Antimicrobial resistance among clinical isolates of haemophilus influenzae in the United States

Study objectives: Increasing rates of β-lactamase production and antimicrobial resistance by respiratory tract pathogens continue to be a concern in the treatment of a variety of infections commonly cared for in the emergency department setting. We determine the current antimicrobial susceptibility patterns and β-lactamase production rates of Haemophilus influenzae and compare them with our results from the previous 6 years. Methods: A total of 5,445 isolates were collected from patients with H influenzae respiratory tract infections between 1994 and 2002. The most recent study was completed using specimens collected during the winter of 2001 to 2002 and included 1,025 strains. β-Lactamase production was assessed with the nitrocefin disc assay; broth microdilution antimicrobial susceptibility testing with 16 antimicrobial agents was performed according to National Committee for Clinical Laboratory Standards guidelines. Results: The rate of β-lactamase production was 36.4% in 1994 and decreased in each of the subsequent study periods (1997, 31%; 1999, 30.8%; and 28.9% in 2001 to 2002). Susceptibility rates of greater than 96% were found with amoxicillin/clavularic acid, cefuroxime, cefpodoxime, cefixime, ciprofloxacin, azithromycin, chloramphenicol, tetracycline, and telithromycin. Susceptibility rates to ampicillin (67.5%), cefprozil (88.1%), and cefaclor (87.3%) were unchanged during the study period. Only trimethoprim/sulfamethoxazole (74.5% versus 100% in 1994) and clarithromycin (60.5% versus 71.2 in 1994) revealed a decreasing rate of susceptibility between 1994 and 2002. Conclusion: Since 1994 to 1995, β-lactamase production has decreased and trimethoprim/sulfamethoxazole and clarithromycin resistance has increased in clinical isolates of H influenzae. The reasons for these trends were not determined but may be a result of a change in clinician prescribing practices away from β-lactam drugs.