Methicillin/oxacillin (Oxa) resistance in Staphylococcus aureus is primarily mediated by the acquired penicillin-binding protein (PBP2a) encoded by mecA. PBP2a acts together with native PBP2 to mediate oxacillin resistance by contributing complementary transpeptidase and transglycosylase activities, respectively. The VraS/VraR two-component regulatory system is inducible by cell-wall antimicrobials (beta-lactams, glycopeptides) and controls transcriptional induction of many cell-wall genes including pbp2 and itself. We investigated the role of VraS/VraR in the phenotypic expression of oxacillin resistance by inactivating vraS in community-acquired MRSA clinical isolates that lack functional genes encoding the mecA regulatory sequences mecI and mecR1. Inactivation of vraS abrogated oxacillin resistance, and complementation with the vraS operon restored the resistance phenotype. mecA transcription increased in the vraS mutants; however, PBP2a abundance was similar to that of the wild type. Although pbp2 transcription decreased in the vraS mutants, overexpression of the pbp2 operon did not restore resistance. These data demonstrate that although expressions of mecA and pbp2 are required for oxacillin resistance, they are not sufficient. Therefore, the vraS/vraR regulatory system plays a crucial role in allowing MRSA to respond to beta-lactams by regulation of a gene target other than the known effectors of methicillin resistance.

译文

金黄色葡萄球菌对甲氧西林/苯唑西林 (Oxa) 的耐药性主要由mecA编码的获得性青霉素结合蛋白 (PBP2a) 介导。PBP2a与天然PBP2一起通过分别贡献互补的转肽酶和转糖基化酶活性来介导苯唑西林抗性。VraS/VraR两组分调节系统可由细胞壁抗菌剂 (β-内酰胺,糖肽) 诱导,并控制许多细胞壁基因 (包括pbp2及其自身) 的转录诱导。我们通过在缺乏编码mecA调节序列mecI和mecr1的功能基因的社区获得性MRSA临床分离株中灭活VraS,研究了vraS/VraR在苯唑西林抗性表型表达中的作用。vraS的失活消除了苯唑西林的抗性,并与vraS操纵子互补恢复了抗性表型。vraS突变体中的mecA转录增加; 然而,PBP2a的丰度与野生型相似。尽管vraS突变体中的pbp2转录降低,但pbp2操纵子的过表达并不能恢复抗性。这些数据表明,尽管mecA和pbp2的表达对于苯唑西林抗性是必需的,但它们还不够。因此,vraS/vraR调节系统在允许MRSA通过调节除甲氧西林抗性的已知效应子以外的基因靶标来响应 β-内酰胺方面起着至关重要的作用。

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