106 growth: synthesis of bacterial protoplasm 



will be able to synthesise these factors. Thus it is possible to 

 "train" Staph, aureus to synthesise thiamine by serial sub- 

 cultivation in media containing progressively less of this 

 factor. 



(e) Organisms exacting towards both amino-acids and 

 GROWTH factors: It has been suggested that an organism 

 becomes exacting when it grows for many generations in a 

 medium in which all growth requirements are provided ready 

 made. If the rate of growth is regulated by the rate of 

 synthesis of some factor, and that factor is provided ready 

 made in the environment, then the organism will be able to 

 grow more rapidly if it utilises the preformed factor than 

 if it is dependent upon the synthetic process. Likewise, a 

 mutant which has lost the ability to synthesise that factor 

 will grow as well as, if not better than, the synthetically 

 competent cell in the rich medium. Rich media would there- 

 fore be expected to select nutritionally-exacting mutants and, 

 in general, the richer the medium, the poorer would become the 

 synthetic abihties of the organisms using it as a natural 

 habitat. Soil organisms can find few or no complex growth 

 factors in their habitat, but organisms that have assumed a 

 parasitic existence in animal tissues are living in an environ- 

 ment rich in all those substances that go to make up proto- 

 plasm. In general, we find that the more parasitic an 

 organism, the more exacting are its growth requirements. 

 Some organisms such as S. haemolyticus are exacting towards 

 several amino-acids and several growth factors. The omission 

 of any one of these amino-acids or factors from the medium 

 renders it sterile towards this organism. The organism is 

 highly parasitic because only in the presence of tissues and 

 tissue products will such an array of factors be found naturally. 

 Parasitism leads to exacting growth requirements, and the 

 exacting nature of the growth requirements makes parasitism 

 obligatory. It does not follow that exacting organisms are 

 all parasitic on man, or that they are pathogenic, as patho- 

 genicity depends upon factors other than parasitism (see 

 Chap. XI). Organisms such as the Lactobacilli from milk or 



