Fluoroquinolone resistance is an emerging problem in companion animal practice. The
present study aimed to determine comparative fluoroquinolone minimum inhibitory
concentrations (MICs) for enrofloxacin, marbofloxacin and pradofloxacin and identify
plasmid-mediated quinolone resistance (PMQR) mechanisms in 41 multidrug-resistant
(MDR) Escherichia coli isolates representing three main clonal groups (CGs) cultured from
extraintestinal infections in dogs. All isolates were resistant to fluoroquinolones and the
PMQR genes qnrA1, qnrB2, qnrS1 and qepA were identified in isolates from each CG. For a
subset of 13 representative isolates, fluoroquinolone chromosomal resistance mechanisms
were characterized. CG1 isolates had three mutations in the quinolone resistance
determining region (QRDR), two in gyrA (Ser TCG-83 ! Leu TTG and Asp GAC-87 ! Asn
AAC) and one in parC (Ser AGC-80 ! Ile ATT), whilst CG2 and CG3 isolates also possessed
an additional mutation in parC (Glu GAA-84 ! Gly GGA) which was reflected in higher
fluoroquinolone MICs compared to CG1. Organic solvent tolerance was demonstrated in 8
of the 13 isolates, and all 13 isolates demonstrated enhanced efflux on the basis of a 4-fold
decrease or greater in the MIC of enrofloxacin when incubated with an efflux pump
inhibitor. A mutation in acrR which can cause overexpression of the AcrAB multidrug
efflux pump was detected in CG1 strains. These findings indicate that fluoroquinolone
resistance in MDR E. coli isolated from extraintestinal infections in dogs is associated with
a combination of target mutations in the QRDRs, transferable PMQR mechanisms and
enhanced efflux