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-O1non-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-O1non-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.