A mutualistic model bacterium is lethal to non-symbiotic hosts via the type VI secretion system

What makes a bacterium pathogenic? Since the early days of germ theory, researchers have categorized bacteria as pathogens or non-pathogens, those that cause harm and those that do not, but this binary view is not always accurate. <i>Vibrio fischeri</i> is an exclusive mutualistic symbiont found within the light organs of Hawaiian bobtail squid. This symbiotic interaction requires <i>V. fischeri</i> to utilize a range of behaviors and produce molecules that are often associated with pathogenicity. This juxtaposition of employing "pathogenic" behaviors for a symbiotic relationship led the field to focus on how <i>V. fischeri</i> establishes a beneficial association with its host. In this study, we observe that <i>V. fischeri</i> induces mortality in zebrafish embryos and <i>Artemia</i> nauplii. Non-lethal doses of <i>V. fischeri</i> lead to zebrafish growth delays and phenotypes indicative of disease. Our data also provide evidence that the conserved type VI secretion system on chromosome I (T6SS1) plays a role in the <i>V. fischeri</i>-induced mortality of zebrafish embryos and <i>Artemia</i> nauplii. These results support the hypothesis that the <i>V. fischeri</i> T6SS1 is involved in eukaryotic cell interactions. Despite its traditional view as a beneficial symbiont, we provide evidence that <i>V. fischeri</i> is capable of harming aquatic organisms, indicating its potential to be pathogenic toward non-symbiotic hosts.IMPORTANCE<i>Vibrio fischeri</i> is best known for its beneficial partnership with the Hawaiian bobtail squid, where it uses molecular tools often associated with disease-causing bacteria. Our research shows that <i>V. fischeri</i> can also cause harm, killing zebrafish embryos and brine shrimp larvae. We pinpoint one of <i>V. fischeri</i>'s two type VI secretion systems (T6SS1) as a key factor in this pathogenicity. These findings reveal that <i>V. fischeri</i> is not strictly a mutualistic microbe but can act like a pathogen under certain conditions. This broadens our understanding of how <i>V. fischeri</i> could interact with different hosts and offers new insights into the dual roles bacteria can play in nature.