Phenotypic and genotypic quinolone resistance in Escherichia coli underlining GyrA83/87 mutations as a target to detect ciprofloxacin resistance

Background: Quinolone resistance (QR) is one component of the MDR emerging in Escherichia coli and is of particular concern given the widespread use of fluoroquinolones.

Objectives: To characterize the QR phenotypes and genotypes in E. coli responsible for bloodstream infections and to propose molecular determinants that could be targeted to predict ciprofloxacin resistance.

Methods: E. coli isolates from blood cultures in three French hospitals were studied for quinolone MICs and characterization of genotypic QR determinants (QRg).

Results: Among 507 isolates tested for MICs, 148 (29.2%) were resistant to quinolones based on EUCAST breakpoints and 143 (28.2%) harboured at least one QRg. QRg were mainly mutations in the QRDR (138 isolates, 27.2%), with 55.8% of these isolates carrying at least three QRDR mutations. gyrA mutations predominated (92.8%) followed by parC (61.6%), parE (32.6%) and gyrB (1.4%) mutations. Only 4.7% of the isolates harboured a plasmid-mediated quinolone resistance (PMQR) gene: aac(6')-Ib-cr (60.0%) or qnr (qnrS, qnrB) (32.0%). For the first time in France, we reported the qepA4 allele of the plasmid-encoded efflux pump QepA. Only five isolates carried PMQR without a QRDR mutation. The positive predictive value (PPV) for ciprofloxacin resistance was 100% for any QRg and 99.2% for gyrA mutations specifically.

Conclusions: QR observed in E. coli isolates involved in bloodstream infections is still mainly due to QRDR mutations, especially at codons GyrA83/87, which could be used as a molecular target to rapidly detect resistance.