In-vitro susceptibility of Haemophilus influenzae to meropenem compared with imipenem, five other beta-lactams, chloramphenicol and ciprofloxacin.

The in-vitro activity of the new carbapenem, meropenem, was compared with that of imipenem, five other beta-lactams, chloramphenicol and ciprofloxacin against 223 isolates of Haemophilus influenzae. This number included 115 ampicillin-susceptible, and 56 beta-lactamase positive isolates and 52 beta-lactamase negative isolates with reduced susceptibility to ampicillin. The activities of chloramphenicol and ciprofloxacin did not vary between groups of isolates delineated by their susceptibility to ampicillin. In contrast with ampicillin and piperacillin, the two carbapenems and three cephalosporins tested showed no reduction in activity against beta-lactamase producing isolates. beta-Lactam concentrations required for inhibition of 50% and 90% of beta-lactamase negative isolates with reduced susceptibility to ampicillin (MIC greater than or equal to 1 mg/l) were increased in comparison with similar concentrations required for the ampicillin-susceptible group. The differences in meropenem and imipenem activities between these two groups were much smaller than those observed for each of the other beta-lactams. MICs of meropenem were consistently the lower of the two carbapenems for all isolates. In addition, there were no notable increases in the concentrations of meropenem required for inhibition of the isolates least susceptible to imipenem (MIC greater than or equal to 4 mg/l), suggesting that differences in penetration and/or target-binding properties exist not only between the carbapenems and other beta-lactams, but also between meropenem and imipenem.