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

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. beta-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 (n = 6588 H. in and moxifloxacin; all other isolates had MICs of less than or equal to 0.5 mg/l and less than or equal to 0.25 mg/l, respectively. beta-Lactamase production was found to confer ampicillin resistance in nearly all isolates. For M. catarrhalis, beta-lactamase-negative isolates had MICs less than or equal to 0.12-0.25 mg/l for ampicillin and less than or equal to0.03-0.12 mg/l for ceftriaxone. In contrast, beta-lactamase production resulted in MICs of less than or equal to 0.12- > 16 mg/l for ampicillin and less than or equal to0.03-4 mg/l for ceftriaxone. All M. catarrhalis had MICs less than or equal to 0.12 mg/l for moxifloxacin and less than or equal to1 mg/l for levofloxacin. In summary, antimicrobial susceptibilities and the prevalence of beta-lactamase production in H. influenzae and M. catarrhalis in the United States has remained essentially unchanged from 1997-1998 to 1999. (C) 2002 Elsevier Science B.V. and the International Society of Chemotherapy. All rights reserved.