402 MICROBIOLOGY OF SOIL 



bined nitrogen. Soon after that he succeeded in isolating his organisms 

 in pure culture, and described them as anaerobic bacilli allied to those 

 of the butyric group. In 1901 our knowledge of Azobacteria was 

 enriched by Beyerinck's discovery of a group of large, obligate aerobic 

 bacteria that he designated as Azotobacter. Since that date it has been 

 found that the ability to fix atmospheric nitrogen is possessed also by 

 certain molds and by various species of bacteria. However, this ability 

 is not only extremely variable, but is also very feeble as compared 

 with that of the members of the two groups described by Winogradski 

 and Beyerinck. These two groups may, therefore, be designated as 

 including the nitrogen-fixing bacteria par excellence. 



ANAEROBIC SPECIES. The species isolated by Winogradski was 

 named by him.B. (Clostridium) pasteurianus (Fig. 131). It was found to 



FIG. 131. B. (Clostridium') pasteurianus, a non-symbiotic nitrogen-fixing organism. 



(After Winogradski from Lipman.) 



grow readily under anaerobic conditions in culture solutions contain- 

 ing dextrose and the necessary mineral salts, but no combined nitrogen. 

 The products of growth included protein, butyric and acetic acids, 

 carbon dioxide and hydrogen. In the presence of other bacteria B. 

 (Clostridium} pasteurianus was found to develop also under aerobic 

 conditions. Subsequently studies by Winogradski and other investi- 

 gators showed that B. (Clostridium) pasteurianus, and varieties of this 

 species are very widely distributed in cultivated soils. More recently 

 Bredeman made a thorough and extended study of anaerobic Azo- 

 bacteria and demonstrated their almost invariable presence in a large 

 number of soil samples from Europe, Asia and America. In his opinion 

 they correspond more or less closely to B. amylobacter described many 

 years before by van Tieghem. 



AEROBIC SPECIES. A more or less pronounced power to fix atmos- 



