| Vibrio cholerae is a Gram-negative bacterial species that can cause diarrheal disease in humans. Cholera is caused by O1 and O139 strains that rely on cholera toxin and toxin co-regulated pilus while non-O1/non-O139 strains can utilize other factors to cause milder disease. Using an amoebal model host, a virulent strain of V. cholerae was identified that employed a novel virulence mechanism, a constitutive type VI secretion system (T6SS). Most V. cholerae strains encode T6SS genes, but only some strains constitutively secrete T6SS substrate proteins. These substrates include Hcp and VgrG proteins, two classes of proteins that resemble components of T4 bacteriophage tail proteins. While T6SS genes are generally well conserved among V. cholerae strains, vgrG loci contain diverse regions. In some strains, this results in different C-terminal extensions, which are putative effector domains. Some strains encode vgrG-1 with a C-terminal actin cross-linking domain (ACD), which is necessary for T6SS-mediated virulence in the models examined.;In cell culture and infant mouse infection models, T6SS can function to translocate the ACD. Translocation of native and heterologous C-terminal effector domains of VgrG-1 can occur, supporting the model of VgrG delivery of effector domains. A survey of phagocytic and non-phagocytic cell lines, pharmacological manipulation of macrophages and artificial induction of opsonophagocytosis in non-phagocytic cells indicate that translocation of VgrG-1 requires uptake of V. cholerae into the host cell. These studies suggest that phagocytes are the natural target of V. cholerae T6SS. To determine whether T6SS functions in the context of an intestinal infection, we used a modified infant mouse model of infection in which a diarrheal response occurs after a high inoculum infection. This diarrheal phenotype is characterized by actin cross-linking in the gut, massive cellular infiltrate in the intestinal lumen and changes in host gene expression consistent with inflammation. T6SS delivery of the ACD can occur in vivo and is associated with an inflammatory response in an intestinal infection.;T6SS is conserved among V. cholerae strains but are regulated differentially, presumably in adaptation to external pressures. These pressures could include predation by environmental phagocytes or immune cell phagocytes within an intestinal infection, contexts in which V. cholerae T6SS could naturally function. |
