Staphylococcus aureus CC395 harbours a novel composite staphylococcal cassette chromosome mec element

Background: CoNS species are likely reservoirs of the staphylococcal cassette chromosome mec (SCCmec) in Staphylococcus aureus. S. aureus CC395 is unique as it is capable of exchanging DNA with CoNS via bacteriophages, which are also known to mediate transfer of SCCmec. Objectives: To analyse the structure and putative origin of the SCCmec element in S. aureus CC395. Methods: The only MRSA CC395 strain described in the literature, JS395, was subjected to WGS, and its SCCmec element was compared with those found in CoNS species and other S. aureus strains. Results: JS395 was found to carry an unusually large 88 kb composite SCCmec element. The 33 kb region downstream of orfX harboured a type V SCCmec element and a CRISPR locus, which was most similar to those found in the CoNS species Staphylococcus capitis and Staphylococcus schleiferi. A 55 kb SCC element was identified downstream of the type V SCCmec element and contained a mercury resistance region found in the composite SCC element of some Staphylococcus epidermidis and S. aureus strains, an integrated S. aureus plasmid containing genes for the detoxification of cadmium and arsenic, and a stretch of genes that was partially similar to the type IVg SCCmec element found in a bovine S. aureus strain. Conclusions: The size and complexity of the SCCmec element support the idea that CC395 is highly prone to DNA uptake from CoNS. Thus CC395 may serve as an entry point for SCCmec and SCC structures into S. aureus.