Activity of Trimethoprim-Sulfamethoxazole Against Chlamydia trachomatis in Vitro

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Abstract
With use of a technique for cell culture, the minimal inhibitory concentrations (MICs) of trimethoprim (TMP) and sulfamethoxazole (SMZ), alone and in combination at ratios of 1:1, 1:5, and 1:20, were determined against five strains of Chlamydia trachomatis. The drugs were added to idoxuridine-treated McCoy cells aftger inoculation of the cells with the pathogen. Cultures were examined after incubation for 48 hr; the MIC was defined as the highest concentration of drug at which no inclusions were seen. Definite end points were observed only with inocula of ⩽103 inclusion-forming units (IFU)/ml. With larger inocula, inclusions were present for all dilutions tested. The MICs for TMP alone were ⩾64 μg/ml for all strains. The MICs for SMZ alone ranged from 2 to 128 μg/ml. There appeared to be no better than an additive effect for all combinations of TMP and SMZ tested, although the effect at 1:20 was greater than that at 1:1 or 1:5. With increasing concentrations of both drugs, the inclusions became progressively fewer, smaller, and pyknotic. Even though definite end points could not be determined, there was a significant reduction in the number of IFU/ml at relatively low concentrations of both TMP and SMZ. The results of this study suggest that the activity of TMP-SMZ against C. trachomatis is primarily that of SMZ alone.

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