The in vitro Demonstration of the Efficacy of Trimethoprim as an Antibacterial Agent in a Comparative Bacteriological Study on the Effects of Trimethoprim, Sulfamethoxazole and the Combination Trimethoprim/Sulfamethoxazole

The minimal inhibitory concentrations of trimethoprim (TMP), sulfamethoxazole (SMZ) and the combination trimethoprim/sulfamethoxazole (TMP/SMZ) in a ratio of 1:20 have been determined for 97 bacterial strains (66 Escherichia coli, 13 Proteus mirabilis, 22 Enterococcus, 12 Staphylococcus, 4 Klebsiella and Enterobacter and 4 others) using the agar-dilution technique. The same strains and an additional 31 strains were tested for sensitivity to TMP/SMZ and SMZ by the agar-diffusion test. 48 of them were also examined for TMP sensitivity by this method. All strains had been isolated from urine of patients suffering from acute urinary tract infections. 43% of these strains were sensitive to SMZ, 90% were sensitive to TMP and 95% to TMP/SMZ. The potentiation of TMP by combination with SMZ was more than 5-fold in 97% of the sulfonamide-sensitive strains and in 54.8% of the sulfonamide-resistant strains. The in vitro results seem to indicate that TMP is almost as useful a bacteriostatic chemotherapeutic as its combination with SMZ. The enhancement of its activity by sulfonamides in man cannot yet be evaluated, because only insufficient experience exists in the treatment with TMP alone. TMP and TMP/SMZ were also tested for bactericidal activities on 5 strains. Against 3 of these strains TMP as well as TMP/SMZ showed only bacteriostatic activities. With the other 2 strains slight bactericidal activities of TMP and TMP/SMZ could be demonstrated within 18 h of incubation. There was no major difference between TMP and TMP/SMZ as far as bactericidal activities were concerned. For sensitivity testing the agar-diffusion technique using two discs that contain the single components or one disc containing 1.25 μg TMP is recommended, since it could be shown that the potentiation of TMP by SMZ cannot sufficiently be demonstrated by the agar-diffusion technique using one disc containing both substances.