Identification of a conserved bacterial protein secretion system in <i>Vibrio cholerae</i> using the <i>Dictyostelium</i> host model system

TLDR

Vibrio cholerae, including O1/O139 cholera strains and non‑O1/non‑O139 isolates, survives eukaryotic predation and relies on poorly defined virulence mechanisms, many of which involve vas‑homologous genes whose disruption attenuates virulence in other Gram‑negative pathogens. The study aims to characterize VAS genes as a prototypic type VI secretion system in non‑O1/non‑O139 V. cholerae. The authors used Dictyostelium discoideum to identify a VAS‑mediated extracellular protein translocation system lacking N‑terminal hydrophobic leaders.

Abstract

The bacterium Vibrio cholerae, like other human pathogens that reside in environmental reservoirs, survives predation by unicellular eukaryotes. Strains of the O1 and O139 serogroups cause cholera, whereas non-O1/non-O139 strains cause human infections through poorly defined mechanisms. Using Dictyostelium discoideum as a model host, we have identified a virulence mechanism in a non-O1/non-O139 V. cholerae strain that involves extracellular translocation of proteins that lack N-terminal hydrophobic leader sequences. Accordingly, we have named these genes "VAS" genes for virulence-associated secretion, and we propose that these genes encode a prototypic "type VI" secretion system. We show that vas genes are required for cytotoxicity of V. cholerae cells toward Dictyostelium amoebae and mammalian J774 macrophages by a contact-dependent mechanism. A large number of Gram-negative bacterial pathogens carry genes homologous to vas genes and potential effector proteins secreted by this pathway (i.e., hemolysin-coregulated protein and VgrG). Mutations in vas homologs in other bacterial species have been reported to attenuate virulence in animals and cultured macrophages. Thus, the genes encoding the VAS-related, type VI secretion system likely play an important conserved function in microbial pathogenesis and represent an additional class of targets for vaccine and antimicrobial drug-based therapies.