2 ELEMENTS OF WATER BACTERIOLOGY 



the power of green plants to build up their own food 

 from compounds like carbon dioxide and nitrates which 

 have no stored potential energy. The food require- 

 ments 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 metabolism. The Prototrophic 

 forms are characterized by minimal nutrient require- 

 ments, including organisms Hke 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 of Metatrophic bacteria includes 

 those forms which require organic matter, nitrogenous 

 and carbonaceous, but are not dependent on the fluids 

 of 'the Uving plant or animal. Finally, the Para trophic 

 bacteria are the true parasites, which exist only within 

 the li\dng tissues of other organisms. These sub- 

 divisions, Hke 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 in that diffuse layer of decomposing plant 

 and animal material which we call the humus, or surface 



