72 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 (deaminization). 

 It is generally stated that bacteria with relatively few exceptions 

 fail to grow with their customary vigor in sugar-free media from which 

 free (atmospheric) oxygen is excluded; the relative absence of available 

 oxygen in such compounds would explain this phenomenon, in part 

 at least. 



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 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 breakdown 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 insignificant 

 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 function of bacteria. 



