NUTRITION OF HETEROTROPHIC BACTERIA 85 



group have lost the power of synthesis of both amino - 

 acids and of the vitamins. Many bacterial growth 

 factors are recognised now as being identical with the 

 vitamins which play a large part in animal nutrition. 

 In fact many of the fundamental metabolic reactions of 

 bacteria are identical with, or very similar to, those 

 of animals. They are non-specific in the sense that the 

 CI. sporogenes factor promotes the gro\\i}h of CI. hotulinum 

 and of CI. tuelchii, and that they are produced by many 

 bacteria having simpler nutritional requirements ; Esch. 

 coli, for example, can synthesise the growth factor 

 required by SUiph. aureus (see Chapter IX). 



As with the other changes of nutritional types, here, 

 again, occurs a group of intermediately placed organisms 

 which link those requiring growth factors with those 

 which do not. These intermediate species either exist 

 as two sorts of strain, one requiring the factor and the 

 other not, or they may be trained, with more or less 

 difficulty, to synthesise their own growth factor instead 

 of requiring it ready made. This seems to be the case 

 with the tubercle bacillus and such organisms as Coryne- 

 bacterium diplitherice, which, when freshly isolated, require 

 complex media containing gro\\i:h factors, but can be 

 trained to grow on synthetic media comprising only 

 known carbon and nitrogen sources. 



It is possible that the various growth stimulants 

 which have been described for certain organisms are 

 really essential growth factors, but which are produced so 

 slowly by the organism concerned that their addition 

 from an outside source causes an increased growth. If 

 the rate of synthesis of the factor were so slow that its 

 concentration were negligible, it would be regarded as an 

 essential gro\\i:h factor which must be supplied in the 

 medium to enable growth to occur. If, on the other 

 hand, the organism produced it so fast that adequate 

 growi^h occurred without the necessity of adding it from 

 outside, neither its stimulating nor essential character 



