SYNTHETIC ABILITIES 109 



hand, with a chemically defined medium, and from that we 

 grow a certain amount of bacterial protoplasm; in most 

 cases we have no knowledge of the intermediate chemistry 

 and metabolism. Exacting nutritional requirements show 

 us what certain organisms cannot do, and we make the 

 assumption that these synthetic disabilities of some organisms 

 are synthetic abilities in others. To what extent is this 

 assumption justified ? 



In the first place, we are able to trace a paralleHsm between 

 exactingness and parasitic existence which suggests that the 

 former arises from constant growth in the presence of complex 

 essential substances of difficult synthesis ; in some cases we are 

 able to demonstrate a recovery of synthetic ability, such as 

 that towards tryptophan of exacting Eher. typhosa, serially 

 subcultivated in media containing progressively less trypto- 

 phan, or that towards thiamin of Staph, aureus subcultivated 

 similarly in media containing progressively less thiamin. 



Secondly, in the case of certain growth factors such as 

 nicotinic acid, thiamin, pantothenic acid, pyridoxal, etc., we 

 are able to show a metabolic impairment of deficient cells 

 which is not found in non-exacting organisms, and in such 

 cases the metabolic impairment is related to those enzymes 

 which, as we know from studies in other tissues, have a 

 structure involving the growth factor as prosthetic group or 

 coenzyme. 



Thirdly, by surveying the parts of a complex growth 

 factor which are required by various exacting organisms, 

 we can often show that some organisms can synthesise 

 certain portions of the molecule but not others, e.g. some 

 strains of the diphtheria organism require ^-alanine as growth 

 factor, while others require pantothenic acid ; pantothenic acid 

 will replace ^-alanine for the former organisms, but ^-alanine 

 cannot replace pantothenic for the latter; consequently the 

 former organisms cannot synthesise ^-alanine, but, given that, 

 can synthesise pantothenic acid, whereas the latter strains 

 have a wider disability in that they cannot accomplish this 

 further step but require the complete pantothenic acid molecule. 



