PHYSICAL AND CHEMICAL AGENTS 43 



THE EFFECTS OF BACTERIAL GROWTH 



As before stated, bacterial growth is so rapid and 

 complex that profound changes always result in the 

 structure and composition of the materials upon 

 which the bacteria are developing. The putrefac- 

 tive change which takes place in the body soon after 

 death, due to the invasion of bacteria, is a familiar 

 example. 



Physical Effects. Heat. — The temperature of 

 organic substances undergoing the process of de- 

 composition is often very much higher than the 

 surrounding atmosphere, as in the heating of damp 

 hay or in manure heaps, both of which are attributed 

 to heat-producing (thermogenic) bacteria. 



Light-producing (photogenic) bacteria cause the 

 phosphorescence sometimes observed upon decaying 

 fish, meat, or wood and more commonly in sea water. 



Chemical Products. — The chemical products of 

 bacteria may arise from their secretions or excretions 

 or from the products resulting from their action upon 

 food substances, the character of which depends upon 

 the chemical contents of the food itself and the 

 species of bacteria concerned. It is believed that 

 most of the chemical changes wrought by bacterial 

 activity are not the direct action of the organisms 



