Antimicrobial Activities of Piperacillin-Tazobactam against Haemophilus influenzae Isolates, Including β-Lactamase-Negative Ampicillin-Resistant and β-Lactamase-Positive Amoxicillin-Clavulanate-Resistant Isolates, and Mutations in Their Quinolone Resistance-Determining Regions

beta-Lactamase-negative ampicillin-resistant (BLNAR) isolates of Haemophilus influenzae have been emerging in some countries, including Japan. The Clinical and Laboratory Standards Institute has only a susceptible MIC breakpoint (<= 1 mu g/ml) for piperacillin-tazobactam and a disclaimer comment that BLNAR H. influenzae should be considered resistant, which was adapted without presentation of data. In addition, fluoroquinolone-resistant H. influenzae isolates have recently been occasionally reported worldwide. To address these problems, we examined susceptibilities to beta-lactams, including piperacillin-tazobactam, and ciprofloxacin by microdilution and disk diffusion ( only for piperacillin-tazobactam) methods, against a total of 400 recent H. influenzae clinical isolates, including 100 beta-lactamase-negative ampicillin-susceptible, beta-lactamase-positive ampicillin-resistant, BLNAR, and beta-lactamase-positive amoxicillin-clavulanate-resistant (BLPACR) isolates each. BLNAR and BLPACR isolates were tested by PCR using primers that amplify specific regions of the ftsI gene. We also detected mutations in quinolone resistance-determining regions (QRDRs) by direct sequencing of the PCR products of DNA fragments. Among beta-lactams, piperacillin-tazobactam exhibited potent activity against all isolates of H. influenzae, with all MICs at <= 0.5 mu g/ml ( susceptible). A disk diffusion breakpoint for piperacillin-tazobactam of >= 21 mm is proposed. We confirmed that all BLNAR and BLPACR isolates had amino acid substitutions in the ftsI gene and that the major pattern was group III-like (87.5%). One ciprofloxacin-resistant isolate ( MIC, 16 mu g/ml) and 31 ciprofloxacin-susceptible isolates ( MICs, 0.06 to 0.5 mu g/ml) had amino acid changes in their QRDRs. Piperacillin-tazobactam was the most potent beta-lactam tested against all classes of H. influenzae isolates. It is possible that fluoroquinolone-resistant H. influenzae will emerge since several clinical isolates carried mutations in their QRDRs.