2 Elements of Water Bacteriology. 



green plants to build up their own food from compounds 

 like carbon dioxide and nitrates which have no stored 

 potential energy. The food requirements of various 

 bacterial types differ, however, widely among themselves. 

 Fischer (1900) has divided the whole group into three 

 great subdivisions according to the nature of their metab- 

 olism. The Prototrophic forms are characterized by 

 minimal nutrient requirements, including organisms like 

 the nitrifying bacteria which require no organic compounds 

 at all but derive their nourishment from carbon dioxide 

 or carbonates, nitrites and phosphates, or from inorganic 

 ammonium salts. A second group, the Metatrophic 

 bacteria, includes those forms which require organic 

 matter, nitrogenous and carbonaceous, but are not depen- 

 dent on the fluids of the living plant or animal. Finally, 

 the Paratrophic bacteria are the true parasites, which 

 exist only within the living tissues of other organisms. 

 These subdivisions, like all groups among the lower plants, 

 are not sharply defined; and the metatrophic bacteria 

 in particular exhibit every gradation, from types which 

 grow in water with a trace of free ammonia, to organisms 

 like the colon bacillus which normally occur on the 

 surface of the plant or animal body, feeding upon the 

 fluids or on the extraneous material collected upon its 

 surface. 



The vast majority of bacteria belong to the second, or 

 metatrophic group, living as saprophytes on dead organic 

 matter wherever it may occur in nature, and particularly 



