Development of trimethoprim-resistance in Salmonella typhi during therapy.

A typhoid patient was infected with a fully-sensitive, plasmidless strain of Salmonella typhi which acquired resistance to kanamycin, sulfamethoxazole and trimethoprim during cotrimoxazole therapy. The resistant post-treatment strain harboured 4 plasmids of 62, 4.1, 3.8 and 3.0 Md in size. Kanamycin-, sulfamethoxazole- and trimethoprim-resistance were borne on a transferable 62 Md plasmid which was compatible with groups FI, FIme, FII, FIV, H1, H2, B, I1, I2, J, K, M, N, T, X, W and P. Sulfamethoxazole-resistance was also borne on the 4.1 Md plasmid. The 3.8 Md plasmid was not transferable; the 3.0 Md plasmid was transferable but did not confer antibiotic resistance. Excluding the strain described here, only 5 out of 35 other S typhi isolates contained plasmids. This is the first report of trimethoprim-resistant S typhi in Hong Kong.