NITROGEN METABOLISM 71 



The catabolic phase is essentially analytic; it is characterized 

 chemically by a series of reactions in which the cleavage of more 

 complex compounds to simpler ones with their simultaneous or sub- 

 sequent oxidation, involving the liberation of energy, is a noteworthy 

 feature. The catabolic phase is chiefly a series of oxidations of carbon 

 and hydrogen. (For illustrative catabolic reactions see infra, pp. 73, 76.) 



IH. NITROGEN METABOLISM. 



Bacteria, like all known living things, contain nitrogen in their 

 substance, and nitrogen in some form is absolutely indispensable for 

 the building up of their structure. Nitrogen, in other words, is an 

 absolutely essential element in the constructive phase of the bacterial 

 cell. The form in which nitrogen must be presented to bacteria in 

 order to be utilizable by them varies with the kind of organism. The 

 nitrogen-fixing bacteria found on the roots of leguminous plants can 

 utilize the nitrogen of the atmosphere; some nitrifying bacteria can 

 utilize the nitrogen of ammonium salts. (These two groups of organ- 

 isms appear to be the only ones which can oxidize nitrogen.) Many 

 bacteria can obtain their nitrogen from amino-acids. The majority 

 of bacteria pathogenic for man and the higher animals are somewhat 

 more exacting in this respect and require more highly organized 

 nitrogen, as peptones and proteoses, while a small group of obligately 

 human pathogenic bacteria, as the gonococcus, grows only in media 

 containing nitrogen as it exists in the highly specialized protein of 

 human origin, at least during their first growth outside the human 

 body on artificial media. 



The vegetative phase of bacterial metabolism is essentially a series 

 of oxidations of carbon and hydrogen; nitrogen can not be oxidized by 

 the great majority of bacteria, and consequently it appears to yield 

 little or no energy to them. When nitrogen-containing compounds 

 as amino-acids, peptones, albumoses, or proteins are utilized for the 

 energy requirements of these organisms, the nitrogen (amino nitrogen) 

 is usually eliminated from the amino-acid complex incidental to the 

 oxidation of the carbon and hydrogen; the nitrogen thus eliminated 

 appears in soluble form in the culture medium as ammonia. This 

 process is true deaminization. Nitrates and even nitrites may be 

 sources of energy to many bacteria, usually, however, because of their 

 valuable oxygen content. To summarize, bacteria must have available 

 nitrogen for their structural needs, but nitrogen, except for the nitrogen- 

 fixing and nitrifying bacteria, is not as a rule a source of energy to 

 them, because the great majority of bacteria can not oxidize it. 



