A reappraisal of the antibacterial action of cotrimoxazole in vitro.

The in-vitro action of the sulphamethoxazole/trimethoprim combination (cotrimoxazole) against coliforms, Proteus spp, and Staphylococcus aureus was re-examined. In nutrient broth the components of cotrimoxazole did not produce bacteriostatic synergy against most sulphonamide-resistant and a significant proportion of sulphonamide-sensitive strains. Any bactericidal action appeared to be due to the trimethoprim component. After addition to urine the combination was only bacteriostatic against most coliforms and Proteus spp over six hours. Further clinical trials are needed to determine the circumstances under which cotrimoxazole is superior to either trimethoprim or sulphonamides against some infections, particularly of the urinary tract.