Nasal carriage of methicillin-resistant Staphylococcus aureus among preclinical medical students: epidemiologic and molecular characteristics of methicillin-resistant S. aureus clones.

Between May 2008 and October 2009, a total of 2103 interns were randomly tested for nasal colonization of S. aureus and methicillin-resistant Staphylococcus aureus (MRSA). The prevalence of S. aureus among staphylococci specimens was 23.1%, and among the total S. aureus the MRSA prevalence was 9.4%. MRSA isolates were further subtyped using genetic element staphylococcal cassette chromosome mec (SCCmec) typing, pulsed-field gel electrophoresis (PFGE) band pattern, and multilocus sequence typing. SCCmec type IVa was the most prevalent strain, at 45.4 %. Eleven PFGE patterns were identified in MRSA strains, with 1 predominant (pulsotype A, 45.5%). Eight strains which belonged to clonal complex 78 carried type IVa SCCmec and produced type 3 coagulase. Panton-Valentine leukocidin (PVL) genes (lukS and F-PV) were identified in 10 (45.4%) MRSA strains; these predominately carried ϕSa2958type and ϕSa108PVL-like type PVL phages. After inducing prophages, 8 strains infected other S. aureus isolates and could generate novel PVL-positive strains of S. aureus. The present study demonstrates that interns can carry certain MRSA strains asymptomatically and contribute to the spread of MRSA between the community and hospital.