CHAPTER V 



Heterotrophic, Aerobic Bacteria Requiring Combined Nitrogen 



General classification. The heterotrophic bacteria requiring combined 

 nitrogen comprise the large numbers of organisms developing on the 

 common plate used for counting bacteria and probably a still greater 

 number of organisms, which develop very slowly or do not develop upon 

 the plate at all. Morphologically they consist of spore-forming and 

 non-spore forming rods, cocci and spirilli. Physiologically they take 

 part in numerous soil processes, especially in the decomposition of 

 both simple and complex organic substances in the soil including pro- 

 teins, their derivatives, and other nitrogen compounds ; celluloses, pen- 

 tosans, and other complex and simple carbohydrates; fats and various 

 other ingredients of natural organic matter. Morphology alone is an 

 insufficient basis for the classification of these organisms. Just as in 

 the general classification, one must consider the various physiological 

 processes in which these bacteria are concerned. The system used 

 here is far from satisfactory, due to insufficient knowledge concerning 

 the organisms themselves. This system is bound to change with the 

 advance of our knowledge. 



The cellulose-decomposing bacteria, the nitrate and sulfate-reducing 

 bacteria, and the urea-decomposing organisms are treated separately, 

 because of their specific physiology and the special methods which 

 are essential for their isolation, cultivation and study. Some of these, 

 especially some of the urea-decomposing forms and nitrate-reducing 

 bacteria, are no doubt modifications of the more general groups consid- 

 ered here. As a general basis of classification, the following one may 

 be used conveniently: 



I. Aerobic bacteria: 



1. Spore-forming rods 



2. Non-spore forming rods 



3. Cocci 



4. Spirilli 



II. Anaerobic bacteria 



The difference between the aerobism and anaerobism of soil bacteria 

 is largely one of degree and not of kind, as will be shown later. The 



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