Susceptibility Testing to Trimethoprim Alone and Combined with Sulphonamides

In routine sensitivity testing by the disc diffusion method, one member for each group of related antibiotics is chosen to represent the group. Each new antibacterial drug is evaluated with respect to its previous antibiotic class disc. In this study, trimethoprim combined with sulphadiazine in the ratio 1:4 was compared when combined with sulpha-methoxazole in the ratio 1:20. Trimethoprim inhibited 86% of the bacterial strains tested at ≤4μg/ml and the sulphonamides 50% of the strains at ≤64 μg/ml. Regression line analysis showed that in sensitivity testing to trimethoprim alone discs with 5 μgcan be used. Trimethoprim combined with sulphamethoxazole or sulphadiazine inhibited 84% respectively 91% of the strains at 0.5–8 μg/ml. Trimethoprim combined with either sulphon-amide produced superimposable regression lines. Therefore, sensitivity testing to trimethoprim/sulphadiazine can be performed by using discs with trimethoprim/sulphamethoxazole and the inhibition zone ≥ 15 mm denotes susceptibility in the treatment of lower urinary tract infections.