Methicillin-resitant Staphylococcus aureus (MRSA) are the most common and lethal causes of infections of hospitalized patients and, over the past decade, have appeared in the community as an increasingly common cause of infections of individuals who both have and do not have healthcare exposures. MRSA are resistant to beta-lactam (penicillin and derivatives) and many additional antibiotics (multiresistant), severely limiting options for treating serious infections. Beta-lactam antibiotics target the enzymes (penicillin binding proteins or PBPs) that maintain the integritiy of the bacterial cell wall. The gene responsible for methicillin resistance (MR), mecA, encodes a new PBP (PBP2a) that can maintain cell wall structure but is resistant to beta-lactam inhibition mecA is carried on a genetic element called a genomic island that inserts into the staphylococcal chromosome at a specific sequence (attB). In addition to housing mecA, this island, called SCCmec, carries genes called ccr that catalyze both insertion and excision of SCCmec. Both the mechanisms of insertion and excision and the epidemiology of various forms fo SCCmec suggest that the element is mobile and has moved among staphylococcal strains multiple times in the past thirty years. However, the genetic types of SA that have acquired SCCmec are limited compared to the wide range of strain types of methicillin susceptible (MS) SA that are available. This proposal seeks to understand how SCCmec is transferred among staphylococcie and the specific genetic requirements of recipient MSSA. In addition, it will investigate how SCCmec can be lost, converting MRSA back to MSSA. The tree Specific Aims are to 1) Elucidate the molecular mechanisms and target sequences required for ccr-mediated SCCmec insertion and excision;2) Assess the frequency of spontaneous SCCmec excision in vitro and in vivo;and 3) Capture excised SCCmec on a conjugative plasmid, transfer SCCmec and mecAto suitable staphylococcal recipients and identify genomic changes associated with acquisition and stable maintenance. In addition, since there is epidemiologic and genomic evidence that S.epidermidis (SE), a less virulent staphylococcal species, also carries SCCmec and can serve as a genetic reservoir for this element, we will perform experiments in both SA and SE. The goal of these studies is to identify sequences in SA and SE that can be used as probes to dissect the epidemiology of the spread of MR. In this manner we can trace the history of the emergence and rapid spread of MRSA, prevent future dissemination of SCCmec and support arguments that reduce antibiotic will favor SCCmec loss from host bacteria and lower the environmental prevalence of the MRSA phenotype.

Public Health Relevance

This project will provide molecular data that will allow researchers to trace the past and future spread of the gene responsible for methicillin resistance in Staphylococcus aureus (MRSA). In addition, it will investigate conditions that promote the loss of this gene that may suggest strategies for reducing the hospital prevalence of MRSA.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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Study Section
Drug Discovery and Mechanisms of Antimicrobial Resistance Study Section (DDR)
Program Officer
Huntley, Clayton C
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Virginia Commonwealth University
Internal Medicine/Medicine
Schools of Medicine
United States
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Ray, M D; Boundy, S; Archer, G L (2016) Transfer of the methicillin resistance genomic island among staphylococci by conjugation. Mol Microbiol 100:675-85
Wang, Lei; Ahmed, Mostafa H; Safo, Martin K et al. (2015) A Plasmid-Borne System To Assess the Excision and Integration of Staphylococcal Cassette Chromosome mec Mediated by CcrA and CcrB. J Bacteriol 197:2754-61
Boundy, Sam; Safo, Martin K; Wang, Lei et al. (2013) Characterization of the Staphylococcus aureus rRNA methyltransferase encoded by orfX, the gene containing the staphylococcal chromosome Cassette mec (SCCmec) insertion site. J Biol Chem 288:132-40
Boundy, Sam; Zhao, Qixun; Fairbanks, Carly et al. (2012) Spontaneous staphylococcal cassette chromosome mec element excision in Staphylococcus aureus nasal carriers. J Clin Microbiol 50:469-71
Wang, Lei; Safo, Martin; Archer, Gordon L (2012) Characterization of DNA sequences required for the CcrAB-mediated integration of staphylococcal cassette chromosome mec, a Staphylococcus aureus genomic island. J Bacteriol 194:486-98
Kleiner, Elizabeth; Monk, Alastair B; Archer, Gordon L et al. (2010) Clinical significance of Staphylococcus lugdunensis isolated from routine cultures. Clin Infect Dis 51:801-3
Wang, Lei; Archer, Gordon L (2010) Roles of CcrA and CcrB in excision and integration of staphylococcal cassette chromosome mec, a Staphylococcus aureus genomic island. J Bacteriol 192:3204-12
Noto, Michael J; Fox, Paige M; Archer, Gordon L (2008) Spontaneous deletion of the methicillin resistance determinant, mecA, partially compensates for the fitness cost associated with high-level vancomycin resistance in Staphylococcus aureus. Antimicrob Agents Chemother 52:1221-9
Noto, Michael J; Kreiswirth, Barry N; Monk, Alastair B et al. (2008) Gene acquisition at the insertion site for SCCmec, the genomic island conferring methicillin resistance in Staphylococcus aureus. J Bacteriol 190:1276-83
Monk, Alastair B; Boundy, Sam; Chu, Vivian H et al. (2008) Analysis of the genotype and virulence of Staphylococcus epidermidis isolates from patients with infective endocarditis. Infect Immun 76:5127-32

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