Apparent plateau in β-lactamase production among clinical isolates of Haemophilus influenzae and Moraxella catarrhalis in the United States: results from the LIBRA Surveillance initiative

Abstract

Haemophilus influenzae ( n=2791) and Moraxella catarrhalis ( n=1249) isolated from patient specimens during 1999 were collected from 290 laboratories participating in a moxifloxacin surveillance study as part of the LIBRA Surveillance intitiative. Isolates were tested for in vitro susceptibility to a panel of agents suitable for the treatment of respiratory tract infections. β-Lactamase production was identified in 32.2% of H. influenzae and 94.2% of M. catarrhalis. These percentages differed by less than 1.5% from results of a study conducted in 1997–1998 and were similar to results from other recent US surveillance studies. Resistance among H. influenzae to trimethoprim-sulphamethoxazole increased considerably, from 2% in the 1997–1998 study ( n=6588 H. influenzae) to 15.5% in the current study. One isolate of H. influenzae had an MIC of 8 mg/l to both levofloxacin and moxifloxacin; all other isolates had MICs of ≤0.5 mg/l and ≤0.25 mg/l, respectively. β-Lactamase production was found to confer ampicillin resistance in nearly all isolates. For M. catarrhalis, β-lactamase-negative isolates had MICs ≤0.12–0.25 mg/l for ampicillin and ≤0.03–0.12 mg/l for ceftriaxone. In contrast, β-lactamase production resulted in MICs of ≤0.12–>16 mg/l for ampicillin and ≤0.03–4 mg/l for ceftriaxone. All M. catarrhalis had MICs ≤0.12 mg/l for moxifloxacin and ≤1 mg/l for levofloxacin. In summary, antimicrobial susceptibilities and the prevalence of β-lactamase production in H. influenzae and M. catarrhalis in the United States has remained essentially unchanged from 1997–1998 to 1999.