Co-trimoxazole sensitivity testing: comparison of separate and combined disk agar diffusion techniques

Five hundred and seven strains of bacteria isolated from the urine of patients with significant bacteriuria (more than 10(8) colony-forming units per litre) were tested for sensitivity to co-trimoxazole by the agar diffusion technique. Each organism was tested with a combined disk containing trimethoprim and sulphamethoxazole in a primary sensitivity test and, at a standardised inoculum, with both a combined disk and separate disks of trimethoprim and sulphamethoxazole. The results show that combined disk testing does not always indicate the sensitivity patterns of the organisms being tested.