Department of Molecular Medicine, University of Pavia, Pavia, Italy.
Department of Pharmaceutical and Pharmacological Sciences, University of Padua, Padua, Italy.
BMC Microbiol. 2024 Jun 22;24(1):221. doi: 10.1186/s12866-024-03374-6.
Group B Streptococcus (GBS) is a commensal of healthy adults and an important pathogen in newborns, the elderly and immunocompromised individuals. GBS displays several virulence factors that promote colonisation and host infection, including the ST-17 strain-specific adhesin Srr2, previously characterised for its binding to fibrinogen. Another common target for bacterial adhesins and for host colonization is fibronectin, a multi-domain glycoprotein found ubiquitously in body fluids, in the extracellular matrix and on the surface of cells.
In this study, fibronectin was identified as a novel ligand for the Srr2 adhesin of GBS. A derivative of the ST-17 strain BM110 overexpressing the srr2 gene showed an increased ability to bind fibrinogen and fibronectin, compared to the isogenic wild-type strain. Conversely, the deletion of srr2 impaired bacterial adhesion to both ligands. ELISA assays and surface plasmon resonance studies using the recombinant binding region (BR) form of Srr2 confirmed a direct interaction with fibronectin with an estimated Kd of 92 nM. Srr2-BR variants defective in fibrinogen binding also exhibited no interaction with fibronectin, suggesting that Srr2 binds this ligand through the dock-lock-latch mechanism, previously described for fibrinogen binding. The fibronectin site responsible for recombinant Srr2-BR binding was identified and localised in the central cell-binding domain of the protein. Finally, in the presence of fibronectin, the ability of a Δsrr2 mutant to adhere to human cervico-vaginal epithelial cells was significantly lower than that of the wild-type strain.
By combining genetic and biochemical approaches, we demonstrate a new role for Srr2, namely interacting with fibronectin. We characterised the molecular mechanism of this interaction and demonstrated that it plays a role in promoting the adhesion of GBS to human cervico-vaginal epithelial cells, further substantiating the role of Srr2 as a factor responsible for the hypervirulence of GBS ST-17 strains. The discovery of the previously undescribed interaction between Srr2 and fibronectin establishes this adhesin as a key factor for GBS colonisation of host tissues.
B 群链球菌（GBS）是健康成人的共生菌，也是新生儿、老年人和免疫功能低下者的重要病原体。GBS 具有多种促进定植和宿主感染的毒力因子，包括以前被证实与纤维蛋白原结合的 ST-17 株特异性黏附素 Srr2。另一种细菌黏附素和宿主定植的常见靶标是纤维连接蛋白，它是一种广泛存在于体液、细胞外基质和细胞表面的多功能糖蛋白。
在这项研究中，纤维连接蛋白被确定为 GBS Srr2 黏附素的一种新配体。与遗传同源野生型菌株相比，过表达 srr2 基因的 ST-17 菌株 BM110 的衍生物显示出增强的纤维蛋白原和纤维连接蛋白结合能力。相反，srr2 的缺失削弱了细菌对两种配体的黏附。使用 Srr2 的重组结合区（BR）形式进行 ELISA 测定和表面等离子体共振研究证实了与纤维连接蛋白的直接相互作用，估计 Kd 为 92 nM。与纤维蛋白原结合缺陷的 Srr2-BR 变体也没有与纤维连接蛋白相互作用，这表明 Srr2 通过先前描述的纤维蛋白原结合的对接-锁定-闩锁机制结合该配体。确定了负责重组 Srr2-BR 结合的纤维连接蛋白结合位点，并将其定位在蛋白质的中央细胞结合域。最后，在纤维连接蛋白存在的情况下，Δsrr2 突变体与人类宫颈阴道上皮细胞黏附的能力明显低于野生型菌株。
通过结合遗传和生化方法，我们证明了 Srr2 的一个新作用，即与纤维连接蛋白相互作用。我们对这种相互作用的分子机制进行了表征，并证明它在促进 GBS 与人宫颈阴道上皮细胞黏附中发挥作用，进一步证实了 Srr2 作为 GBS ST-17 株高毒力的因素的作用。Srr2 与纤维连接蛋白之间以前未描述的相互作用的发现确立了这种黏附素是 GBS 定植宿主组织的关键因素。
