ESMOND E. SNELL 



437 



the presence of an inhibitor for the process, and in either case b will 

 appear as a nutritive essential for growth, since it can no longer be 

 formed within the cell or is formed in amounts insufficient for growth. 

 The diagram represents only one mechanism through which an inhibitor 

 may operate to make a previously nonessential growth factor become 

 essential for growth. Several others can be readily visualized. 



0123456 8 10 12 



MILLIMICROGRAMS OF PYRIDOXAL PER 6 ML 



Figure 2. Inhibition of growth of Saccharornyces carlsbergensis 4228 by 

 thiamin and its reversal by vitamin Be- Curves i, 2, 3, and 4 represent the 

 growth response to pyridoxal in the presence of 0, o.i, i, or 107 of thiamin, 

 respectively, per 6 ml. of medium (25). 



This latter explanation for the occurrence of certain nutritive re- 

 quirements in bacteria has been insufficiently emphasized in the past 

 apparently because of a disinclination to admit that normal cellular 

 metabolites might act as inhibitors for certain metabolic reactions. Yet 

 several such cases are now known as the few instances cited above 

 illustrate. The concept is novel only in viewing the inhibitor as a sub- 

 stance normally present within the cell. It has been recognized for several 



