CHAPTER VIII. 

 CHEMICAL ENVIRONMENT (Continued). 



GENERAL FOOD RELATIONSHIPS. METABOLISM. 



The foregoing brief review of the chemical composition 

 of the bacterial cell illustrates the variety of compounds 

 which necessarily occurs, but affords no definite clue as to 

 the source of the elements which enter into these compounds. 

 These elements come from the material which the organism 

 uses as food. Under this term are included elements or 

 compounds which serve as building material, either for new 

 cell substance or to repair waste, or as sources of energy. 



An organism which is capable of making use of an element 

 in the free state is said to be prototrophic for that particular 

 element. Thus aerobes and facultative anaerobes are proto- 

 trophic for O. The "root-tubercle bacteria" of leguminous 

 and other plants and certain free living soil organisms are 

 prototrophic for N.^ 



On the other hand, if the element must be secured from 

 compounds, then the organism is metatrophic in respect to 

 the element in question. Should the compound be inorganic, 

 the term autotrophic is applied to the organism, and hetero- 

 trophic if the compound is organic. It is very probable that 

 anaerobes, exclusive of a few nitrogen absorbers, are meta- 

 trophic for all the elements they utilize. With the excep- 

 tion of the anaerobes it seems that all bacteria are mixo- 

 trophic, that is, prototrophic for one or two elements and 

 auto- or heterotrophic for the others.^ 



' The sulphur bacteria are partially prototrophic for S; probably the iron 

 bacteria also for Fe. Some few soil bacteria have been shown to be capable 

 of utilizing free H, and it seems certain that the bacteria associated with 

 the spontaneous heating of coal may oxidize free C. So far as known no 

 elements other than these six are directly available to bacteria. 



' Only a few kinds of bacteria so far as known are proto-autotrophic. 

 The nitrous and nitric organisms of Winogradsky, which are so essential 

 in the soil, and which might have been the first of all organisms so far as 

 their food is concerned, and some of the sulphur bacteria are examples. 



