Mec gen

The mecA gene is a component of a novel mobile gene element, the chromosomal staphylococcal gene cassette (SCCmec). The associated specific cassette chromosome recombinases ccrA and ccrB enable the element to be transferred from one bacterium to another. The SCCmec elements found so far vary in their structure. So far, 4 SCCmec types have been found: SCCmec types I to IV.

MecA encodes for the penicillin-binding protein PBP2a. Staphylococci possess five different membrane-bound penicillin-binding proteins (PBPs). These penicillin-binding proteins are necessary for the construction of the peptidoglycan cell wall of the bacteria. They cause the cross-linking of the peptidoglycan layers of the cell wall through transpeptidation. Betalactam antibiotics have a 1000-fold reduced affinity for the new PBP2a and can therefore no longer block it. As a result, cell wall synthesis continues undisturbed and the bacteria are not killed (methicillin-resistant S. aureus = MRSA). The methicillin resistance determined by the mecA gene leads to the loss of all penicillins, cephalosporins and carbapenems.

1. Hanssen AM et al (2006) SCCmec in staphylococci: genes on the move. FEMS Immunology & Medical Microbiology. 46: 8–20.
2. Hiramatsu K et al (2001) The emergence and evolution of methicillin-resistant Staphylococcus aureus. Trends in Microbiology 9: 486-493.
3. Katayama Y et al (2001) Genetic organization of the chromosome region surrounding mecA in clinical staphylococcal strains: role of IS431-mediated mecI deletion in expression of resistance in mecA-carrying, low-level methicillin-resistant Staphylococcus haemolyticus. Antimicrobial agents and chemotherapy 45: 1955-1963.