Antimicrobial susceptibility of Haemophilus influenzae

Selected clinical isolates of Haemophilus influenzae were tested for their susceptibility to seven antibiotics by a microtiter broth dilution and an agar diffusion method. Eleven of 40 strains tested were resistant to ampicillin, the drug of choice, by both methods. All the strains tested were susceptible to chloramphenicol, and all but one were susceptible to tetracycline. Of the other four antibiotics tested, the ranges of minimal inhibitory concentrations (MICs) were: 0.5 to 4 mug of gentamicin per ml, 0.5 to 4 mug of erythromycin per ml, and 2 to 16 mug of clindamycin per ml. MICs of ampicillin for both the susceptible and resistant strains were markedly affected by inoculum concentration. The ampicillin MICs of the resistant strains were also affected by the time of incubation.