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Antimicrob. Agents Chemother. doi:10.1128/AAC.00997-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

QnrD, a novel gene conferring transferable quinolone resistance in Salmonella enterica serovars Kentucky and Bovismorbificans of human origin

Research group for Antimicrobial Resistance and molecular epidemiology; National Food Institute, Technical University of Denmark; Copenhagen V, Denmark; Department of Veterinary Pathobiology, Faculty of Life Science, University of Copenhagen, 1870 Frederiksberg C, Denmark; Branch for Enteric Disease Control and Prevention, Institute for Infectious Disease Control and Prevention Henan, Center for Disease control and Prevention, Nongye East Road, East New District, Zhengzhou, Henan, 450016 PR China

^* To whom correspondence should be addressed. Email: licav{at}food.dtu.dk.

	Abstract

In a previous study, four Salmonella isolates from humans in the Henan province of China showed reduced susceptibility towards ciprofloxacin (MIC 0.125-0.25 µg/ml) but were susceptible to nalidixic acid (MIC 4-8µg/ml). All isolates were negative for known qnr genes (A, B and S), aac(6')Ib-cr and mutations in gyrA and parC. Plasmid DNA was extracted from all four isolates, transformed into TG1 and DH10B E. coli by electroporation and transformants were selected on 0.06 µg/ml ciprofloxacin containing Brain and Heart Infusion agar plates. Resistance to ciprofloxacin could be transferred by electroporation and a similar 4,270 bp plasmid was found in all transformants.

By sequence analysis the plasmid was found to carry an open reading frame (ORF), which had similarities to other qnr genes and encoding a 214 amino acid pentapeptide repeat protein. This gene, designated as qnrD showed 48% similarity to qnrA1, 61% to qnrB1 and 41% to qnrS1. Further subcloning of the qnrD coding region into the constitutively expressed tetA gene of the pBR322 vector showed that the gene conferred an increase of the MIC for ciprofloxacin by a factor of 32 (from MIC=0.002 to MIC=0.06 µg/ml).For comparison, qnrA1 and qnrS1 were also subcloned into pBR322 and transformed into DH10B cells conferring an MIC 0.125 and 0.5 µg/ml, respectively. A phylogenetic analysis of all known qnr sequences was performed and showed that qnrD was closer related to the qnrB variants, but formed an independent cluster.

This is to our knowledge the first description of this qnrD gene.