444 PLANT GROWTH SUBSTANCES 



may suffice to permit growth by supplying the same deficiencies through 

 different mechanisms. 



In the above discussion the attempt has been made to give, at least 

 in outline, the present status of research in bacterial nutrition. The 

 valuable leads that a study of the interrelationships between these 

 nutrients is providing the biochemist should be apparent. The utility of 

 bacteria as test organisms in the microbiological determination of the 

 substances which they require for growth is also well known. One of 

 the most important of the gains to be derived from a study of bacterial 

 nutrition is the recognition of new substances which function as essential 

 growth factors, for such substances have, in the past, always proved to 

 be substances of general importance in metabolism. For example, the 

 recent discovery that D-alanine (10,32) and putrescine or the related 

 compounds, spermine and spermidine (8), are essential for growth of 

 some bacteria lends new importance to these long-known compounds. 

 It had not been previously known whether any of the D-amino acids 

 played essential roles in metabolism; it now appears most certain that in 

 some bacteria, at least, D-alanine is essential for growth (10). Similarly, 

 although putrescine had long been known as a decomposition product 

 of arginine, it was not known to play any essential role in metabolism. 

 Such a role now seems certain from the observation that the compound 

 serves as an essential growth factor for Hemophilus parainjliienzae (8). 



Elucidation of the chemical nature of the several unidentified growth 

 factors reported as essential for growth of various species of bacteria may 

 similarly be expected to contribute materially to our knowledge of bio- 

 chemistry and metabolism, for these unidentified substances, like the 

 growth factors of Table i, are generally distributed in natural materials 

 and undoubtedly have general metabolic significance. Several of the 

 better defined unidentified growth factors are listed in Table 3. 



In summary, we have emphasized that the growth factors required 

 by bacteria comprise those compounds, such as the amino acids and the 

 water-soluble vitamins, which are of general importance as essential 

 metabolites in all Jiving organisms. A nutritional requirement for one, 

 several, or many of these growth factors may arise through cumulative, 

 random mutations which result in the loss by the organism of the capacity 

 to synthesize the growth factors which it requires under a given set of 

 environmental conditions. This inability to synthesize a growth factor 

 may result either from loss of one of the enzymes necessary for its 



