We investigated the genetic relatedness of 5 community-acquired (CA) Staphylococcus aureus isolates obtained from 4 consecutive pediatric patients presenting with sepsis syndrome and severe pneumonia during a 3-week period in 2000. Two patients were infected with methicillin-susceptible S. aureus (MSSA), and 2 were infected with methicillin-resistant S. aureus (MRSA). The pulsed-field gel electrophoresis patterns for the 2 CA-MRSA isolates were identical to each other, as were the patterns for the 3 CA-MSSA isolates. A 2-band difference reflecting the presence of a staphylococcal cassette chromosome mec (SCCmec) element distinguished the CA-MRSA isolates from the CA-MSSA isolates. The small, mobile type IV SCCmec element was present in the CA-MRSA isolates. These data suggest that an insertion or, less likely, a deletion of the SCCmec type IV element occurred in a highly virulent S. aureus background. Staphylococcal toxin genes sea, seh, lukS-PV, and lukF-PV were detected in all isolates. Also, in all isolates, was a partial homolog of seo (seo'). The relationship among these patient isolates strengthens the assumption that CA-MRSA infections may be caused by isolates closely related to MSSA isolates.

Among methicillin-resistant Staphylococcus aureus isolates, a staphylococcal chromosomal cassette containing the mecA gene (SCCmec) is integrated into the chromosome at a unique site. SCCmec also contains unique ccrAB recombinase genes mediating its integration and excision from the genome and is flanked by characteristic left and right direct-and inverted-repeat sequences. A few non-mecA-containing SCC elements that have the other molecular features described above have recently been described. The origin of these cassettes is not clear. We have identified two new members of the SCC family integrated within orfX in Staphylococcus epidermidis strain ATCC 12228, neither of which carries mecA. One is a 57-kb element flanked by a unique 28-bp SCC direct repeat. It was called the SCC composite island (SCC-CI) because it carries a 19-kb SCC element (SCCpbp4) nested within it. SCCpbp4 contains pbp4 and tagF genes, as well as one pair of ccrAB genes (allotype 2) flanked by classical SCC-specific terminal repeats. External to SCCpbp4, SCC-CI contains a second pair of ccrAB genes (allotype 4), three IS431 elements, and genes mediating resistance to heavy metals. Genes mediating restriction-modification that may facilitate horizontal transfer are also present within SCC-CI, both within and outside SCCpbp4. Several novel arrangements of the SCC direct and inverted repeats were identified. Several long stretches of homology with other SCCs were found within and outside SCCpbp4. In view of the fact that SCC-CI was found in a commensal species, it may represent a reservoir for sequences involved in genetic shuffling between staphylococci and may contribute to the diversity found in SCC elements.