Decline of EMRSA-16 amongst methicillin-resistant Staphylococcus aureus causing bacteraemias in the UK

Methicillin-resistant Staphylococcus aureus (MRSA) emerged in 1961 and became dramatically more prevalent as agents of bacteraemia in the UK in the mid-1990s in England, Wales and Northern Ireland. By the end of the 1990s, .40% of all S. aureus bacteraemias were due to MRSA, though this proportion (and the total number) of MRSA bacteraemias has since declined and stood at 20% in 2008. The rise of MRSA in the 1990s and early 2000s correlated with the emergence and spread of two epidemic strains, designated EMRSA-15 and EMRSA-16, which, in 1999–2000, accounted for 95.6% of all UK MRSA bacteraemias; with 60.2% due to EMRSA-15 and 35.4% to EMRSA-16. EMRSA-15 and EMRSA-16 differ genetically, belonging to distinct multilocus sequence type (MLST) clonal complexes (CCs), namely CC22 (ST22) for EMRSA-15 and CC30 (ST36) for EMRSA-16. They also differ in their staphylococcal cassette chromosome mec (SCCmec) types, with EMRSA-15 typically harbouring SCCmecIV whereas EMRSA-16 typically has SCCmecII. They may differ in resistance profile too, but both lineages are usually resistant to fluoroquinolones and macrolides, and the use of these antibiotics has been described as a risk factor for colonization or infection. Since its first emergence in the UK, EMRSA-15 has become disseminated in Europe, Australia, the Middle East and the Far East; EMRSA-16 has been reported widely, inand outside of the UK, but it is not perceived to be as successful as EMRSA-15. Here, we report trends for EMRSA-15 and EMRSA-16 among MRSA isolated as part of the BSAC Bacteraemia Surveillance Programme between 2001 and 2007.