H. A. BARKER 



which the compound in question constitutes the main energy 

 source. In such a medium only organisms that can muhiply at 

 the expense of the added substrate become numerous. Aerobic 

 or anaerobic species can be selected by supplying or removing 

 oxygen, and other conditions such as the nitrogen source, the pH, 

 or the temperature may be varied so as to favor specific groups of 

 bacteria. Bymaking several transfers in the appropriate medium, 

 organisms with the desired substrate specificity and environmental 

 requirements can be "enriched" to the extent that their isolation 

 in pure culture can be achieved by conventional methods. 



The application of the enrichment culture method to the 

 study of the fermentation of nitrogenous compounds, for example, 

 has resulted in the isolation of several previously unknown 

 bacteria that are potentially useful for metabolic studies. With 

 uric acid as an enrichment substrate under anaerobic conditions, 

 two closely related purine-fermentating bacteria, Clostridium 

 acidi-urici and C. cylindrosporum, can be isolated from soil (3). 

 These bacteria show a high degree of biochemical specialization, 

 since they decompose uric acid, xanthine, and guanine with 

 great vigor but do not attack a variety of other common sub- 

 strates (2). Enrichment cultures with orotic acid have yielded 

 an organism, Zymobacterium oroticum, which has been used to 

 study the enzymatic synthesis and degradation of the pyrimidine 

 ring (18,35). Other bacteria fermenting uracil, thymine, or 

 allantoin have been obtained by the same method. A number 

 of amino acid fermenting bacteria have also been isolated from 

 enrichment cultures. Examples of such organisms are Clostrid- 

 ium tetanomorphum, which attacks histidine and glutamate (34), 

 C. propionicum, which ferments alanine, serine, and threonine (8), 

 and Diplococcus glycinophilus, which uses only glycine and certain 

 of its di- and tripeptides (8). 



Many similarly specialized bacteria, fermenting other com- 

 pounds, undoubtedly can be obtained by the enrichment 

 method. It is a useful tool with which to isolate organisms 

 favorable for the study of a variety of biochemical reactions. 



The remainder of this article will be devoted to a discussion 



74 



