Analysis of In Vitro Antibacterial Activities of the Combination of Trimethoprim and Sulfamethoxazole on Clinical Isolates in Japan

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Abstract
Antimicrobial activities of trimethoprim (TMP) and sulfamethoxazole (SMZ) in vitro were critically studied in a total of 44 laboratory strains and 449 clinical isolates in Japan. The potentiation of the antimicrobial activity of SMZ by TMP was semiquantitatively assessed, by use of the fractional-inhibitory-concentration index as a parameter. Significant potentiation was found, irrespective of the sensitivities of the organisms to SMZ alone, in practically all laboratory strains tested except Pseudomonas. The distribution of the MIC of the combination of TMP and SMZ, which were combined in a wide range of ratios for the checkerboard method of titration, demonstrated significant potentiation in the majority of 449 fresh clinical isolates. More than 50% of isolates originally resistant to SMZ alone gave fractional-inhibitory-concentration indices indicative of moderate potentiation. The degree of this potentiation appeared to depend to some extent on the primary sensitivities of the respective organisms to SMZ alone.

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