Enrofloxacin Promotes Plasmid-Mediated Conjugation Transfer of Fluoroquinolone-Resistance Gene qnrS

Abstract

This study aimed to determine the effect of enrofloxacin (ENR) on the transfer of the plasmid-mediated quinolone resistance (PMQR) gene <i>qnrS</i> from opportunistic pathogen <i>Escherichia coli</i> (E2) to <i>Salmonella</i> Enteritidis (SE211) and to analyze the resistance characteristics of SE211-<i>qnrS</i> isolates. The plasmid carrying <i>qnrS</i> gene of E2 was sequenced by Oxford Nanopore technology. The plasmid carrying <i>qnrS</i> gene belonged to incompatibility group IncY. <i>In vitro</i>, the transfer experiment of IncY plasmid was performed by the liquid medium conjugation method. The conjugation transfer frequency of the IncY plasmid was 0.008 ± 0.0006 in the absence of ENR, 0.012 ± 0.003 in 1/32 MIC_ENR, 0.01 ± 0.008 in 1/8 MIC_ENR, and 0.03 ± 0.015 (Mean±SD) in 1/2 MIC_ENR, respectively. After inoculation of <i>E. coli</i> E2 and SE211, chickens were treated with different doses of ENR (3.03, 10, and 50 mg/kg b.w.) for 7 days consecutively. To screen the SE211-<i>qnrS</i> strains from intestinal tract of chickens, the resistance genes and susceptibility of isolates were identified. The amount of <i>E. coli</i> E2 and the copy number of <i>qnrS</i> gene in the chicken intestinal tract were determined by colony counting and qPCR, respectively. <i>In vivo</i>, more SE211-<i>qnrS</i> strains were isolated from the treated group compared with the untreated group. SE211-<i>qnrS</i> strains not only obtained IncY plasmid, but also showed similar resistance phenotype as E2. In conclusion, ENR treatment can promote the spread of a IncY-resistance plasmid carrying the <i>qnrS</i> fluoroquinolone-resistance gene in <i>Escherichia coli</i> and the development of drug-resistant bacteria.