tandem amplification of the staphylococcal cassette chromosome mec element can drive high-level methicillin resistance in methicillin-resistant staphylococcus aureus
Gallagher, Laura A. Coughlan, Simone Black, Nikki S. Lalor, Pierce Waters, Elaine M. Wee, Bryan Watson, Mick Downing, Tim Fitzgerald, J. Ross Fleming, Gerard T. A.
Gallagher, Laura A.
Coughlan, Simone
Black, Nikki S.
Lalor, Pierce
Waters, Elaine M.
Wee, Bryan
Watson, Mick
Downing, Tim
Fitzgerald, J. Ross
Fleming, Gerard T. A.
Repository DOI
Publication Date
2017-07-17
Type
Article
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Citation
Gallagher, Laura A. Coughlan, Simone; Black, Nikki S.; Lalor, Pierce; Waters, Elaine M.; Wee, Bryan; Watson, Mick; Downing, Tim; Fitzgerald, J. Ross; Fleming, Gerard T. A.; O'Gara, James P. (2017). tandem amplification of the staphylococcal cassette chromosome mec element can drive high-level methicillin resistance in methicillin-resistant staphylococcus aureus . Antimicrobial Agents and Chemotherapy 61 (9),
Abstract
Hospital-associated methicillin-resistant Staphylococcus aureus (MRSA) strains typically express high-level, homogeneous (HoR) beta-lactam resistance, whereas community-associated MRSA (CA-MRSA) more commonly express low-level heterogeneous (HeR) resistance. Expression of the HoR phenotype typically requires both increased expression of the mecA gene, carried on the staphylococcal cassette chromosome mec element (SCCmec), and additional mutational event(s) elsewhere on the chromosome. Here the oxacillin concentration in a chemostat culture of the CA-MRSA strain USA300 was increased from 8 mu g/ml to 130 mu g/ml over 13 days to isolate highly oxacillin-resistant derivatives. A stable, small-colony variant, designated HoR34, which had become established in the chemostat culture was found to have acquired mutations in gdpP, clpX, guaA, and camS. Closer inspection of the genome sequence data further revealed that reads covering SCCmec were similar to 10 times overrepresented compared to other parts of the chromosome. Quantitative PCR (qPCR) confirmed >10-fold-higher levels of mecA DNA on the HoR34 chromosome, and MinION genome sequencing verified the presence of 10 tandem repeats of the SCCmec element. qPCR further demonstrated that subculture of HoR34 in various concentrations of oxacillin (0 to 100 mu g/ml) was accompanied by accordion-like contraction and amplification of the SCCmec element. Although slower growing than strain USA300, HoR34 outcompeted the parent strain in the presence of subinhibitory oxacillin. These data identify tandem amplification of the SCCmec element as a new mechanism of high-level methicillin resistance in MRSA, which may provide a competitive advantage for MRSA under antibiotic selection.
Funder
Publisher
American Society for Microbiology
Publisher DOI
10.1128/aac.00869-17
Rights
Attribution-NonCommercial-NoDerivs 3.0 Ireland