ANALOGOUS SYSTEMS WITH OTHER ESSENTIAL METABOLITES 163 



In other species susceptible to the action of sulphonaiuides, the evidence for ^-amino- 

 benzoic acid as an essential metabolite is indirect. The acid appears to be present in yeast ; 

 and it can be recognized in extracts of bacteria either chemically or biologically, utilizing 

 either bacteria for which it is an essential nutrient, or bacteria like Mirick's, which oxidize 

 it. But in both cases the specificity of the test is open to question. Landy and his 

 colleagues (1943), using Acetobacter suboxydans for microbiological assay and checking 

 their results by chemical methods, found 0-042^g per ml. in the culture fluid of Staph, 

 aureus. They found also that strams which had been made resistant to sulphathiazole 

 by training in sulphathiazole-broth contained 70 times as much ^j-aminobenzoic acid as 

 the normal strains. These workers correlate this resistance with the power to synthesize 

 an excess of ^-aminobenzoic acid (see also Spink et al. 1944, Housewright and Koser 1944). 

 Sulphonamide-resistant strains of Bad. coli, Sh. shigce and V. choleroe, on the other hand, 

 showed no associated increase in content of j9-aminobenzoic acid. 



Stokinger and his colleagues (1942) found that sulphonamide-resistant gonococci 

 contained more ^-aminobenzoic acid than susceptible strains, but since azochloramide, 

 which inhibits ^-aminobenzoic acid, had no effect on resistance, they concluded that 

 increased resistance was not a function of increased synthesis of the acid. 



Analogous Systems with Other Essential Metabolites. 



The most impressive evidence for the Woods-Fildes hypothesis, apart from the 

 observations upon which it was originally founded, lies in the success of its exten- 

 sion to other systems. Fildes (1941) applied it indirectly to the metabolism of a 

 strain of Salm. typhi which needed tryptophan as an essential nutrient. Indole 

 could replace the tryptophan in basal media, and the essential tryptophan was 

 synthesized from it. It was found that indoleacrylic acid inhibited growth of the 

 organism in indole-containing, but not in tryptophan-containing media. Trypto- 

 phan antagonized the inhibition, but there was no constant ratio between antagoniz- 

 ing and inhibiting concentrations of the two substances. Fildes supposed that 

 the indoleacrylic acid competed for the services of an enzyme with a product 

 intermediate between indole and the trjrptophan. Indoleacrylic acid had the 

 same action on the strain after it had been trained to grow in a medium containing 

 ammonia as the sole source of nitrogen, suggesting that the hypothetical inter- 

 mediate substance was still an essential metabolite whose activity could be specific- 

 ally inhibited. 



More direct tests of the hypothesis were made by Mcllwain (1940, 1941a, 

 1942a, h). 



Para-aminobenzoic acid is of the form R-COOH, and sulphanilamide, of the form 

 R-SOgNHa, is an analogue. Mcllwam prepared analogues of various bacterial nutrients, 

 either of the form R-SOgNHa or R-SOsH, and showed that they inhibited the growth of 



bacteria for which the nutrient was essential. Thus nicotinic acid Jf \ 



COOH is an 

 essential nutrient for Skiph. aureus and Proteus vulgaris, and pyridine sulphonamide 



<iy . .^ 



SO2NH2 is an inhibitor of their growth in a basal medium containing a nicotinic 

 acid. The same relationship held between aminocarboxyhc acids like glycine, alanine 

 and valine, which were known to be concerned in the growth of the test organisms, often 

 as essential nutrients, and their sulphonic acid analogues. Again, in confirmation of 

 the work of SneU (1941), pantothenic acid (see p. 67) 



HOCH2C(CH3)2CH(OH)CONHCH2CH2COOH 



