Sensitivity of Pseudomonas Aeruginosa To Sulphonamides and Trimethoprim and the Activity of the Combination Trimethoprim: Sulphamethoxazole

Abstract

The activities of 3 sulfonamides and trimethoprim against strains of P. aeruginosa were studied. Sulfadiazine had the most activity, sulfadimidine had little and the activity of sulfamethoxazole was intermediate. According to their sensitivity to sulfamethoxazole, strains were divided into 2 groups: highly resistant (16%, MIC [minimum inhibitory concentration] > 1000 .mu.g/ml) and moderately resistant (84%, MIC .ltoreq. 1000 .mu.g/ml). The former were resistant on disk testing to sulfatriad 300 .mu.g. Sulfamethoxazole and trimethoprim did not act in synergy against them. The moderately resistant strains were sensitive to sulfatriad; trimethoprim and sulfamethoxazole showed marked synergy against them in agar-plate dilution tests. The concentrations of trimethoprim and sulfamethoxazole necessary for synergy lay for each drug within the range of concentrations at which they were found in urine, and the ratio of their MIC when acting in synergy was similar to the ratio of their concentrations in urine. A disk containing trimethoprim and sulfamethoxazole in a ratio of 1:2 rather than 1:20 would probably be more appropriate when testing strains from urine for their sensitivity to co-trimoxazole.