PHYSICAL AND CHEMICAL AGENTS 31 



exist in living t issues^ certain species being adapt- 

 able to either condition, which are known as facul- 

 tative bacteria. Nearlv all of tho-di sease-producing: 

 (p athogenic) bacteria belong to the latter class . 



Some para sitic species are unable to liv e _except 

 in the living tissues ^ j3l_certain hosts^ as the leprosy 

 bacillus, which does not grow outside of the human 

 body; also, the ^dinary bac ter ia found in th a soil, 

 and water can notbe 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^^mtre- 

 f action are due to th e sa prophytic ba cte£i&; while 

 the parasitic_forms found in the livin g tissues pr o- 

 dime the cha,ngeR which we cj.ll 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 9ver again 

 in different forms, making what is known as the 

 Jood cucl e. Were it not for bacterial activity, vege- 



