118 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



numbers of bacteria by a definite percentage, say one half. 

 If at one temperature, for example, half of the bacteria are 

 killed in ten minutes, and at another temperature one half 

 are killed in five minutes, it is evident that the second 

 temperature is far more destructive than the first. It will 

 be noted that the time required to kill half the bacteria 

 is mathematically the converse of the generation time. 



In summary, it may be emphasized that all effects of 

 environment upon microorganisms may be manifested in 

 growing cultures by changes in the length of the generation 

 time, changes in morphology, and in the physiologic and 

 cultural reactions. Likewise the effect of environment upon 

 the rate of death of bacteria may be noted by comparing 

 the length of time necessary to kill a definite percentage of 

 the microorganisms present. 



In the present chapter the effect of the physical environ- 

 ment will be discussed. The most important of the physical 

 agencies are light, heat, electricity, the radiations from 

 Rontgen tubes and from radium, pressure, osmotic pressure 

 and desiccation. 



Effect op Light 



Light affects organisms in several ways. Certain species 

 of bacteria, particularly those which contain a purple color- 

 ing matter known as bacteriopurpurin, are attracted by 

 light ; when motile they swim in the direction of the light 

 source. When growing, for example, in a suitable medium 

 illuminated from one side, they will move toward the 

 brighter side and collect on the surface of the glass. The 

 effect of any agency upon the direction of movement of an 

 organism is termed a iaxy. The power of light to attract 

 is termed pfwtotaxis. Most of the bacteria, however, do 

 not grow well in the presence of light ; in fact, light exerts 

 a destructive influence. 



Light in general interferes with the growth of micro- 



