Identification of Qnr and AAC(6')-1b-cr plasmid-mediated fluoroquinolone resistance determinants in multidrug-resistant Enterobacter spp. isolated from extraintestinal infections in companion animals.

Fluoroquinolone resistance is becoming more common in veterinary medicine. Resistance is due to a combination of chromosomal and plasmid-mediated fluoroquinolone resistance (PMQR) mechanisms. The aim of the present study was to screen 17 multidrug-resistant Enterobacter isolates obtained from opportunistic infections in companion animals for chromosomal and plasmid-mediated fluoroquinolone resistance determinants and to determine if they are co-located with other antimicrobial resistance genes including beta-lactamases. Phenotypic tests (biochemical identification, organic solvent tolerance testing) were combined with genotypic analysis (PCR, pulsed field gel electrophoresis, sequencing, plasmid isolation and southern blot hybridization) to characterize the molecular basis for fluoroquinolone resistance. Antimicrobial susceptibility was determined by broth microdilution for fluoroquinolone antimicrobials (enrofloxacin, ciprofloxacin, moxifloxacin, marbofloxacin and pradofloxacin) and by disk diffusion for other antimicrobials. Sixteen isolates were resistant to at least one of the five fluoroquinolones tested. Fourteen isolates possessed PMQR determinants which were identified as qnrA1 (n=3) or qnrB2 (n=11), often in combination with aac(6')-1b-cr (n=6). The PMQR genes were localized to large, transferable MDR plasmids often associated with an extended-spectrum beta-lactamase and quinolone resistance was co-transferred with bla(SHV-12) for 10 of the 14 qnr-positive strains. Three isolates had wild-type topoisomerases, 11 had a single point mutation in gyrA (Ser83Phe or Tyr), and three had two mutations; one in gyrA (Ser83Ile) and one in parC (Ser80Ile). PMQR genes in clinical veterinary Enterobacter isolates are co-located with beta-lactamases and other resistance genes on large transferable plasmids. PMQR genes contribute to fluoroquinolone resistance when combined with topoisomerase mutations and efflux.