Molecular genetics of methicillin-resistant Staphylococcus aureus.

A large and growing proportion of Staphylococcus aureus clinical isolates are methicillin resistant, and are resistant to practically all beta-lactam antibiotics. Methicillin-resistant S. aureus (MRSA) strains harbor mecA, which is carried by a unique mobile genetic element, staphylococcal cassette chromosome mec (SCCmec) integrated into the S. aureus chromosome. The mecA gene encodes a methicillin-insensitive transpeptidase, the production of which confers resistance to otherwise inhibitory concentrations of beta-lactam antibiotics. Several distinct clones have been identified among MRSA that apparently have been generated by integration of distinct types of SCCmec. While MRSA are primarily nosocomial pathogens, recent observations indicate that other MRSA clones are colonizing a significant proportion of healthy individuals in the community as well. Community-acquired MRSA (C-MRSA), may become a new threat to humans, and international cooperation of researchers and clinicians will be of cardinal importance in addressing this problem.