The assembly of a hybrid type IV secretion system by a Crohn’s disease-associated Escherichia coli strain

Type IV secretion systems (T4SSs) are central to bacterial pathogenesis due to their versatile functions. While traditionally known for their role in DNA transfer via conjugation and secretion of effector proteins, T4SSs have been shown to mediate biofilm formation in various bacteria. These biofilms are critical for the fitness of adherent-invasive strains of Escherichia coli (AIEC), which are commonly isolated from Crohn’s disease patients and are known for propelling gut inflammation. Many AIEC strains carry F-like plasmids encoding the IncF subgroup of T4SSs. Unlike minimized systems that comprise 12 core components, the IncF family has evolved into an expanded T4SS through the acquisition of additional genes that enhance conjugation. Here, we show that a biofilm-forming AIEC strain harbors an unusual IncF plasmid that lacks two conserved components otherwise considered essential for T4SS functionality. We demonstrate that this strain forms a natural hybrid T4SS, where the two components missing in the plasmid are supplied by a co-residing chromosomal T4SS present on an integrative and conjugative element (ICE). Using biochemical assays, we show that this functional machine is a mosaic of IncF and ICE-encoded proteins that co-operatively drive pilin polymerization and biofilm formation on epithelial cells. Furthermore, we show that a subpopulation of bacteria expresses the IncF and ICE-encoded genes in response to host cells, leading to the assembly of biofilms that promote AIEC fitness in the gut. Together, these findings uncover a crosstalk between two co-residing and evolutionary distant mobile genetic elements to form a hybrid T4SS that mediates biofilm biogenesis by a Crohn’s disease-associated pathogen.

第四型分泌系统（Type IV secretion systems，T4SSs）凭借其多功能性，在细菌致病机制中居于核心地位。既往研究认为T4SS的核心功能为通过接合作用介导DNA转移与效应蛋白分泌，现有研究表明，该系统还可介导多种细菌形成生物被膜。这类生物被膜对黏附侵袭性大肠埃希菌（adherent-invasive Escherichia coli，AIEC）的生存适应性至关重要；AIEC常从克罗恩病（Crohn’s disease）患者体内分离得到，其可驱动肠道炎症反应。多数AIEC菌株携带编码IncF亚型T4SS的F型质粒。与仅包含12种核心组分的简化型T4SS不同，IncF家族通过获取额外的增强接合功能的基因，进化为扩张型T4SS。本研究发现一株可形成生物被膜的AIEC菌株携带一种罕见的IncF质粒，该质粒缺失了两种通常被认为是T4SS发挥功能所必需的保守组分。研究证实，该菌株可形成天然杂合T4SS：质粒缺失的两种组分由共存于整合接合元件（integrative and conjugative element，ICE）上的染色体编码T4SS提供。通过生化实验分析，我们发现该功能性分泌系统是IncF与ICE编码蛋白的嵌合体，二者协同驱动菌毛聚合并在上皮细胞表面形成生物被膜。此外，研究发现部分细菌亚群可响应宿主细胞表达IncF及ICE编码基因，进而组装生物被膜，提升AIEC在肠道内的生存适应性。综上，本研究揭示了两种共存且进化距离较远的可移动遗传元件之间的交叉互作，它们可形成杂合T4SS，介导克罗恩病相关病原菌的生物被膜生物发生。