Abstract
BackgroundMethicillin-resistance in staphylococci is conferred by the mecA gene, located on the genomic island Staphylococcal Cassette Chromosome mec (SCCmec). SCCmec mobility relies on the Ccr recombinases, which catalyze insertion and excision form the host’s chromosome. Although being a crucial step in its horizontal transfer, little is known about the dynamics of SCCmec excision.ResultsA quantitative PCR-based method was used to measure the rate of SCCmec excision by amplifying the chromosome–chromosome junction and the circularized SCCmec resulting from excision. SCCmec excision rate was measured in methicillin-resistant Staphylococcus aureus (MRSA) strain N315 at various growth times in broth cultures. In the present experimental settings, excision of SCCmec occurred at a rate of approximately 2 × 10−6 in MRSA N315.ConclusionThis work brings new insights in the poorly understood SCCmec excision process. The results presented herein suggest a model in which excision occurs during a limited period of time at the early stages of growth.Electronic supplementary materialThe online version of this article (doi:10.1186/s13104-015-1815-3) contains supplementary material, which is available to authorized users.
Highlights
Methicillin-resistance in staphylococci is conferred by the mecA gene, located on the genomic island Staphylococcal Cassette Chromosome mec (SCCmec)
Amplicons qEx and qCirc were used to detect excisants and circularized SCCmec, respectively. These were compared to the total number of chromosomes, which was determined by targeting a region of the rlmH gene, present in both Methicillin-resistant S. aureus (MRSA)- and MSSA-like cells
Rate of SCCmec excision in standard experimental conditions Quantitative PCR was used to measure the amounts of both unoccupied chromosomal sites and SCCmec circular forms by specific amplification of the reconstituted chromosomal insertion site obtained after SCCmec excision and the junction formed upon SCCmec circularization, respectively (Fig. 1). These were compared to the total number of chromosomes, which was determined by targeting a region of the rlmH gene, present in both MRSA- and MSSA-like cells (Fig. 1)
Summary
Methicillin-resistance in staphylococci is conferred by the mecA gene, located on the genomic island Staphylococcal Cassette Chromosome mec (SCCmec). Many of the virulence factors encoded by S. aureus are located within mobile genetic elements, which come in addition to the well-conserved core genome, and account for the majority of genetic variation between the isolates [2,3,4]. This is often the case for antibiotic resistance genes, which can be found on plasmids, transposons or genomic islands [4].
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