Nadarajah Jeya, Lee Mark J S, Louie Lisa, Jacob Latha, Simor Andrew E, Louie Marie, McGavin Martin J
University of Toronto Department of Laboratory Medicine and Pathobiology, and Sunnybrook and Women's College Health Sciences Centre, Department of Microbiology, Toronto, ON, Canada.
University of Calgary, Calgary, AB, Canada.
J Med Microbiol. 2006 Dec;55(Pt 12):1675-1683. doi: 10.1099/jmm.0.46700-0.
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.
耐苯唑西林金黄色葡萄球菌临界株（BORSA）的苯唑西林最低抑菌浓度（MIC）值为1 - 8微克/毫升，但缺乏编码低亲和力青霉素结合蛋白（PBP）2a的mecA基因。评估了BORSA表型与特定遗传背景的关系，以及正常PBP2中的氨基酸序列变异。在38株BORSA中，26株具有共同的基因组DNA脉冲场凝胶电泳（PFGE）图谱，属于多位点序列分型（ST）25。其他分离株在基因上具有多样性。对三株BORSA的pbp2完整序列进行了测定，它们分别对应ST25、ST1和ST47，这三株是根据缺乏blaZ编码的β-内酰胺酶而挑选出来的。还对另外七株ST25分离株的pbp2编码必需转肽酶结构域的片段进行了测序。所有BORSA的转肽酶结构域均发生了氨基酸替换，与克隆类型无关。Gln(629)-->Pro替换在所有ST25 BORSA中都很常见，但大多数可以通过转肽酶结构域中的其他独特替换相互区分。ST1和ST47分离株在转肽酶结构域中也有独特的替换。将ST25或ST1分离株的pbp2在金黄色葡萄球菌RN6390中进行质粒介导表达，可使其苯唑西林MIC从0.25微克/毫升增加到4微克/毫升，而敏感菌株ATCC 25923的pbp2则无此作用。因此，不同BORSA谱系的PBP2中不同的氨基酸替换导致了临界耐药性。主要的ST25谱系与包含耐甲氧西林金黄色葡萄球菌（MRSA）的五个克隆复合体中的任何一个均无关联，这表明ST25不易获得mecA介导的耐药性。
