An intrinsic pattern of reduced susceptibility to fluoroquinolones in pediatric isolates of Streptococcus pyogenes

A total of 116 clinical isolates collected in 2003 from a tertiary pediatric hospital and a primary pediatric department in Chicago, IL, were screened for reduced susceptibility to selected fluoroquinolones by disc diffusion. Correlation between reduced susceptibility and point mutations in the quinolone resistance-determining region of parC and gyrA genes was evaluated, and point mutations were compared with other reports of isolates derived from adult or mixed patient populations. Nine percent of isolates had reduced susceptibility to 1 or more of these fluoroquinolones by Etest: ciprofloxacin, levofloxacin, and moxifloxacin. A single point mutation (Ser-79) in parC seemed responsible for the reduced susceptibility. Resistant Streptococcus pyogenes isolates were compared using M/emm type, repetitive sequence-based PCR (rep-PCR), and pulsed-field gel electrophoresis (PFGE). Rep-PCR provided no more separation of strains than M/emm typing, and PFGE results with SgrAI were more discriminatory than with SmaI. The majority of these isolates were M/emm type 6. PFGE analysis using SgrAI demonstrated 2 different resistant strains among the M/emm type 6 isolates. The findings suggest that a population of S. pyogenes with an intrinsic reduced susceptibility to fluoroquinolones exists in pediatric clinical isolates. Monitoring of amino acid changes in both parC and gyrA will assist in the prediction of emergence of high-level fluoroquinolone resistance.