title: 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.
abstract: Borderline oxacillin-resistant Staphylococcus aureus (BORSA) exhibit oxacillin MIC values of 1-8 microg 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 beta-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 Gln(629)-->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 microg 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.
authors.affiliation: University of Toronto Department of Laboratory Medicine and Pathobiology, and Sunnybrook and Women's College Health Sciences Centre, Department of Microbiology, Toronto, ON, Canada.
authors.forename: Jeya
authors.initials: J
authors.lastname: Nadarajah
authors.affiliation: University of Toronto Department of Laboratory Medicine and Pathobiology, and Sunnybrook and Women's College Health Sciences Centre, Department of Microbiology, Toronto, ON, Canada.
authors.forename: Mark J S
authors.initials: MJS
authors.lastname: Lee
authors.affiliation: University of Toronto Department of Laboratory Medicine and Pathobiology, and Sunnybrook and Women's College Health Sciences Centre, Department of Microbiology, Toronto, ON, Canada.
authors.forename: Lisa
authors.initials: L
authors.lastname: Louie
authors.affiliation: University of Toronto Department of Laboratory Medicine and Pathobiology, and Sunnybrook and Women's College Health Sciences Centre, Department of Microbiology, Toronto, ON, Canada.
authors.forename: Latha
authors.initials: L
authors.lastname: Jacob
authors.affiliation: University of Toronto Department of Laboratory Medicine and Pathobiology, and Sunnybrook and Women's College Health Sciences Centre, Department of Microbiology, Toronto, ON, Canada.
authors.forename: Andrew E
authors.initials: AE
authors.lastname: Simor
authors.affiliation: University of Calgary, Calgary, AB, Canada.
authors.forename: Marie
authors.initials: M
authors.lastname: Louie
authors.affiliation: University of Toronto Department of Laboratory Medicine and Pathobiology, and Sunnybrook and Women's College Health Sciences Centre, Department of Microbiology, Toronto, ON, Canada.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: McGavin
mesh_terms: D019943:Amino Acid Substitution; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics*; D002170:Canada; D024881:Drug Resistance, Bacterial; D008969:Molecular Sequence Data; D010068:Oxacillin / Q000494:pharmacology*; D046915:Penicillin-Binding Proteins / Q000235:genetics*; D010458:Peptidyl Transferases / Q000235:genetics; D020641:Polymorphism, Single Nucleotide; D013203:Staphylococcal Infections / Q000382:microbiology; D013211:Staphylococcus aureus / Q000737:chemistry / Q000187:drug effects* / Q000235:genetics*
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D046915:Penicillin-Binding Proteins; D010458:Peptidyl Transferases; D010068:Oxacillin