162 ANTIBACTERIAL SUBSTANCES FOR TREATMENT OF INFECTIONS 



an aromatic amino compound led Woods (1940) to test the antagonizing action 

 of jo-aminobenzoic acid. It proved to be a powerful antagonize! ; in a defined 

 basal medium containing streptococci, one molecule of the acid antagonized the 

 bacteriostatic action of 26,000 molecules of sulphanilamide. The probable presence 

 of jo-aminobenzoic acid in yeast was later established by Rubbo and Gillespie 

 (1940), Blanchard (1941) and Mcllwain (19426). (See also Ratner et al. 1944.) 



Subsequent observers confirmed Woods' findings (see Green and Bielschowsky 

 1942, Rubbo and Gillespie 1942, Wood 1942) and showed that the ratio of molar 

 bacteriostatic concentrations of sulphanilamide to the antagonizing concentration 

 of jo-aminobenzoic acid varied between 26,000 : 1 and 1000 : 1. The acid antag- 

 onizes all the sulphonamides, and Wood (1942) observed that the greater the 

 bacteriostatic activity of the sulphonamide, the smaller is the ratio, i.e. the greater 

 its abihty, molecule per molecule, to counteract the antagonizing effect of jo-amino- 

 benzoic acid. 



The Woods-Fildes Hypothesis. 



The structural similarity of sulphanilamide and _p-aminobenzoic acid led Woods 

 (1940) and Fildes (1940a) to propound an hypothesis of sulphonamide action that 

 has proved exceedingly fruitful in the investigation of antibacterial substances. 

 The Woods-Fildes hypothesis made precise the previously held notion that sulphon- 

 amides in some way interfered with an essential metabolic function of the bacterium. 

 Woods and Fildes contend that ^-aminobenzoic acid is an essential metabolite 

 for susceptible bacteria, whose growth is inhibited when sulphonamides, by reason 

 of their structural similarity to the natural metabolite, block the enzyme system 

 concerned with this particular part of the essential metabolism of the organism. 

 Inhibitor and antagonist compete for the enzyme system, and bacteriostasis 

 results when the inhibitor is successful in the competition. 



The degree of competition in a given organism may be expressed by the ratio 

 Cj/Cji, Ci being the molar concentration of inhibitor that is just bacteriostatic 

 in the presence of a molar concentration C^ of the corresponding metabolite 

 (Mcllwain 19426). Thus the ratio, for which Mcllwain proposed the name " anti- 

 bacterial index," was found by Woods to be 26,000 for Str. pyogenes, sulphanilamide 

 and p-aminobenzoic acid. Among other things, it can be used to predict the 

 chemotherapeutic efficiency of a drug, if the concentration of the metabolite 

 {i.e. the antagonist) in the tissues of the animal is known (see below, p. 164). 



The facts of ^-aminobenzoic acid antagonism to the sulphonamides have been 

 amply confirmed both in vitro and in the living animal. The Woods-Fildes hypo- 

 thesis of its action is widely accepted. However, it is not consistent with all 

 the phenomena of sulphonamide action, and for this reason we shall examine both 

 the corroborative evidence and the inconsistencies in some detail. 



para-aminobenzoic Acid as Essential Metabolite. 



In two species of bacteria there is no doubt that ^-aminobenzoic acid is an essential 

 metabolite, for it is demonstrably an essential nutrient. Both CI. acetobutylicum (Rubbo 

 et al. 1941, Rubbo and Gillespie 1942, Lampen and Peterson 1941) and Acetobader sub- 

 oxydans (Lampen et al. 1942) require the acid for growth, and both are susceptible to 

 an inhibiting action of sulphonamides that is reversed by excess of the acid (see also 

 Kuhn and Schwartz 1941, WiedUng 1941, IsbeU 1942, Landy and Dicken 1942). In 

 one other species, a soil bacillus isolated by Mirick (1943), an enzyme system capable 

 of oxidizing ^-aminobenzoic acid has been demonstrated ; the growth of the organism 

 was inhibited by sulphapyridine. 



