Abstract
ObjectivesMethicillin resistance in Staphylococcus spp. results from the expression of an alternative penicillin-binding protein 2a (encoded by mecA) with a low affinity for β-lactam antibiotics. Recently, a novel variant of mecA known as mecC (formerly mecALGA251) was identified in Staphylococcus aureus isolates from both humans and animals. In this study, we identified two Staphylococcus sciuri subsp. carnaticus isolates from bovine infections that harbour three different mecA homologues: mecA, mecA1 and mecC.MethodsWe subjected the two isolates to whole-genome sequencing to further understand the genetic context of the mec-containing region. We also used PCR and RT–PCR to investigate the excision and expression of the SCCmec element and mec genes, respectively.ResultsWhole-genome sequencing revealed a novel hybrid SCCmec region at the orfX locus consisting of a class E mec complex (mecI-mecR1-mecC1-blaZ) located immediately downstream of a staphylococcal cassette chromosome mec (SCCmec) type VII element. A second SCCmec attL site (attL2), which was imperfect, was present downstream of the mecC region. PCR analysis of stationary-phase cultures showed that both the SCCmec type VII element and a hybrid SCCmec-mecC element were capable of excision from the genome and forming a circular intermediate. Transcriptional analysis showed that mecC and mecA, but not mecA1, were both expressed in liquid culture supplemented with oxacillin.ConclusionsOverall, this study further highlights that a range of staphylococcal species harbour the mecC gene and furthers the view that coagulase-negative staphylococci associated with animals may act as reservoirs of antibiotic resistance genes for more pathogenic staphylococcal species.
Highlights
A wide range of staphylococcal species harbour the mecA gene encoding an alternative penicillin-binding protein 2a (PBP2a), which has a low affinity for b-lactam antibiotics and allows cell wall synthesis to occur in the presence of b-lactam antibiotics.[1,2,3,4] mecA, along with its cognate regulators mecI-mecR1, are acquired as part of a larger mobile element known as staphylococcal cassette chromosome mec (SCCmec).[5]
Coagulase-negative staphylococcal species are thought to be the source of mecA for methicillin-resistant Staphylococcus aureus (MRSA), with a number of studies having identified likely in vivo transfer events from a coagulase-negative staphylococcal species to S. aureus.[8,9,10]
The evolutionary origins of the mecA gene are thought to lie in the common ancestor of Staphylococcus fleurettii, Staphylococcus vitulinus and Staphylococcus sciuri,[11,12,13] further supported by experimental evidence that the mecA1 gene of S. sciuri is capable of mediating high-level b-lactam resistance in S. aureus.[13]
Summary
A wide range of staphylococcal species harbour the mecA gene encoding an alternative penicillin-binding protein 2a (PBP2a), which has a low affinity for b-lactam antibiotics and allows cell wall synthesis to occur in the presence of b-lactam antibiotics.[1,2,3,4] mecA, along with its cognate regulators mecI-mecR1, are acquired as part of a larger mobile element known as staphylococcal cassette chromosome mec (SCCmec).[5]. SCCmec elements insert into the chromosome at the 3′ end of the orfX by site-specific recombination mediated by the CcrA and CcrB recombinases encoded on SCCmec.[6,7] Coagulase-negative staphylococcal species are thought to be the source of mecA for methicillin-resistant Staphylococcus aureus (MRSA), with a number of studies having identified likely in vivo transfer events from a coagulase-negative staphylococcal species to S. aureus.[8,9,10] The evolutionary origins of the mecA gene are thought to lie in the common ancestor of Staphylococcus fleurettii, Staphylococcus vitulinus and Staphylococcus sciuri,[11,12,13] further supported by experimental evidence that the mecA1 (pbpD) gene of S. sciuri is capable of mediating high-level b-lactam resistance in S. aureus.[13]
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