PHYSICAL AND CHEMICAL AGENTS 31 



exist in living tissues, certain species being adapt- 

 able to either condition, which are known as facul- 

 tative bacteria. Nearly all of the disease-producing 

 (pathogenic) bacteria belong to the latter class. 



Some parasitic species are unable to live except 

 in the living tissues of certain hosts, as the leprosy 

 bacillus, which does not grow outside of the human 

 body ; also, the ordinary bacteria found in the soil 

 and water cannot be grown in animal tissues. 



The activities of bacteria are so rapid and complex 

 that profound changes are produced in the materials 

 upon which they grow and find their food. The 

 changes which we know as fermentation and putre- 

 faction are due to the saprophytic bacteria, while 

 the parasitic forms found in the living tissues pro- 

 duce the changes which we call disease, and are 

 frequently the cause of death. 



The saprophytes, acting upon the highly organized 

 tissues of dead animals and vegetables, resolve 

 them into the simpler elements, water, carbon dioxide, 

 and ammonia, in which form they are appropriated 

 as nourishment by the higher plants. The higher 

 plants in turn furnish food for the animal kingdom, 

 and thus the food supply is used over and over again 

 in different forms, making what is known as the 

 food cycle. Were it not for bacterial activity, vege- 



