ESSENTIAL NUTRIENTS 65 



three- and four-carbon organic acids, and ammonium salts as a source of nitrogen. 

 One strain of Sahn. typhi failed to grow on any of the media, nor would it grow when 

 glucose was substituted for the organic acids as a carbon-source ; but it grew when 

 provided with tryptophan. The variation in the tryptophan requirements of dif- 

 ferent strains oi^alm. typhi was studied by Fildes and his colleagues (Fildes, Glad- 

 stone and Knight 1933, Fildes and Knight 1933). The organism grew in a basal 

 medium containing sodium citrate, magnesium sulphate, phosphate buffer and 

 glucose, together with fourteen amino-acids : alanine, glycine, valine, glutamic acid, 

 asparagine, tyrosine, phenylalanine, proline, histidine, arginine, leucine, lysine, 

 cystine and tryptophan. Any one of the first ten could be omitted without affecting 

 growth ; the omission of leucine or cystine caused delay in growth, and in the 

 absence of tryptophan there was no growth. The concentration of tryptophan 

 necessary for growth in the presence of other sources of nitrogen was as little as 

 0-00064 per cent. Next, two strains of Sahn. typhi were trained to grow in the 

 absence of tryptophan, with ammonium chloride as the sole source of nitrogen ; 

 they had evidently adapted themselves to synthesize the amino-acid, for it was 

 demonstrable in the bacterial cell by the glyoxylic reaction. It would seem therefore 

 that tryptophan is an essential constituent of the typhoid bacillus ; that most strains 

 when first removed from media that contain it are unable to synthesize it ; but that 

 they can be trained to do so by gradually accustoming them to a medium from which 

 it is absent. Several other species were examined by the same method (see also 

 Fildes 1935, Fildes and Eichardson 1935), and from these observations Fildes and 

 his colleagues concluded that tryptophan was essential in the metabolic processes of 

 the bacteria studied (an " essential metabolite "). They divided bacteria into three 

 groups : (1) Those that can synthesize tryptophan ; such as autotrophic bacteria, 

 and, among the potentially pathogenic organisms examined. Bad. coli, Ps. pyocyanea 

 and Salm. typhi-murium : (2) Those that are unable to synthesize tryptophan 

 for themselves, but can be trained to do so ; e.g., certain strains of Salm. typhi, 

 C. diphthericB, Staph, aureus and probably Myco. tuberculosis : (3) Those that cannot 

 synthesize tryptophan, yet for which it is an essential nutrient ; to this group 

 belong CI. tetani, CI. sporogenes, CI. botulinum and B. anthracis. As a result of 

 analyses of cultures of Sahn. typhi, Burrows (1939a, b) concludes that in no circum- 

 stances is tryptophan an essential nutrient for the organism, but that for those strains 

 whose synthetic powers are small, tryptoj^han acts as a stimulant of growth. 



The distinction between a stimulant and an essential nutrient is important, 

 for if we are to define bacteria studied in this manner by their synthetic abilities, 

 either manifest or latent, we may regard organisms as devoid of a particular 

 synthetic ability, which in fact may have it but in a degree insufiicient to allow 

 growth to take place ; and later researches may demonstrate this ability, either 

 by successful training of the organism, or by developing more sensitive methods 

 of demonstrating small amounts of the metaboUte in question. 



Again, the medium containing minimum requirements for growth may not 

 permit the development of the full characteristics of the parent strain. Gladstone 

 (1937) succeeded in training certain strains of Staph, aureus to utilize ammonia 

 as a sole source of nitrogen. Neither coagulase nor a-hsemolysin (see Chapter 25) 

 was produced in this medium ; for the production of optimum quantities of haemo- 

 lysin, arginine, glycine, proline, phenylalanine and valine were necessary. The 

 trained Staph, aureus exhibited all its initial metabolic properties when trans- 

 ferred to ordinary rich laboratory media. 



P.B. D 



