Susceptibility testing to trimethoprim alone and combined with sulphonamides

In routine sensitivity testing by the disc diffusion methods, 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 sulphamethoxazole in the ratio 1:20. Trimethoprim inhibited 86% of the bacterial strains tested at smaller than or equal to microgram/ml and the sulphonamides 50% of the strains at smaller than or equal to 64 microgram/ml. Regression line analysis showed that in sensitivity testing to trimethoprim alone discs with 5 micrograms can be used. Trimethoprim combined with sulphamethoxazole or sulphadiazine inhibited 84% respectively 91% of the strains at 0.5-8 microgram/ml. Trimethoprim combined with either sulphonamide produced superimposable regression lines. Therefore, sensitivity testing to trimethoprim/sulphadiazine can be performed by using discs with trimethoprim/sulphamethoxazole and the inhibition zone greater than or equal to 15 mm denotes susceptibility in the treatment of lower urinary tract infections.