68 



Occurrence of Microorganisms in Soil 



purple bacteria oxidize H2S, S, sulfite, and thiosulfate to sulfate. In 

 the absence of oxidizable sulfur compounds, the purple bacteria can 

 develop in the presence of organic compounds under anaerobic 

 conditions, but only in the presence of radiant energy. 



Other Autotrophic Bacteria. Among the other autotrophic bac- 

 teria should be mentioned those that oxidize hydrogen, carbon mon- 

 oxide, and ferrous iron; not all of these bacteria are strictly soil 

 inhabitants, although certain conditions make some of them abundant 

 in the soil. 



Heterotrophic Bacteria 



Heterotrophic bacteria comprise the great majority of soil organ- 

 isms. They depend on organic materials for then energy sources, 

 and are primarily concerned with the decomposition of cellulose and 



hemicelluloses, starches and sug- 

 ars, proteins and other nitroge- 

 nous materials, fats and waxes. 

 These bacteria vary greatly in 

 structure and physiology, in abun- 

 dance, and in importance. Some 

 are aerobic; others are anaerobic. 

 Some are spore-forming; others 

 are non-spore-forming. Some are 

 Gram-positive; others are Gram- 

 negative. Some are able to fix 

 atmospheric nitrogen; others de- 

 pend upon fixed forms of organic 

 or inorganic nitrogen. 



Spore-Forming Bacteria. The 

 soil harbors a large number of 

 spore-forming bacteria. The three 

 most common forms can be read- 

 ily recognized by the gelatin plate 

 method. Bacillus mycoides is a rapidly liquefying form; it produces 

 large filamentous to rhizoid colonies. Bacillus cereus liquefies gela- 

 tin almost as rapidly and usually forms round colonies with entire 

 edges; the surface membrane contains granules that tend to be ar- 

 ranged concentrically. Bacillus megatherium liquefies gelatin more 

 slowly; its colonies are seldom more than 1 cm in diameter and are 

 characterized by a flocculent center composed of white opaque gran- 

 ules, surrounded by a zone of clear hquefied gelatin; the smaller 



OOooOOo 



Fig. 30. Heterotrophic, spore-form- 

 ing bacterium, Bacillus megatherium: 

 a, young cells showing flagella; h, 

 young cells showing connections into 

 chains of rods; c, older cells; d, vari- 

 ations in size and shape of spores 

 ( from Conn ) . 



