PHYSIOLOGY OF MICROORGANISMS 59 



i<ms arc parasitic. This classification of the organisms as 

 they grow in nature does not hold under laboratory condi- 

 tions, since many of the parasitic bacteria can be grown on 

 dead matter. The parasitic organisms find favorable con- 

 ditions for growth in the body of plant or animal. If these 

 conditions can be duplicated or approximated in the labora- 

 tory, the organism will grow independent of the living 

 host. 



The bacteria are of far greater importance in causing 

 decomposition than are the yeasts and molds. This rests 

 largely on their cosmopolitan nature as far as growth is con- 

 cerned. They grow over a wider range of temperature 

 than any other group. They grow with or without air, and 

 can make use of the most varied materials as food. These 

 facts, coupled with the extreme rapidity of growth, insures 

 the predominance of bacteria over other organisms when- 

 ever the reaction or concentration of the food material per- 

 mits them to develop. 



Metabiosis. ( )r<ranic matter is decomposed by the agency 

 of all forms of life that do not use the energy directly from 

 the sun. The decomposition is complete; each element is 

 returned to that form in which it was when it was utilized 

 by the "-reeii plant. No one form is able to cause the com- 

 plete decomposition of a mixture of organic substances. 

 The process is usually carried on by a sequence of forms, 

 the second using the by-products of the first as food mate- 

 rial. The by-products of the second supply food and 

 energy for a third, and the process goes on, with a gradual 

 dissipation of energy and the formation of more and more 

 simple compounds. This action of a sequence of organisms 

 is termed metabiosis. It is of great importance, not only in 

 nature, but also in the fermentation industries. 



The gradual degradation of organic matter by a series of 

 organisms is usually known only in part, either with refer- 



