Insights on evolution of virulence and resistance from the whole-genome analysis of a predominant methicillin-resistant Staphylococcus aureus clone sequence type 239 in China

Earlier, we reported that ST239 was the15-year predominant methicillin-resistant Staphylococcus aureus(MRSA) clone in China. In this study, MRSA strain CN79 belonging to ST239 and isolated from blood was used to determine the whole genome sequence. Comparative genomics analysis was done between MRSA CN79and 25 sequenced S. aureus in the NCBI GenBank database. A total of 2,734 protein-encoding genes were identified in the MRSA CN79 genome, which carries 11antibiotic resistance genes and 65 virulence genes. Two prophages phiCN79A and phiNM3-like were found on the MRSA CN79 genome. MRSA CN79 carries 30 specific genes that are absent from the 25 sequenced S. aureus genomes. Most of them were prophage-related genes.Several antibiotic resistance genes, such as b-lactamase and ABC-type multidrug transport system gene, were located on the genomic island mSab. The antibiotic resistance genes, such as tet(M), ermA1, and blaZ, were also located on different transposons. The virulence genes sea,map, hlb, and sak are located on phiNM3-like prophage and the exotoxin genes are carried on the genomic island mSaa. These results suggest that ST239 MRSA strains are widespread owing to horizontal acquisition of the mobile genetic elements harbored antibiotic resistance genes and virulence genes in response to environmental selective pressures, such as antibiotics and the human immune system during evolution.