120 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



given off is largely blue, violet and ultra-violet. Relatively 

 short exposures of living cells to this light will kill them. 



Effect of Heat 



For every organism there exists a group of temperature 

 relationships. The lowest temperature at which any appre- 

 ciable growth will develop is called an organism's minimum 

 temperature. That temperature at which it grows most 

 rapidly is its optimum. The highest temperature to which 

 it will grow is its mxiximum. The numter of degrees dif- 

 ference between the maximum and minimum growth tem- 

 peratures is termed the growth temperature range. An 

 additional temperature relationship has been termed the 

 thermal death point. 



The Minimum Temperature. — As the medium in which 

 an organism is growing is cooled, in general the generation 

 time increases until it reaches a point where there is prac- 

 tically no multiplication ; below this temperature there will 

 be no growth and the bacteria will die off more or less 

 rapidly, depending upon circumstances. If other condi- 

 tions are constant, the colder the microorganisms, the less 

 slowly do they die. Dried bacteria, for example, may be 

 exposed to the temperature of liquid air without being 

 killed. 



The minimum growth temperature varies greatly with 

 different bacteria. Some microoganisms can multiply at 

 temperatures below 0° C. Such multiplication, for exam- 

 ple, has been observed in butter and in brine in cold storage. 



Optimum Growth Temperature. — As the temperature of 

 a culture is increased from the minimum the generation 

 time of the microorganisms will decrease in length, that is, 

 the bacteria will multiply more and more rapidly. This 

 continues with increase in temperature to a certain point; 

 heating beyond this will then again increase the generation 

 time, that is, slow down the growth rate. That tempera- 



