ESMOND E. SNELL 



known that this interchangeability results from the fact that vita- 

 min Be is required for synthesis of this amino acid; when the 

 latter is supplied preformed, the requirement for vitamin Be 

 disappears, provided each of the L-amino acids required for 

 growth also is supplied (24,83), This remains the only known 

 instance of a D-amino acid being essential for growth, although 

 these substances, once thought not to occur naturally, have now 

 been frequently encountered in various microbial products, such 

 as antibiotics and capsules. A closer investigation (74) shows 

 the D-alanine occurs chiefly in the cell wall, of which it is appar- 

 ently an essential component, and in soluble cell extractives. 

 Recent investigation of the composition of bacterial cell walls 

 shows them to be rich in carbohydrate materials, including 

 amino sugars (55). It is possible that the requirement of 

 Lactobacillus bifidus {or A^-acetylglucosamine derivatives (19a,31) 

 and of some lactobacilli, grown with pentoses, for traces of glucose 

 (39) may reflect a requirement for these substances as structural 

 materials for cell wall formation. 



/?-Hydroxybenzoic acid and D-lactic acid (or its homologues) 

 are examples of growth requirements found so far only in artifi- 

 cially induced mutants; this does not lessen their importance 

 but is mentioned to illustrate the utility of mutant organisms in 

 the detection of previously unrecognized essential metabolites. 

 The technique has been especially valuable in permitting detec- 

 tion and isolation of intermediates in biosynthetic reactions — 

 compounds which normally have such a transient existence that 

 their natural occurrence would be difficult to detect in any other 

 way. Space limitations do not permit tabulation and discussion 

 of the many compounds discovered in this fashion — each of which 

 can serve as essential organic nutrients for certain mutant 

 organisms under a restricted set of conditions. 



Appropriate peptides stimulate or are even essential for 

 growth of many bacteria, and recent investigations have begun 

 to throw light on the reasons these compounds show activities 

 not shared by their component amino acids. Three model in- 

 stances have so far been observed : ( 7) assimilation of one amino 



106 



