Trends in Microbiology
ReviewThe emergence and evolution of methicillin-resistant Staphylococcus aureus
Section snippets
Integration of the SCCmec element into the S. aureus chromosome
In recent years, significant advances have been made in defining in simple genetic terms how MRSA arose. MRSA was ‘born’ when it acquired a large genetic element known as the staphylococcal cassette chromosome mec (SCCmec) 6, 7, 8 (Fig. 1). SCCmec is a 21–67 kb fragment of DNA that integrates in the chromosome of MRSA at a unique site (attBscc) located near the S. aureus origin of replication (Fig. 2). attBscc is found in an open reading frame (ORF) of unknown function, designated orfX, that is
Evolution towards high methicillin resistance
Pre-MRSA The acquisition of SCCmec is the first common genetic event for S. aureus to become fully β-lactam resistant. However, at least two more genetic events are involved in the evolution of clonotype-II-A MRSA, which is by far the most dominant MRSA in Japanese hospitals. This clone possesses a class A mec complex. Unlike the bla regulatory gene system (blaI-blaR1) for β-lactamase induction, the mec regulatory gene system (mecI-mecR1) involved in the regulation of mecA gene transcription
Evolution towards vancomycin resistance
Because of the limited space, only an outline of vancomycin resistance is given here; for more details see 14, 22, 25.
Acknowledgements
This work was supported by the Core University System Exchange Programme under the Japan Society for the Promotion of Science (JSPS), coordinated by the University of Tokyo Graduate School of Medicine and Mahidol University. The study was also partly supported by a Grant for International Health Cooperation Research (11C-4) from the Ministry of Health and Welfare.
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