Materials and Methods Bacterial Strains Community-acquired Methicillin- Resistant Staphylococcus Aureus Carrying Panton-valentine Leukocidin Genes: Worldwide Emergence

cillin-resistant Staphylococcus aureus (MRSA) have been reported worldwide. We assessed whether any common genetic markers existed among 117 CA-MRSA isolates Western Samoa by performing polymerase chain reaction for 24 virulence factors and the methicillin-resistance determinant. The genetic background of the strain was analyzed by pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST). The CA-MRSA strains shared a type IV SCCmec cassette and the Panton-Valentine leukocidin locus, whereas the distribution of the other toxin genes was quite specific to the strains from each continent. PFGE and MLST analysis indicated distinct genetic backgrounds associated with each geographic origin, although predominantly restricted to the agr3 background. Within each continent, the genetic background of CA-MRSA strains did not correspond to that of the hospital-acquired MRSA. M ethicillin-resistant Staphylococcus aureus (MRSA) are identified as nosocomial pathogens throughout the world (1). Established risk factors for MRSA infection include recent hospitalization or surgery, residence in a long-term–care facility, dialysis, and indwelling percuta-neous medical devices and catheters. Recently, however, cases of MRSA have been documented in healthy community dwelling persons without established risk factors for MRSA acquisition. Because they are apparently acquired in the community, these infections are referred to as community acquired (CA)-MRSA (2). CA-MRSA infections have been reported in North America, Europe, Australia, and New Zealand (3–5). The recent genomic sequence of a CA-MRSA isolate (6) indicated the presence not only of a novel smaller variant of the methicillin-resistance locus (SCCmec IVa, according to Baba et al. designation [6]), but also that of the locus for the Panton-Valentine leuko-cidin (PVL). The PVL locus is carried on a bacteriophage and is present in only a small percentage of S. aureus isolates from France, where this locus is associated with skin infections, and occasionally, severe necrotizing pneumonia (7,8). In a recent study, we found that CA-MRSA infections in France are caused by a single clone producing the PVL (3). Analysis of a set of CA-MRSA strains from the United States and Australia confirmed the presence of SCCmec IVa in most of them, and genetic comparison of the CA-MRSA by multi-locus sequence typing (MLST) indicated that they belonged to five clonal complexes, two of which predominated (4). This finding suggested that CA-MRSA have arisen from diverse genetic backgrounds rather than the worldwide spread of a single clone (4). The aim of this study was to determine whether the PVL gene represents a stable marker of the CA-MRSA strains …