ESMOND E. SNELL 



nutritional importance of several others (/?-aminobenzoic acid, 

 inositol, nicotinic acid). Since 1942, as illustrated in Table I and 

 discussed more fully below, microorganisms (chiefly those of a 

 single group, the lactic acid bacteria) have played a dominant 

 role as test organisms in the discovery and isolation of new vita- 

 mins and related factors. 



But the discovery, isolation, and chemical characterization 

 of a new vitamin represent only the more glittering facets of a 

 many-sided problem. How is the vitamin distributed in nature? 

 In what forms does it occur? What essential role does it play 

 in metabolism? Investigation of these and related questions 

 also has been immensely furthered by use of microorganisms. 

 Since its introduction by Snell and Strong (71) in 1939 the 

 technique of comparing the growth response of microorganisms 

 to tissue extracts and to pure vitamin as a quantitative method 

 for estimation of vitamins in natural materials (reviews, 2,64,65) 

 has become fully accepted and has increased greatly our knowl- 

 edge of the distribution of the vitamins in nature. Complica- 

 tions sometimes arise in such assays from the presence of previ- 

 ously unsuspected vitamin derivatives or of metabolically related 

 substances, and the unraveling of such difficulties can provide 

 information relative to the latter two questions posed above. 



The study of metabolic roles of the vitamins also has been 

 facilitated greatly through use of artificially induced nutritional 

 mutants of microorganisms (reviews, 13,36) and through detailed 

 study of the action of inhibitors structurally related to vitamins — 

 inhibition analysis (reviews, 58,81). It is the thesis of this 

 essay that bacterial nutrition still has much to contribute to the 

 study of nutrition of higher animals. Some directions in which 

 we may look for such contributions will be pointed out in the 

 sequel. 



Vitamins and Growth Factors Identified since 1942 



A brief and incomplete review of progress in recognition of 

 vitamins and growth factors since the similar review by Peterson 



88 



