Aerobic Bacteria 



195 



lots of: sterile media, the organism was gradually purified from most 

 of the contaminating forms and was finalK isolated in pure culture 

 on mannitol agar. 



Although five species of Azotobacter are now known, Lohnis and 

 Smith recognized only two species, Az. chroococcum and Az. a^lis. 

 They considered Az. bcijcrinckii Lipman and Az. vitreum Lohnis to 

 be varieties of Az. aii,i]is. Khuver has shown, however, that Az. aailis 



Fig. 81. Azotobacter chroococcum, young culture (from Beijerinck). 



is quite distinct from Az. vinelandii and is found only in canal waters, 

 whereas the latter is characteristic of soil. Winogradsky proposed 

 the name Azomonas for the noncystogenic organisms found in waters. 

 Azotobacter indicum is certainly a distinct form. 



Azotobacter represents a highly interesting group of strictly aerobic 

 organisms. Its temperature range lies between 10° C and 40° C, 

 with an optimum at 30-35 °C. It is highly sensitive to acidity, with 

 an optimum at pH 7-8. A large number of organic compounds can 

 be used as sources of energy and carbon. They include fatty acids 

 and oxyacids, higher and lower alcohols, and mono-, di-, and poly- 

 saccharides. 



The fixation of nitrogen by Azotobacter is brought about by a sys- 

 tem of enzymes designated as "azotase." Nitrogenase, one of the 

 components of this system, is capable of combining directly with 

 elementary nitrogen. This enzyme complex has not been isolated, 



