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Antimicrobial Agents and Chemotherapy, April 2008, p. 1221-1229, Vol. 52, No. 4
0066-4804/08/$08.00+0     doi:10.1128/AAC.01164-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Spontaneous Deletion of the Methicillin Resistance Determinant, mecA, Partially Compensates for the Fitness Cost Associated with High-Level Vancomycin Resistance in Staphylococcus aureus{triangledown}

Michael J. Noto,1 Paige M. Fox,1 and Gordon L. Archer2*

Department of Microbiology and Immunology,1 Internal Medicine, Virginia Commonwealth University School of Medicine, McGuire Hall Room 103, 1112 East Clay Street, Richmond, Virginia2

Received 4 September 2007/ Returned for modification 2 November 2007/ Accepted 14 January 2008

Treatment of infections caused by Staphylococcus aureus is often confounded by the bacterium's ability to develop resistance to chemotherapeutic agents. Methicillin-resistant S. aureus (MRSA) arises through the acquisition of staphylococcal chromosomal cassette mec (SCCmec), a genomic island containing the methicillin resistance determinant, mecA. In contrast, resistance to vancomycin can result from exposure to the drug, a mechanism that is not dependent upon a gene acquisition event. Here we describe three MRSA strains that became resistant to vancomycin during passage in the presence of increasing concentrations of the drug. In each case two derivative strains were isolated, one that had lost mecA and one that retained mecA during passage. Strain 5836VR lost mecA by the site-specific chromosomal excision of SCCmec, while the other two strains (strains 3130VR and VP32) deleted portions of their SCCmec elements in a manner that appeared to involve IS431. Conversion to vancomycin resistance caused a decrease in the growth rate that was partially compensated for by the deletion of mecA. In mixed-culture competition experiments, vancomycin-resistant strains that lacked mecA readily outcompeted their mecA-containing counterparts, suggesting that the loss of mecA during conversion to vancomycin resistance was advantageous to the organism.

* Corresponding author. Mailing address: Internal Medicine, Virginia Commonwealth University, P.O. Box 980565, 1101 East Marshall Street, Richmond, VA. 23298-0565. Phone: (804) 828-0678. Fax: (804) 828-5022. E-mail: garcher{at}vcu.edu

{triangledown} Published ahead of print on 22 January 2008.

Antimicrobial Agents and Chemotherapy, April 2008, p. 1221-1229, Vol. 52, No. 4
0066-4804/08/$08.00+0     doi:10.1128/AAC.01164-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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