Trimethoprim/sulphamethoxazole resistance in Escherichia coli and Klebsiella spp. urinary isolates.

The susceptibility of 40 Escherichia coli and 35 Klebsiella spp. urinary isolates to trimethoprim/sulphamethoxazole (cotrimoxazole) was determined. The determination was based on the activity of the standard multodiscs-cotrimoxazole (25 micrograms), (Oxoid); sulphafurazole (500 micrograms), (Mastring-S); and the minimum inhibitory concentration of trimethoprim and sulphamethoxazole singly against the isolates. Thirty-two (80%) of 40 isolates of Escherichia coli, and 26 (74%) of 35 isolates of Klebsiella spp. were resistant to cotrimoxazole. All the isolates were resistant to more than 500 mg/l sulphamethoxazole. Twenty of the 32 (62%) cotrimoxazole-resistant E. coli strains and 18 of the 26 (69%) cotrimoxazole-resistant Klebsiella strains were resistant to more than 2000 mg/l trimethoprim. These high level trimethoprim-resistant strains invariably carried transferable resistance to at least the sulphonamides, ampicillin and tetracycline, which could be transferred en bloc to known sensitive recipients by the process of conjugation. The high incidence of the R-plasmid-mediated resistance to high levels of trimethoprim suggests the presence of a selective pressure from the increased, and probably the uncontrolled, use of trimethoprim/sulphonamide proprietary formulations in the society.