Characterization of a strain of community-associated methicillin-resistant Staphylococcus aureus widely disseminated in the United States

A highly stable strain of Staphylococcus aureus with a pulsed-field gel electrophoresis type of USA300 and multilocus sequence type 8 has been isolated from patients residing in diverse geographic regions of the United States. This strain, designated USA300-0114, is a major cause of skin and soft tissue infections among persons in community settings, including day care centers and correctional facilities, and among sports teams, Native Americans, men who have sex with men, and military recruits. The organism is typically resistant to penicillin, oxacillin, and erythromycin (the latter mediated by msrA) and carries SCCmec type IVa. This strain is variably resistant to tetracycline [mediated by tet(K)]; several recent isolates have decreased susceptibility to fluoroquinolones. S. aureus USA300-0114 harbors the genes encoding the Panton-Valentine leucocidin toxin. DNA sequence analysis of the direct repeat units within the mec determinant of 30 USA300-0114 isolates revealed differences in only a single isolate. Plasmid analysis identified a common 30-kb plasmid that hybridized with blaZ and msrA probes and a 3.1-kb cryptic plasmid. A 4.3-kb plasmid encoding tet(K) and a 2.6-kb plasmid encoding ermC were observed in a few isolates. DNA microarray analysis was used to determine the genetic loci for a series of virulence factors and genes associated with antimicrobial resistance. Comparative genomics between USA300-0114 and three other S. aureus lineages (USA100, USA400, and USA500) defined a set of USA300-0114-specific genes, which may facilitate the strain's pathogenesis within diverse environments.