This 

 is the highest known temperature for development. The 

 minimum for existence lies at or near the absolute zero 

 ( — 273°) since certain organisms have been subjected to the 

 temperature produced by the sudden evaporation of liquid 

 hydrogen ( — 256° to — 265°) and have remained alive. Whether 

 they could withstand such temperatures indefinitely is not 

 known. The minimum for development is near the freez- 

 ing-point of water, since reproduction by division has been 

 observed in the water from melting sea-ice at a temperature 

 of — 1.5°. Thus bacteria as a class have a range for existence 

 of about 373° (-273° to -|-100°) and for development of 

 94.5° ( — 1.5° to +93°) certainly much wider ranges than 

 any other group of organisms.^ 



The optimum temperature for development varies within 

 rather wide limits for different organisms. In general it 

 may be stated that the optimum temperature is approxi- 

 mately that of the natural habitat of the organism, though 

 there are, exceptions. The optimum of the "hot spring" 

 bacteria just mentioned is apparently that of the springs 

 ■(93° in this case). Many soil organisms are known 

 whose optimum is near 70° (a temperature rarely, if ever, 

 attained in the soil), but only when groirn in air or oxygen; 



1 With the possible exception of blue green algse which have been found 

 with bacteria in the above-mentioned hot springs. Seeds of many plants 

 have been subjected to as low temperatures as those above-mentioned with- 

 out apparent injury. 



