160 ANTIBACTERIAL SUBSTANCES FOR TREATMENT OF INFECTIONS 



The bacteriostatic action varies directly as the concentration of the drug, 

 and inversely as the concentration of the bacteria. This latter relation cannot 

 be simply explained in terms of the amount of sulphonamide available per bacterium, 

 for in any given test only a small fraction of the drug is taken up by the bacteria 

 (Mcintosh and Whitby 1939, Kohn and Harris 1941, Rose and Fox 1942, Pike 

 and Acton 1942). With organisms that grow optimally at 37° C. a rise in tem- 

 perature above this level greatly increases the sulphonamide activity. White 

 (1939) found that only 1/lOOth of the amount active at 37° C. was required at 

 39° C. The degree of bacteriostatic action is greatly influenced by the medium 

 used for the test. In general, the richer the medium, the less effective the drug. 

 From the point of view of precision work in bacterial metabohsm the variability 

 of common ingredients of routine media, like peptone, tissue extracts and blood 

 fluids, is enormous. Nevertheless, in carefully prepared routine media, comparative 

 tests of sulphonamide inhibition are valid, though absolute measures are unreliable, 

 owing to the variable amounts of substances that antagonize the action of the 

 drug (see, for example, MacLeod 1940, Lewis and Snyder 1940, MacLeod and 

 Mirick 1942). Antagonizers are present in peptone (Lockwood 1938, Weld and 

 Mitchell 1939), in serum, and in extracts and hydrolysates of animal tissue (Landy 

 and Dicken 1942, Lewis 1942). Reliable results are possible only in defined media 

 where the antagonistic effect of each ingredient may be determined with certainty. 

 It is generally believed that in low concentration the sulphonamide drugs exert 

 only a bacteriostatic effect, but the careful observations of Wolff and Julius (1939) 

 suggest that they are lethal to actively dividing organisms, and that at other 

 stages of the culture cycle bacterial growth is merely inhibited. The analogy 

 to penicillin is very striking (see p. 183). 

 The In Vitro Resistance of Bacteria to Sulphonamides. 



The resistance of bacteria to a sulphonamide is measured in terms of the 

 minimum concentration of the drug that prevents the growth in a defined medium 

 of a standard inoculum of the strain under test. Without careful standardization 

 of the medium and conditions of test (see, for example, Strauss et al. 1941) these 

 measures serve only as rough indications. Comparative tests, however, show that, 

 although bacteria as a whole are generally susceptible to the action of one or 

 other of the sulphonamides, there are wide variations in susceptibihty, both from 

 one species to another, and from strain to strain within a species. Thus, although 

 the species Str. pyogenes is relatively susceptible to sulphanilamide, and Staph, 

 aureus relatively resistant, yet in an examination of a large number of strains 

 of both species, there would be a wide overlap of susceptible strains of Staph, 

 aureus and resistant strains of Str. pyogenes. Strain- variations in natural resistance 

 have been noted by a number of observers (see, for example. Green 1940, Green 

 and Bielschowsky 1942, Poston and Orgain 1942, Felsenfeld 1943) and must be 

 taken into account in the therapy of infections by sulphonamides, since clearly 

 the assumption of a general level of susceptibihty for a given species may have 

 serious consequences if applied consistently to the treatment of all infections by 

 that species. 



Of even greater importance is the demonstration that resistance to sulphon- 

 amides can be acquired in the test-tube. Bacteria may be trained by serial sub- 

 culture in media containing non-bacteriostatic concentrations of the sulphonamide 

 (Green 1940, MacLeod 1940, Strauss et al. 1941, Schmidt et al. 1942, Kirby and 

 Rantz 1943, Kirby 1943, Lankford et al. 1943, Harris and Kohn 1943, Mcintosh 



