116 BACTERIOLOGICAL CHEMISTRY 



compounds originally. It is possible that intestinal 

 bacteria may serve as a source of vitamins for human 

 and animal nutrition, since it has been observed that 

 vitamin deficiency symptoms often develop when the 

 growth of intestinal bacteria is suppressed by the use of 

 such di-ugs as sulphaguanidine or sulphasuxidine. 



The Lactobacilli require a considerable proportion 

 of the known growth factors for their adequate growth 

 on synthetic media. This fact is used for the detection 

 and estimation of the amounts of such substances present 

 in various extracts or foods. Thus Landy and Dickens 

 have shown that L. casei will grow well on a sjmthetic 

 medium containing the appropriate amino -acids and 

 mineral salts together with aneurin, biotin, nicotinamide, 

 pantothenic acid, pyridoxine and riboflavin. The amount 

 of growth can be estimated by titrating the lactic acid 

 formed. If any one of the growth factors is omitted 

 from the medium growth does not occur. If the missing 

 factor is added in amounts less than that required for 

 maximum growth, the degree of growth is proportional 

 to the quantity of the factor added. The growth of L. 

 casei on the medium lacking one of the factors can, 

 therefore, be used as a test for the presence of that factor 

 in anji^hing added to the medium ; by comparing the 

 amount of growth in presence of the addendum with 

 that occurring on the full medium the test can be made 

 quantitative. By omitting each growth factor in turn 

 from the medium an analysis of the factors in an extract 

 can be made. 



Another valuable method of assaying growth factors 

 and essential amino -acids has recently developed from 

 the use of mutants of the mould Nenrosi)ora crassa 

 obtained by the action of X-rays on the asexual spores 

 and crossing them with the heterothallic strain of opposite 

 character. A series of mutants has been obtained wliich 



