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J. Bacteriol. doi:10.1128/JB.01484-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Inferring a population structure for Staphylococcus epidermidis from multilocus sequence typing (MLST) data

Laboratório de Genética Molecular, Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa (ITQB/UNL), Rua da Quinta Grande, nr. 6, APT127, 2780-156 Oeiras, Portugal; Department of Infectious Disease Epidemiology, Faculty of Medicine, Imperial College London, Old Medical School Building, St Mary's Hospital, Norfolk Place, London W2 1PG, UK; UEI Micobactérias, Instituto de Higiene e Medicina Tropical, Universidade Nova de Lisboa (IHMT, UNL), Rua da Junqueira, nr. 96, 1349-008 Lisboa, Portugal; Centro de Recursos Microbiológicos, Faculdade de Ciências e Tecnologia (FCT/UNL), Quinta da Torre, 2829-516 Monte de Caparica, Portugal; Laboratory of Microbiology, The Rockefeller University, New York 10021, USA

^* To whom correspondence should be addressed. Email: lencash{at}mail.rockefeller.edu.

	Abstract

Despite its importance as a human pathogen, information on population structure and global epidemiology of S. epidermidis is scarce and the relative importance of the mechanisms contributing to clonal diversification is unknown. In this study, we addressed these issues by analysing a representative collection of S. epidermidis isolates from diverse geographic and clinical origins using multilocus sequence typing (MLST). Additionally, we characterized the mobile element (SCCmec) carrying the genetic determinant of methicillin resistance.

The 217 S. epidermidis isolates from our collection were split by MLST into 74 types, suggesting a high level of genetic diversity. Analysis of MLST data using the eBURST algorithm revealed the existence of nine epidemic clonal lineages that were disseminated worldwide. One single clonal lineage (CC2) comprised 74% of the isolates whereas the remaining isolates were clustered into 8 minor clonal lineages and 13 singletons. According to our evolutionary model, SCCmec was acquired at least 56 times by S. epidermidis. Although geographic dissemination of S. epidermidis strains and the value of index of association between the alleles (I_A^S=0.2898, P<0.05), support the clonality of S. epidermidis species, examination of the sequence changes at MLST loci during clonal diversification showed that recombination gives rise to new alleles approximately twice as frequently as point mutations. We suggest that S. epidermidis has a population with an epidemic structure, where nine clones have emerged upon a recombining background, and evolved quickly through frequent transfer of genetic mobile elements, including SCCmec.

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