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An integrated type VI secretion / type I fimbriae system co-regulates attachment and secretion as part of a two component system-regulated bacterial lifestyle switch
Type VI secretion systems (T6SS) allow bacteria to inject effector proteins into neighboring cells. While these systems can target a wide range of cell types, they require direct contact with their target cells. Although attachment is therefore essential for the successful injection of effectors, the coordination of secretion and attachment remains poorly understood. The single T6SS of Yersinia enterocolitica is encoded across two neighboring operons, both of which harbor genes for type I fimbriae. While the system is inactive under standard laboratory conditions, it is activated by a single nucleotide polymorphism in the histidine kinase of a two-component system. Beyond the integrated T6SS/T1F system, this strain displays strong upregulation of other attachment systems and a concurrent drastic downregulation of flagella and chemotaxis, indicating a switch from a motile, planktonic lifestyle to an adherent, secretion-active state. Transmission electron and fluorescence microscopy revealed increased pilus production and a dense arrangement of T6SS structures. Proteomics analyses enabled us to delineate the regulons involved in this lifestyle switch and to identify a T6SS effector nuclease. Given that the genetic arrangement is conserved in various pathogenic bacteria, the integration of T6SS and fimbrial attachment systems and the genetic switch described herein may be a common feature allowing bacteria to adapt to changing conditions.
