ABSTRACT
Objective To investigate the mechanism of fluoroquinolone resistance in clinical isolates of Enterococcus faecium. Methods The MICs of six fluoroquinolones(norfloxacin,ciprofloxacin,ofloxacin,levofloxacin,gatifloxacin and moxifloxacin) against 35 clinical isolates of E.faecium from eight hospitals in Tianjin were determined by agar dilution method in the absence or presence of multidrug resistance efflux pump inhibitor reserpine.The quinolone-resistance determining region(QRDR)of parC and gyrA were amplified and sequenced.Results No less than twofold decrease in MIC values of the six fluoroquinolones in the presence of reserpine was observed in 35,29,1,0,6 and 2 of the 35 strains of E.faecium respectively.One fluoro- quinolone-susceptible isolate and five fluoroquinolone-resistant isolates were selected randomly to analyze the QRDR of parC and gyrA.All five fluoroquinolone-resistant isolates had single amino acid alteration in both GyrA and ParC.Ser-80 in ParC was substituted by lie(4 isolates)or Arg(1 isolates).Glu-87 in GyrA was replaced by Lys(2 isolates)or Gly(2 isolates). The other one had an Ser-83-to-Ile substitution.The one fluoroquinolone-suseeptible isolate had no alteration in the QRDR of either ParC or GyrA.Conclusions Both target alteration and active efflux are responsible for the resistance to fluoroquinolone in clinical isolates of E.faecium.