64 



BACTERIA AND SOIL FERTILITY 



a few degrees above the optimum, but in the case of the sapro- 

 phytes it may be many degrees above. For the heat-loving bac- 

 teria it may be as high as 89° C, v^hereas for the disease pro- 

 ducers it lies betvi^een 40° and 50° C. The grov^th of some 

 disease-producing organisms at a high temperature for some time 

 causes them to lose their disease-producing power but not their 

 power of calling out anti-bodies. Hence, this means is sometimes 

 used in the preparation of vaccines. Most of the psychrophilic 

 organisms will not grow at a temperature above 30 '^ C. 



The temperature relations for the three classes of organisms 

 may be seen from the following table: 



Class 



Temperature (C) 



Where Found 





Minimum 



optimum 



Maximum 





Psychrophilic Bacteria 



(Cold-loving) 

 Mesophilic Bacteria 

 Thermophilic Bacteria 



(Heat-loving) 







25-45 



15-20 

 37 



30 



43 



85 



Water, soil 



Bodies of animals 

 Bodies of animals, 

 manure, hot springs 



The growth-temperature range of an organism is the number 

 of degrees between the minimum and maximum. This is nar- 

 row for some organisms, as for instance the disease-producing or- 

 ganisms, and wide for many of the saprophytes. 



Cold. — Although bacteria cannot function below 0° C. yet 

 they are extremely resistant to very low temperatures. The com- 

 mon bacteria of the sail and water as well as the diphtheria and 

 typhoid bacilli have been exposed for days to the temperature of 

 liquid air (about — 190° C.) without destroying them or sen- 

 sibly changing their properties. Others have been exposed to the 

 temperature of liquid hydrogen (about — 250° C) with the same 

 results. Freezing causes a great decrease in the number of micro- 

 organisms in water. This is probably due to the drying of the 

 cell and also to the great pressure generated within the water on 



