Combined action of sulphamethoxazole and trimethoprim against clinically-isolated sulphonamide-resistant bacteria.

42 strains of gram-negative bacteria, isolated from clinical material, resistant to trimethoprim (TM), sulphamethoxazole (SMZ) or both, have been tested for synergistic sensitivity to TM + SMZ. The combination showed no synergy against strains 'highly resistant' to SMZ (MIC greater than 1 mg/ml). Indifference was invariably observed. However, synergy was seen with strains 'moderately resistant' to SMZ (MIC 100--1,000 microgram/ml). Our results indicate that present techniques for the testing of urinary tract isolates for sensitivity to co-trimoxazole need revision.