NITROGEN METABOLISM 69 
substance, thus, CgHa.CHO + CH3.CHO = CeHs.CHOH.CO.CHg. 
This reaction is brought about by the acti\ity of an enzyme which 
Neuberg has named "carboHgase." 
The cataboHc phase is essentially analytic; it is characterized 
chemically by a series of reactions in which the cleavage of more com- 
plex compounds to simpler ones with their simultaneous or subsequent 
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, p. 75.) 
m. 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 and the 
sulphur bacteria can utilize the nitrogen of ammonium salts. (These 
two groups of organisms appear to be the only ones which can oxidize 
elementary nitrogen.) Many bacteria can obtain their nitrogen from 
amino-acids.' The majority of bacteria pathogenic for man and the 
higher animals are somew^hat 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 special- 
ized 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 cannot 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 deamination. 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 nitro- 
gen-fixing sulphur and nitrifying bacteria is not, as a rule, a source of 
energy, because the great majority of bacteria cannot oxidize it. 
' Tamura (Ztschr. f. physiol. Chem., 1913, 88, 190) has identified proteins, purins 
and lecithin bodies in the substance of bacteria grown upon non-protein media. 
