University of Kentucky College of Medicine , Lexington, Kentucky, USA.
mSphere. 2023 Aug 24;8(4):e0030323. doi: 10.1128/msphere.00303-23. Epub 2023 Jul 27.
Belonging to the two-partner secretion family of proteins, contact-dependent growth inhibition (CDI) systems mediate interbacterial antagonism among closely related Gram-negative bacteria. The toxic portion of a large surface protein, BcpA/CdiA, is delivered to the cytoplasm of neighboring cells where it inhibits growth. Translocation of the antibacterial polypeptide out of the producing cell requires an associated outer membrane transporter, BcpB/CdiB. Some bacteria, including many species, encode multiple distinct CDI systems, but whether there is interaction between these systems is largely unknown. Using complex species as a model, here we show that related BcpB transporters exhibit considerable secretion flexibility and can secrete both cognate and non-cognate BcpA substrates. We also identified an additional unique CDI system capable of mediating interbacterial competition and demonstrated that its BcpB transporter has similar relaxed substrate specificity. Our results showed that two BcpB transporters (BcpB-2 and BcpB-3) were able to secrete all four of the BcpA toxins, while one transporter (BcpB-1) appeared unable to secrete even its cognate BcpA substrate under the tested conditions. This flexibility provided a competitive advantage, as strains lacking the full repertoire of BcpB proteins had decreased CDI activity. Similar results were obtained in , suggesting that secretion flexibility may be a conserved feature of CDI systems. Together these findings suggest that the interaction between distinct CDI systems enhances the efficiency of bacterial antagonism. IMPORTANCE The complex (Bcc) is a group of related opportunistic bacterial pathogens that occupy a diverse range of ecological niches and exacerbate disease in patients with underlying conditions. Contact-dependent growth inhibition (CDI) system proteins, produced by Gram-negative bacteria, contain antagonistic properties that allow for intoxication of closely related neighboring bacteria via a secreted protein, BcpA. Multiple unique CDI systems can be found in the same bacterial strain, and here we show that these distinct systems interact in several Bcc species. Our findings suggest that the interaction between CDI system proteins is important for interbacterial toxicity. Understanding the mechanism of interplay between CDI systems provides further insight into the complexity of bacterial antagonism. Moreover, since many bacterial species are predicted to encode multiple CDI systems, this study suggests that interactions between these distinct systems likely contribute to the overall competitive fitness of these species.
属于双伴侣分泌家族的蛋白质，接触依赖性生长抑制（CDI）系统介导密切相关的革兰氏阴性细菌之间的细菌拮抗作用。大表面蛋白 BcpA/CdiA 的毒性部分被递送到邻近细胞的细胞质中，在那里它抑制生长。抗菌多肽从产生细胞中的易位需要相关的外膜转运蛋白 BcpB/CdiB。一些细菌，包括许多种，编码多个不同的 CDI 系统，但这些系统之间是否存在相互作用在很大程度上是未知的。在这里，我们使用复杂的物种作为模型，表明相关的 BcpB 转运蛋白表现出相当大的分泌灵活性，可以分泌同源和非同源的 BcpA 底物。我们还鉴定了一种额外的独特的 CDI 系统，能够介导细菌间竞争，并证明其 BcpB 转运蛋白具有类似的宽松底物特异性。我们的结果表明，两种 BcpB 转运蛋白（BcpB-2 和 BcpB-3）能够分泌所有四种 BcpA 毒素，而一种转运蛋白（BcpB-1）在测试条件下似乎甚至不能分泌其同源的 BcpA 底物。这种灵活性提供了竞争优势，因为缺乏全套 BcpB 蛋白的菌株的 CDI 活性降低。在 中也得到了类似的结果，这表明分泌灵活性可能是 CDI 系统的一个保守特征。这些发现表明，不同 CDI 系统之间的相互作用增强了细菌拮抗作用的效率。重要的是，复杂的（Bcc）是一组相关的机会性病原体细菌，它们占据了广泛的生态位，并在有潜在疾病的患者中加重疾病。接触依赖性生长抑制（CDI）系统蛋白由革兰氏阴性细菌产生，含有拮抗性质，通过分泌蛋白 BcpA 使密切相关的邻近细菌中毒。在同一细菌菌株中可以发现多个独特的 CDI 系统，在这里我们表明这些不同的系统在几个 Bcc 物种中相互作用。我们的研究结果表明，CDI 系统蛋白之间的相互作用对于细菌间毒性很重要。了解 CDI 系统之间相互作用的机制为细菌拮抗作用的复杂性提供了进一步的见解。此外，由于许多细菌物种预计编码多个 CDI 系统，因此该研究表明，这些不同系统之间的相互作用可能有助于这些物种的整体竞争适应性。
