70 BACTERIAL METABOLISM 
IV. CARBON METABOLISM. 
Carbon is an important structural element for bacteria, and it 
is equally indispensable as a source of energy, for the oxidation of 
carbon is an important feature of the catabolic activity of the majority 
of microorganisms. The reduced form in which this element is present 
in amino-acids and other protein derivatives appears to be particularly 
adapted for structural purposes; for fuel purposes it is less available, 
possibly because of the necessity of introducing free oxygen into the 
carbon complex to provide the requisite energy for the vegetative 
activities of bacteria, as well as the additional amount of work required 
to eliminate the nitrogen of the amino-acid molecule (deamination). 
It is generally stated that bacteria with relatively few exceptions fail 
to grow w^ith their customary vigor in sugar-free media from which 
free (atmospheric) oxygen is excluded; the relative absence of avail- 
able oxygen in such compounds would explain this phenomenon, in 
part at least. 
Fats also are apparently utilized directly by comparatively few 
bacteria, although Eijkman' found that certain microbes produced an 
adipocere-like substance when they were cultivated upon fat-contain- 
ing agar. Usually, when bacteria act upon fat, they first hydrolyze the 
substance and then ferment the glycerin leaving the fatty acids of the 
fat molecule untouched. 
The carbohydrate molecule, which contains no nitrogen and in 
which the carbon is already partially oxidized, can be utilized for 
fuel purposes by most bacteria with less expenditure of energy for 
its preparation than can be the case with most amino-acids, peptones, 
or proteins; for this reason it is very probable that the carbon of 
utilizable carbohydrate is acted upon by many bacteria in preference 
to protein carbon. In this sense utilizable carbohydrate protects or 
shields protein or protein derivatives from bacterial attack for their 
fuel requirements; it does not protect protein from bacterial break- 
down to supply their structural requirements, however. 
The net result of this selective protective action of carbohydrates 
for protein is important because the amount of material required to 
provide energy for the bacterial cell far exceeds the amount of material 
required to build up the bacterial cell.'^ The chemical transformations 
incidental to the anabolic phase of bacterial metabolism are insignifi- 
cant in amount and ordinarily not noticeable; on the contrary, the 
chemical transformations associated with the catabolic phase of 
bacterial metabolism are relatively very considerable in amount; and 
the nature and extent of those chemical reactions which are associated 
with the transformation of material for energy are important not 
only for the identification of bacteria; they collectively comprise the 
important specific functions of bacteria. 
1 Centralbl. f. Bakt., 1901, 29, 847. 
2 See page 253 for the relations between the structure of the various carbohydrate 
molecules and their utilization for energy by bacteria. 
