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

Abstract

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 beta-lactamase. LY127935 inhibited 99% of the strains at a concentration of 0.125 microgram/ml; the remaining strain was inhibited by this antibiotic at 0.25 microgram/ml. beta-Lactamase-producing strains were inhibited by ampicillin at greater than or equal to 2 microgram/ml. beta-Lactamase-negative strains were all inhibited by ampicillin at less than or equal to 1 microgram/ml, except for one nontypable strain which required 2 microgram of ampicillin per ml for inhibition. All strains were susceptible to chloramphenicol at less than or equal to 4 microgram/ml. beta-Lactamase-producing strains were less susceptible (geometric mean = 4.702 microgram/ml) to ticarcillin than were strains which did not produce beta-lactamase (geometric mean = 0.331 microgram/ml). LY127935 susceptibility was not influenced by increasing inoculum size, as ws ampicillin susceptibility. Combinations of LY127935 and chloramphenicol or ampicillin were not antagonistic in vitro.