Staphylococcal cassette chromosome mec: Recent advances and new insights

Staphylococcal cassette chromosome (SCC) elements are complex mobile genetic elements that often carry antimicrobial resistance and in some cases virulence-associated genes. In addition to SCCmec, which harbours the methicillin resistance gene mec, many different SCC elements have been identified in staphylococci. Recent findings have significantly enhanced our understanding of the diversity of SCCmec elements and their contribution to the evolution of MRSA and are the focus of this short review. This includes the identification of (i) novel mec genes and allelic variants, (ii) an extensive array of ccr and mec complex genes as well as SCCmec, SCC and pseudo SCC/SCCmec elements and composite islands (CIs) in staphylococci, (iii) potential mec, SCC and SCCmec precursors among distinct coagulase-negative staphylococcal species, and (iv) SCC encoded virulence-associated genes. Due to their complex nature and increasing diversity, detailed characterisation of SCC and SCCmec elements and CIs represents a unique challenge but is vital for effective epidemiological typing and tracking of MRSA and other staphylococci and to enhance our understanding of the origins and evolution of MRSA.