Antimicrobial Susceptibility Breakpoints and First-Step parC Mutations in Streptococcus pneumoniae: Redefining Fluoroquinolone Resistance

摘要

Clinical antimicrobial susceptibility breakpoints areused to predict the clinical outcome of antimicrobial treat-ment. In contrast, microbiologic breakpoints are used toidentify isolates that may be categorized as susceptiblewhen applying clinical breakpoints but harbor resistancemechanisms that result in their reduced susceptibility to theagent being tested. Currently, the National Committee forClinical Laboratory Standards (NCCLS) guidelines utilizeclinical breakpoints to characterize the activity of the fluoro-quinolones against Streptococcus pneumoniae. To deter-mine whether levofloxacin breakpoints can identify isolatesthat harbor recognized resistance mechanisms, we exam-ined 115 S. pneumoniae isolates with a levofloxacin MIC of>2 μg/mL for first-step parC mutations. A total of 48 (59%)of 82 isolates with a levofloxacin MIC of 2 μg/mL, a levelconsidered susceptible by NCCLS criteria, had a first-stepmutation in parC. Whether surveillance programs that uselevofloxacin data can effectively detect emerging resist-ance and whether fluoroquinolones can effectively treatinfections caused by such isolates should be evaluated