In vitro susceptibility of 104 clinical isolates of Haemophilus influenzae to moxalactam (LY127935), ampicillin, chloramphenicol, and ticarcillin

A total of 104 strains of Haemophilus influenzae isolated from pediatric patients over a 1-year period were tested for susceptibility to moxalactam (LY127935), ampicillin, chloramphenicol, and ticarcillin. Of these strains, 30 produced β-lactamase. LY127935 inhibited 99% of the strains at a concentration of 0.125 μg/ml; the remaining strain was inhibited by this antibiotic at 0.25 μg/ml. β-Lactamase-producing strains were inhibited by ampicillin at ≥ 2 μg/ml. β-Lactamase-negative strains were all inhibited by ampicillin at ≤ 1 μg/ml, except for one nontypable strain which required 2 μg of ampicillin per ml for inhibition. All strains were susceptible to chloramphenicol at ≤ 4 μg/ml. β-Lactamase-producing strains were less susceptible (geometric mean = 4.702 μg/ml) to ticarcillin than were strains which did not produce β-lactamase (geometric mean = 0.331 μg/ml). LY127935 susceptibility was not influenced by increasing inoculum size, as was ampicillin susceptibility. Combinations of LY127935 and chloramphenicol or ampicillin were not antagonistic in vitro.