86 NUTRITION AND METABOLISM. 



alcoholic fermentation when submerged in the liquid. A number of 

 bacteria can grow without sugar if oxygen is present, because they can ob- 

 tain their energy by oxidation. In the absence of oxygen, however, they 

 can multiply only if some fermentable material like sugar is present, 

 because this is the only source of energy available under anaerobic 

 conditions. 



The amount of energy produced by the microbial cells is generally 

 larger than the amount required for growth. The excess energy is trans- 

 formed to heat which causes a rise of temperature in the culture medium. 

 The formation of heat will be discussed more in detail in one of the follow- 

 ing paragraphs. 



