CHAPTER VII 

 FACTORS INFLUENCING BACTERIAL GROWTH 



The principal factors, in addition to food, governing the 

 growth and development of micro5rganisms are temperature, 

 light, electricity, pressure, shaking, and presence or absence of 

 injurious substances. 



These may affect bacteria in any one of several v^ays: (i) 

 They may actually destroy the microorganism, thus leaving the 

 medium sterile. (2) They may increase or decrease the rate of 

 grov^th and metabolism in general. (3) They may change the 

 products w^hich that specific organism produces. In this way a 

 microorganism which produces a disease in a very severe form 

 may be so modified that it is produced in a very mild form. This 

 is used extensively in producing artificial immunity in animals. 

 (4) They may change the shape and structure of the organism. 



Temperature. — All life is dependent upon a suitable tempera- 

 ture. The body of man and the higher animals possesses a com- 

 plicated mechanism which maintains the body temperature nearly 

 constant throughout life, independent of the surrounding tem- 

 perature. The temperature of the body of cold-blooded animals 

 and plants is dependent upon the temperature of their surround- 

 ings. When the temperature changes from normal, however, 

 we may have a slowing-up of the growth process, a total cessation 

 of growth or death, depending upon the magnitude of the change. 

 The extent of the variation necessary to produce these changes 

 varies with difiFerent organisms. Every organism, however, has 

 an optimum, minimum, and maximum, growth-temperature, as 

 well as a thermal death-point. 



The optimum temperature is the one best suited to the p-rowth 

 of the microorganism. This varies widely with different species. 

 Cold water and soils contain numerous bacteria, the optimum 



