Activity of trimethoprim-sulfamethoxazole against Chlamydia trachomatis in vitro.

With use of a technique for cell culture, the minimal inhibitory concentration (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 after inoculation of the cells with the pathogen. Cultures were examined after incubation for 48 hr; the MIC was defined as the highest concentration of drugs at which no inclusions were seen. Definite end points were observed only with inocula of less than or equal to 10(3) inclusion-forming units (IFU)/ml. With larger inocula, inclusions were present for all dilutions tested. The MICs for TMP alone were greater than or equal to 64 micrograms/ml for all strains. The MICs for SMZ alone ranged from 2 to 128 micrograms/ml. There appeared to be no better than an additive effect for all combinations of TMP and SMZ tested, although the effect of 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.