BACTERIAL FERMENTATIONS 



valine is oxidized, and another amino acid such as glycine or 

 proline is reduced. 



The main inorganic oxidants used by anaerobic bacteria 

 are nitrate, sulfate, and carbonate. The ability to use one of 

 these compounds as a major oxidant is generally restricted to 

 specialized groups of bacteria. Nitrate is reduced to nitrous 

 oxide and nitrogen by denitrifying bacteria, sulfate is converted 

 to hydrogen sulfide by sulfate-reducing bacteria, and carbon 

 dioxide is reduced either to methane by the strictly anaerobic 

 methane-producing bacteria (32) or to acetic acid by organisms 

 such as C. aceticum (37) and C. thermoaceticum (38). Inorganic 

 oxidants are advantageous in that they make possible the utiliza- 

 tion of a wider variety of organic compounds, and the more 

 complete oxidation of individual substrates, than is possible in a 

 true fermentation. 



Selection of Organisms 



The selection of an organism capable of attacking a given 

 compound or performing a particular reaction sequence can 

 often be made by consulting a reference book on systematic 

 bacteriology. However, because bacteriologists have generally 

 shown a strong preference for the use of a limited number of 

 substrates, of which the sugars are most prominent, it sometimes 

 happens that no organism is known which possesses a highly 

 developed ability to attack a particular compound in which an 

 investigator is interested. Under these circumstances a suitable 

 organism frequently may still be obtained by use of the enrich- 

 ment culture method. 



The advantages of the enrichment culture method, origi- 

 nally developed by Winogradsky and Beijerinck in the last 

 century, appear not to be widely recognized; certainly, the 

 method has not been applied as extensively as might be desirable. 

 The method depends on the principle that organisms capable of 

 decomposing a given compound can be obtained from a mixed 

 population such as that in soil by using a culture medium in 



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