EFFECT OF PHYSICAL ENVIRONMENT 121 



ture at which the generation time is the least or that 

 temperature at which the organisms are multiplying most 

 rapidly is the optimum. 



Bacteria are sometimes divided into several groups using 

 the optimum temperature as the basis for classification. 

 Those bacteria which grow best at relatively low tempera- 

 tures, such as the temperature of the ice box, are termed 

 psychrophilic. Such bacteria are found in cold water, or 

 in lakes, the arctic regions, etc. Not many of them are of 

 economic importance. Those bacteria which prefer to grow 

 at moderate temperatures, such as room temperature or 

 blood temperature, are termed mesophilic. The mesophilic 

 bacteria may be again subdivided into those which grow 

 best at' temperatures of 20-25°, that is, at ordinary room 

 temperature, and those which grow best at blood heat at 

 37.5°. Among the organisms belonging to the latter group 

 are those which are the disease producers in man and ani- 

 mals. A few bacteria are known which have optimum tem- 

 peratures decidedly above blood heat, that is, above 37.5° 

 Some species of bacteria will grow only at temperatures of 

 45° to 60°. They are termed thermophilic. Such organ- 

 isms are not uncommon in the soil, and are of significance in 

 food canning. 



The Maximum Temperature. — As the temperature is in- 

 creased above the optimum, the generation time of bacteria 

 increases, that is, the rate of growth decreases, until at last 

 it comes practically to a standstill. The highest tempera- 

 ture at which there is appreciable growth is termed the 

 maximum. Here again much variation may be found 

 among microorganisms. Certain of the psychrophilic forms 

 will not grow at temperatures above that of the room. 

 Many of the pathogenic bacteria have maximum tempera- 

 tures between 40° and 45° C. Certain of the thermophilic 

 bacteria will continue to grow at comparatively high tem- 

 peratures, even 65° to 70°, that is, they will grow at a 



