216 BACTERIOLOGICAL CHEMISTRY 



hydrazines, are very toxic. In the presence of glucose, 

 ammonium salts are more readily utilised than amines. 

 The presence of ammonium salts stimulates the utilisation 

 of amides ; some acids, for example malic acid, which 

 are not normally fermented can be fermented when 

 present as the ammonium salt. 



In the commercial production of protein by yeasts, 

 using molasses as the substrate, the replacement of up to 

 50 per cent, of the nitrogen of the molasses by ammonium 

 salts leads to an increased yield, but a complete replace- 

 ment restricts growth ; the optimum concentration of 

 ammonium salts has been shown to be that which causes 

 least swelling of the proteins. 



Amides. — The amides, particularly urea, can act as 

 nitrogen source for bacteria and yeasts, probably being 

 utilised via ammonia as an intermediate. Formamide 

 appears to be more readily utilised than other amides, 

 probably because of the constant presence of ammonia 

 in its solutions. 



Amino- Acids. — The majority of amino -acids are 

 effective as nitrogen sources, although some, for example 

 tryptophane and tyrosine, may be toxic if present in any 

 great quantity owing to the end products, indole and 

 phenol respectively, formed under appropriate conditions. 

 The open chain amino -acids are more easily attacked than 

 those containing ring systems (tyrosine, tryptophane, 

 histidine). Some organisms, for example C. diphthericB, 

 H. influenzce and Lactobacilli, require complex mixtures 

 of amino-acids for their growth, although most will 

 grow on a single simple amino -acid. For the exacting 

 organisms some amino acids are essential, but others are 

 not. Thus for the growth of L. arabinosus on a synthetic 

 medium arginine, cystine, glutamic acid, ^soleucine, 

 leucine, methionine, phenylalanine, tryptophane, tyrosine 

 and valine are essential ; in addition other amino-acids 

 are also required but the need can be supplied by one of 

 several, whilst omission of any one of those listed causes 



