84 BACTERIOLOGICAL CHEMISTRY 



The amino -acid most often demanded as essential by 

 the " exacting " strains is tryptophane, 



li-CH2.CH.c00H 



I II II I 



%/\/ NH^ 



NH 



p-indole-a-amino-propionic acid. It is one of the most 

 complicated of the amino -acids, so that it is, perhaps, 

 not surprising that it should be the most difficult to 

 synthesise, and accordingly be one of the earliest to be 

 required in a preformed condition. In many cases this 

 lost synthetic power may be restored by " training " the 

 organism by repeated subculture on media containing 

 less and less tryptophane and more and more ammonium 

 salt, until it can grow again in the entire absence of the 

 amino -acid. This so-called " training " may not be a 

 true change in the metabolism of the organism, but may 

 be a concentration, by selection, of a few individual cells 

 in the " exacting " strain which have not lost their 

 S3nithetic power. Those cells which have lost the power 

 will die out under the adverse conditions, until ultimately 

 only non-exacting organisms are left ; the acquisition 

 of the ability to use ammonium salts is, according to this 

 view, only apparent, the power really being present all the 

 time in a small proportion of the bacteria. 



Further along the route to complete loss of synthetic 

 power are those organisms which, in addition to needing 

 organic carbon and organic nitrogen in the form of one 

 or more amino-acids, require the so-called " growth 

 factors " or bacterial vitamins, as they are sometimes 

 termed. The best-known organisms in this group are 

 Staphylococcus aureus, Clostridium sporogenes, CI. botu- 

 linum and Lactobacillus casei. It seems possible, if not 

 probable, that the organisms of the other groups can 

 produce their own vitamins, but that those in the present 



