Activation and biological role of the T6SS/T1F secretion/adhesion system in the human pathogen Yersinia enterocolitica

Bacterial secretion systems are crucial for the virulence and competition of many bacterial pathogens. Among these, the type VI secretion system (T6SS) functions as a contractile molecular nanomachine, delivering toxic effector proteins into eukaryotic or prokaryotic target cells. The T6SS consists of 13 highly conserved core components, typically organized in gene clusters. The human pathogen Yersinia enterocolitica harbors a special type of T6SS cluster that also incorporates genes for type I fimbriae (T1F), structures typically involved in surface adhesion. We found that these genes are co-regulated, and their activation leads to a distinctive switch in bacterial cell biology and attachment. To determine the biological role and function of the T6SS/T1F cluster we aimed to characterize the expression and activation conditions of the T6SS/T1F cluster and identify T6SS effector proteins. Live cell microscopy of bacteria expressing functional fluorescent labeled TssB, a core component of the T6SS sheath allowed to visualize the expression and assembly of the T6SS under various conditions. We identified regulatory elements that strongly increased the expression levels of T6SS/T1F components. Proteomic analysis in these strains led to the identification of the first T6SS effector in Y. enterocolitica and its putative immunity protein. Examining the role of this effector yielded insights into bioactivity and target specificity of the T6SS/T1F system and its function in Y. enterocolitica virulence.