GROWTH FACTORS 115 



parent substances of prosthetic groups of enzymes or 

 CO -enzymes which are essential for the metabolism of 

 micro-organisms. It is this which, probably, accounts 

 for the fact that most growth factors are needed only in 

 very small amounts, since they remain in circulation, 

 as it were, and are used over and over again. If they 

 are not available the particular metabolic process in 

 which the corresponding enzjine or co -enzyme is in- 

 volved is brought to a standstill and the organism fails 

 to grow. It is almost certainly true that all organisms 

 make use of these substances in their metabolism. Some 

 organisms, however, appear to be able to make them all 

 for themselves, whilst others need one or more of them 

 to be provided from outside sources. It is only when a 

 substance has to be supplied ready made to a micro- 

 organism that it is regarded as being a growth factor for 

 that organism. An organism, although capable of 

 synthesising a particular gro^^i;!! factor, may do so 

 relatively slowly so that it is in a chronic state of deficiency 

 and accordingly develops only poor gro\\i}h. The addi- 

 tion of the metabolite then has a stimulating effect on 

 growth. Fildes has suggested that, in reality, most, if 

 not all, of these compounds can be regarded as " essential 

 metabolites." 



That bacteria really can synthesise " essential meta- 

 bolites " themselves is showrti by the fact that TJiio- 

 bacillus thio-oxidans , for instance, when grown 

 autotrophically on sulphur containing media with no 

 organic material initially present contains aneurin, 

 biotin, nicotinic acid, pantothenic acid, pyridoxine and 

 riboflavin. The same is true of Esch. coli, Proteus 

 vulgaris, Aerohacter aerogenes, Alkaligenes fcecalis, B. 

 antJiracis, B. mesentericus , B. vidgatus, Vibrio comyna 

 and Serratia marcescens in which aneurin, biotin, nicotinic 

 acid and other substances have been detected after 

 their growth on synthetic media containing none of these 



