ESMOND E. SNELL 435 



(35), and most of this can be lecovered from the cells. This behavior 

 typifies that of other essential growth factors. They are essential com- 

 pounds for formation of protoplasm and as such are absorbed from the 

 medium with great efficiency and incorporated into the cell. Many 

 other closely related lactic acid bacteria synthesize the riboflavin which 

 they require (36); L. casei does not and must have it preformed for 

 growth. 



In the past the inability of an organism to synthesize a growth factor 

 has been most frequently explained by assuming that the mutation which 

 gave rise to the nutritional requirement eliminated one of the essential 



TABLE 2 



The comparative requirement of H. parainfluenzae for purine bases in the 

 presence and absence of guanine (9) 



Additions to basal medium* Turbidity f 



None 70 



Guanine hydrochloride 94 



Hypoxanthine 73 



Guanine hydrochloride + hypoxanthine 74 



Guanine hydrochloride + adenine sulfate 70 



*ioo M^g. of the indicated compound to each 10 ml. of medium. 



tPer cent of incident light transmitted, uninoculated medium = 100. 



enzymes concerned in synthesis of the growth factor. Thus we might 

 suppose that L. casei lacked one of the enzymes necessary for ribo- 

 flavin synthesis. So far as is now known this is a sufficient explanation 

 for this case. In many cases, however, this explanation does not suffice. 

 Several examples are now known where a bacterium synthesizes a given 

 growth factor in one medium but does not do so in another. For 

 example (Table 2), if Hemophilus parainfluenzae 7901 is cultured in a 

 defined medium which contains guanine, it must be supplied with either 

 hypoxanthine or adenine to permit growth (9). The simplest explanation 

 of this finding assumes that hypoxanthine (or adenine) cannot be syn- 

 thesized under these conditions and consequently must be supplied to 

 permit synthesis of nucleic acid, which is in turn necessary for growth. 

 Yet if guanine is omitted from this medium the organism grows well 

 in the absence of both hypoxanthine and adenine and must under these 

 conditions synthesize its own purine bases. Failure to synthesize adenine 



