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J Med Microbiol 55 (2006), 1675-1683; DOI: 10.1099/jmm.0.46700-0
© 2006 Society for General Microbiology
ISSN 1473-5644

Identification of different clonal complexes and diverse amino acid substitutions in penicillin-binding protein 2 (PBP2) associated with borderline oxacillin resistance in Canadian Staphylococcus aureus isolates

Jeya Nadarajah1, Mark J. S. Lee1, Lisa Louie1, Latha Jacob1, Andrew E. Simor1, Marie Louie2 and Martin J. McGavin1

1 University of Toronto Department of Laboratory Medicine and Pathobiology, and Sunnybrook and Women's College Health Sciences Centre, Department of Microbiology, Toronto, ON, Canada

2 University of Calgary, Calgary, AB, Canada

Correspondence
Martin J. McGavin
martin.mcgavin{at}sri.utoronto.ca

Received 26 April 2006
Accepted 8 August 2006

Borderline oxacillin-resistant Staphylococcus aureus (BORSA) exhibit oxacillin MIC values of 1–8 µg ml–1, but lack mecA, which encodes the low-affinity penicillin-binding protein (PBP)2a. The relationship of the BORSA phenotype with specific genetic backgrounds was assessed, as well as amino acid sequence variation in the normal PBP2. Among 38 BORSA, 26 had a common PFGE profile of genomic DNA, and were multilocus sequence type (ST)25. The other isolates were genetically diverse. Complete pbp2 sequences were determined for three BORSA, corresponding to ST25, ST1 and ST47, which were selected on the basis of lacking blaZ-encoded ß-lactamase. The essential transpeptidase-domain-encoding segment of pbp2 was also sequenced from seven additional ST25 isolates. Amino acid substitutions occurred in the transpeptidase domain of all BORSA, irrespective of clonal type. A Gln629->Pro substitution was common to all ST25 BORSA, but most could be distinguished from one another by additional unique substitutions in the transpeptidase domain. The ST1 and ST47 isolates also possessed unique substitutions in the transpeptidase domain. Plasmid-mediated expression of pbp2 from an ST25 or ST1 isolate in S. aureus RN6390 increased its oxacillin MIC from 0.25 to 4 µg ml–1, while pbp2 from a susceptible strain, ATCC 25923, had no effect. Therefore, different amino acid substitutions in PBP2 of diverse BORSA lineages contribute to borderline resistance. The predominant ST25 lineage was not related to any of the five clonal complexes that contain meticillin-resistant S. aureus (MRSA), suggesting that ST25 cannot readily acquire mecA-mediated resistance.

Abbreviations: BORSA, borderline oxacillin-resistant S. aureus; CC, clonal complex; MLST, multilocus sequence typing; MRSA, meticillin-resistant S. aureus; MSSA, meticillin-susceptible S. aureus; PBP, penicillin-binding protein; ST, sequence type.

The GenBank/EMBL/DDBJ accession numbers of pbp2 sequences determined from strains MA15 and MSH12 in this study are AF540019 and AF540020, respectively. Accession numbers AF540021–AF540028 were assigned for the respective partial nucleotide sequences of pbp2 from additional BORSA isolates MA4, MA6, MA8, MA9, MA13, MA14, MSH7 and OGH2.




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