Shared reservoir of ccrB gene sequences between coagulase-negative staphylococci and methicillin-resistant Staphylococcus aureus

Methicillin resistance in Staphylococcus aureus and coagulase-negative staphylococci (CoNS) is caused by expression of the low-affinity penicillin-binding protein (PBP) 2a encoded by the mecA gene. This gene is carried on the staphylococcal cassette chromosome mec (SCCmec) of which several types and subtypes have been described. CoNS and S. aureus share SCCmec types and it has been suggested that CoNS are a potential reservoir of mecA for S. aureus. Evidence for this is mainly based on PCR typing of SCCmec or on sequence-based methods including only a limited number of strains. In this study, we determined the genetic relatedness of ccrB sequences contained in SCCmec elements of a spatio-temporally diverse and comprehensive collection of methicillin-resistant CoNS and S. aureus. Part of the ccrB genes of 367 methicillin-resistant CoNS and 94 methicillin-resistant S. aureus (MRSA) were sequenced and compared. The data revealed that 92 of 94 (98) MRSA isolates carried ccrB genes, involving different ccrB alleles, which were indistinguishable from ccrB genes of methicillin-resistant CoNS. In total, 273 of 367 (74) CoNS shared ccrB gene sequences with MRSA. The high rate of identical ccrB sequences in a geographically, temporally and genotypically diverse set of S. aureus and CoNS isolates indicates frequent horizontal transfer of SCCmec between CoNS and S. aureus, which may have contributed to the emergence of MRSA.