False resistance of salmonellae to trimethoprim-sulphamethoxazole in disc diffusion method.

Apparent resistance to trimethoprim-sulphamethoxazole (TMP-SMZ) in disc diffusion method was found in 14 strains of Salmonella typhi and 2 strains of S. paratyphi A grown on Mueller Hinton agar (MHA) of one manufacturer. On MHA from another manufacturer and on diagnostic sensitivity test agar (DSTA), these 16 strains were sensitive to TMP-SMZ. The minimum inhibitory concentration of trimethoprim (TMP) assayed by agar dilution method, on DSTA medium for these 16 strains was in the range of 0.01 to 0.04 micrograms/ml. Thus, the apparent resistance was a spurious result due to the unsatisfactory quality of the batch of MHA of the first manufacturer. Presumably the medium contained thymine or its derivatives which act as antagonists to TMP and sulphonamide drugs.