GROWTH FACTORS 117 



can grow on a complete medium but which fail to grow 

 on a medium lacking a single constituent specific to 

 each mutant. This is due to a lack of synthetic ability 

 to form the particular compound, occasioned by destruc- 

 tion of the controlling gene. Mutants failing to sjoithesise 

 p-Sbinmo benzoic acid, aneurin, choline, inositol, nicotinic 

 acid, pantothenic acid, pyridoxine and the essential 

 amino-acids arginine, leucine, lysine, methionine, proline, 

 threonine, tryptophan and valine are known. The gro^vth 

 of such mutants is proportional to the amount of the 

 specific substance (up to the amount necessary for 

 optimum gro^vth) which is added to an otherwise adequate 

 medium. 



These and other mutants are also extremely valuable 

 for elucidating the mechanism of certain metabolic 

 reactions (see p. 343). 



In contrast with the term " Antibiotics " which has 

 recently come into use for those substances produced by 

 micro-organisms which inhibit the gro^vth of other 

 organisms, it has been suggested that those substances 

 capable of stimulating metabolism which are produced 

 by micro-organisms, and which are usually highly specific 

 in their action, should be called " Biotics " as an alter- 

 native to growth factors. 



A list of some micro-organisms and their requirements 

 of growth factors is given in Table 8. It must be 

 realised that different strains of a given species may 

 vary in their ability to synthesise one or more of the 

 biotics mentioned so that the lists given cannot apply 

 rigidly to all strains. Some of the strains may also 

 require growth factors of still unknown composition in 

 addition to those listed. Considerable variation of this 

 sort is found amongst strains of (7. diphtherice, Lacto- 

 bacillus, Streptococcus and the yeasts. 



