94 Perspectives in Microbiology 



ess, generally are found in abundant yield only when the 

 concentration of the substrate exceeds a relatively high 

 minimum level. The best explanation of this appears to be 

 that the presence of excess substrate maintains the forma- 

 tion of intermediate products at a rate faster than they can 

 be transformed; hence they accumulate. Elsewhere, I have 

 described this as "shunt metabolism." The essential result 

 is a higher yield of a particular product per unit of sub- 

 strate consumed, up to a certain maximum. Although it 

 has been conventional to cultivate fungi in media whose 

 substrate content is 1 to 5 per cent or more, especially 

 where accumulation of intermediates is sought, it is not 

 often that bacteria are cultivated in media containing in 

 excess of 0.5 to 1 per cent of a single carbon source of any 

 type. Especially in aerobic oxidation one might expect, if 

 the bacterium is furnished a high substrate concentration, 

 to find products of incomplete oxidation accumulating. 

 Their nature would afford good clues as to the mode of 

 degradation of the substrate, and thereby expedite the solu- 

 tion of the problem, which otherwise might be difficult 

 of access in media of low substrate concentration. An out- 

 standing illustration of this principle exists in the discov- 

 ery that numerous species of Pseudomonas and Phytomonas 

 produce high yields of various hexonic and pentonic acids 

 when the carbohydrate content of the medium is high (9, 

 11). That this phenomenon is not limited to formation 

 of sugar acids is evidenced by the accumulation of 20 

 per cent molar yield of a-ketoglutaric acid from 5 per cent 

 glucose by a strain of Pseudomonas fluorescens (10). Un- 

 doubtedly the same principle would apply, and awaits ex- 

 ploitation, with respect to intermediate oxidation prod- 

 ucts of substrates other than sugars. It is already known 

 that various strains of P. fluorescens, when furnished ade- 

 quate ethanol as the substrate, carry out an acetification 

 process indistinguishable from that formerly thought re- 

 stricted to the genus Acetobacter (13). 



