The activities of three sulphonamides and trimethoprim against strains of have been studied. Sulphadiazine had most activity, sulphadimidine had little, and the activity of sulphamethoxazole was intermediate. According to their sensitivity to sulphamethoxazole, strains were divided into two groups: “highly resistant” (16%, MIC>1000 μg per ml) and “moderately resistant” (84%, MIC≤ 1000 μg per ml). The former were resistant on disk testing to Sulphatriad 300 μg. Sulphamethoxazole and trimethoprim did not act in synergy against them.

The moderately resistant strains were sensitive to Sulphatriad; trimethoprim and sulphamethoxazole showed marked synergy against them in agar-plate dilution tests. The concentrations of trimethoprim and sulphamethoxazole necessary for synergy lay for each drug within the range of concentrations at which they have been found in urine, and the ratio of their MICs when acting in synergy was similar to the ratio of their concentrations in urine.

It is suggested that a disk containing trimethoprim and sulphamethoxazole in a ratio of 1 : 2 rather than 1 : 20 would be more appropriate when testing strains from urine for their sensitivity to co-trimoxazole.


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