In vitro susceptibility of Haemophilus influenzae to trimethoprim-sulfamethoxazole.

Thirty-four strains of Haemophilus influenzae (20 ampicillin sensitive and 14 ampicillin resistant), mostly type b isolates from cerebrospinal fluid or blood, were tested for susceptibility to a combination of trimethoprim-sulfamethoxazole (TMP-SMZ) in a ratio of 1 part TMP to 19 parts SMZ. All strains were very susceptible to the TMP-SMZ combination, with minimal inhibitory concentrations ranging from 0.007-0.14 to 0.06-1.18 mug of TMP-SMZ per ml. There was little difference in the susceptibility of the ampicillin-sensitive and ampicillin-resistant strains to TMP-SMZ.