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Antimicrobial Agents and Chemotherapy, September 2007, p. 3254-3258, Vol. 51, No. 9
0066-4804/07/$08.00+0     doi:10.1128/AAC.00274-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Role of a qnr-Like Gene in the Intrinsic Resistance of Enterococcus faecalis to Fluoroquinolones

Service de Microbiologie and EA 2128 Interactions hôte et microorganismes des épithéliums, Hôpital Côte de Nacre, Université de Caen Basse-Normandie, 14033 Caen cedex, France

Received 23 February 2007/ Returned for modification 21 May 2007/ Accepted 2 July 2007

Fluoroquinolones are poorly active against enterococci. Recently, plasmid-borne resistance to fluoroquinolones due to the qnr gene was reported in members of the Enterobacteriaceae family. The gene encodes a pentapeptide repeat protein that protects DNA gyrase from inhibition by fluoroquinolones. We have identified in the genome of Enterococcus faecalis V583 a qnr-like gene, named E. faecalis qnr (qnr_{E. faecalis}), encoding a putative pentapeptide repeat protein that shares 25% identity with Qnr. To assess its potential role in the intrinsic resistance of E. faecalis to fluoroquinolones, qnr_{E. faecalis} was inactivated in E. faecalis JH2-2 by insertion of the thermosensitive vector pG1KT. This strain was then complemented with qnr_{E. faecalis} cloned in the multicopy plasmid pORI23. The effects of its overexpression were also studied. Inactivation of the qnr_{E. faecalis} gene resulted in twofold decreases in the MICs of ofloxacin and ciprofloxacin. When the gene was complemented or overexpressed, MICs of fluoroquinolones increased four- to nine-fold, leading to MICs of ofloxacin and ciprofloxacin equal to 32 µg/ml and 8 µg/ml, respectively. The E. faecalis Qnr (Qnr_{E. faecalis}) protein was produced and purified. Qnr_{E. faecalis} protein protected Escherichia coli DNA gyrase from inhibition by ofloxacin. The qnr_{E. faecalis} gene was then introduced into E. coli DH10B, Staphylococcus aureus RN4220, and Lactococcus lactis IL-1419 to study its heterologous expression. MICs of the various fluoroquinolones tested increased 4- to 16-fold, showing that Qnr_{E. faecalis} conferred resistance to fluoroquinolones in various bacterial backgrounds. Overexpression of qnr_{E. faecalis} in enterococci or mobilization of the gene to other bacterial species may be anticipated as a possible new mechanism for fluoroquinolone resistance.

Published ahead of print on 9 July 2007.

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• Jacoby, G., Cattoir, V., Hooper, D., Martinez-Martinez, L., Nordmann, P., Pascual, A., Poirel, L., Wang, M. (2008). qnr Gene Nomenclature. Antimicrob. Agents Chemother. 52: 2297-2299 [Full Text]  
• Rodriguez-Martinez, J. M., Velasco, C., Briales, A., Garcia, I., Conejo, M. C., Pascual, A. (2008). Qnr-like pentapeptide repeat proteins in Gram-positive bacteria. J Antimicrob Chemother 61: 1240-1243 [Abstract] [Full Text]