Quantitative inhibition of Haemophilus influenzae by trimethoprim/sulfamethoxazole.

We evaluated the inhibitory effect of trimethoprim (TMP) and sulfamethoxazole (SMZ), alone and in combination, against 34 strains of Haemophilus influenzae. Growth inhibition was determined after incubation for 18 h by comparing viable counts of cultures in drug-containing medium with corresponding counts of control cultures in drug-free medium. In a modified, thymidine-deficient Levinthal broth, the numbers of colony-forming units of all the isolates tested were reduced 100-fold or more by TMP/SMZ (1.25/25 mug/ml) as compared with growth without drug. Inhibition was significantly greater with TMP/SMZ than with either TMP or SMZ alone. Ampicillin-susceptible and ampicillin-resistant strains were equally susceptible to TMP/SMZ. Growth of nontypable strains was inhibited more than growth of type b organisms.