Failure to demonstrate an advantage in combining sulphamethoxazole with trimethoprim in an experimental model of urinary infection.

Co-trimoxazole was found to have a predominantly bacteriostatic effect upon 28 urinary isolates of Enterobacteriaceae in nutrient broth and was never bactericidal in artificially infected urine. The components of co-trimoxazole were tested individually and trimethoprim was found to be at least as effective as co-trimoxazole in nutrient broth and in urine. Trimethoprim alone produced some bactericidal effect in urine but this was antagonized by sulphamethoxazole. Laboratory tests for evaluating these drugs may give a misleading impression of their activity in vivo. Further clinical comparisons should therefore be made between trimethoprim and cotrimoxazole to determine when trimethoprim should be used in preference to the combination.