92 growth: synthesis of bacterial protoplasm 



various amino-acids have been isolated and their content of 

 the isotopic-N determined; those containing the highest 

 amount of isotope must be those which are formed first, as a 

 result of the fixation process. Glutamic acid is found to 

 contain the highest content of isotopic-N and so, presumably, 

 is the first amino-acid formed after fixation and must act as 

 precursor of the other amino-acids. The organisms will grow 

 on ammonia as N-source but if such fixed-nitrogen is provided 

 in the medium, then fixation immediately ceases. If the 

 organism is supplied with ammonia enriched with isotopic-N, 

 it can utilise this immediately and the isotope is found in the 

 cell-protein within three minutes of contact. The distribution 

 of the isotope in the amino-acids of the cell is the same as that 

 obtained when the cell is fixing isotopic-Ngi this result 

 suggests that ammonia is a primary product of the fixation 

 reaction. When growth of Azotobacter is occurring by fixation 

 of atmospheric nitrogen then the presence of traces of iron, 

 calcium, and molybdenum are essential. The greater require- 

 ment of molybdenum for growth on gaseous nitrogen than for 

 growth on ammonia-nitrogen suggests that this metal has a 

 function in the fixation process. 



Nitrogen fixation is not confined to Azotobacter, but is also 

 a property of some photo synthetic bacteria and certain 

 Clostridia such as CI. pastorianmn, which was the first nitrogen- 

 fixing organism to be isolated from soil. In addition, the 

 Rhizobaceae, the root-nodule bacteria, can perform nitrogen- 

 fixation, but only when living in symbiosis with the host-plant 

 (see Chap. X). 



(6) Non-exacting organisms : This is probably the largest 

 of the sub-groups and consists of those organisms which can 

 synthesise their nitrogen requirements from ammonia or 

 nitrate, their carbon from a simple organic source such as 

 glucose or lactate, and obtain their energy from the degradation 

 of organic matter. Esch. coli is a typical example, in that it 

 can grow luxuriantly in a medium consisting of nutrient salts, 

 including ammonium ions, and either glucose or lactate as 



