90 growth: synthesis of bacterial protoplasm 



Heterotrophic bacteria 



The other major group of bacteria is the heterotrophic 

 group; these organisms obtain their carbon mainly from 

 organic sources, their nitrogen from either or both inorganic 

 and organic sources, and the energy for their synthetic processes 

 by the degradation (oxidation, fermentation, etc.) of energy- 

 rich organic material. Heterotrophic bacteria are thus 

 related in their general metabolism to animals, while the 

 autotrophic bacteria are related to plants. It is possible that 

 the two higher forms of existence may have evolved from the 

 related groups of micro-organisms. 



Heterotrophic bacteria can be subdivided on nutritional 

 grounds as follows (see Table IX) : 



(a) Organisms able to utilise atmospheric nitrogen: 

 The ability to trap atmospheric nitrogen and transform it into 

 inorganic or organic nitrogenous compounds within the cell 

 is called "nitrogen fixation," and is a property possessed' 

 by comparatively few species. Such organisms are of great 

 importance in agriculture, since the natural fertilisation of 

 the soil is a result of their activities. The most important 

 member of this group is Azotobacter, a strict aerobe found 

 free-living in the soil. This organism can grow in the complete 

 absence of " fixed-nitrogen " as long as it is provided with 

 atmospheric nitrogen and a source of carbon in the form of 

 fermentable carbohydrate. When growth occurs under such 

 conditions there is a quantitative relation between the amount 

 of carbohydrate fermented and the amount of nitrogen fixed. 

 Despite much research, we still have little or no definite 

 knowledge of the chemistry of the fixation process, and the 

 primary product of fixation has not yet been identified. Claims 

 have been made that atmospheric Ng is first reduced by the 

 organism to either NH3 or NHgOH but these claims have yet 

 to be satisfactorily substantiated. Studies in which Azoto- 

 bacter has been exposed to gaseous nitrogen, enriched with 

 isotopic-Ng, have shown that the isotope appears in the amino- 

 acids of the organism within a few minutes of exposure. The 



