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. is an exclusive mutualistic symbiont found within the light organs of Hawaiian bobtail squid. This symbiotic interaction requires 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 establishes a beneficial association with its host. In this study, we observe that induces mortality in zebrafish embryos and nauplii. Non-lethal doses of 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 -induced mortality of zebrafish embryos and nauplii. These results support the hypothesis that the T6SS1 is involved in eukaryotic cell interactions. Despite its traditional view as a beneficial symbiont, we provide evidence that is capable of harming aquatic organisms, indicating its potential to be pathogenic toward non-symbiotic hosts.IMPORTANCE 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 can also cause harm, killing zebrafish embryos and brine shrimp larvae. We pinpoint one of 's two type VI secretion systems (T6SS1) as a key factor in this pathogenicity. These findings reveal that is not strictly a mutualistic microbe but can act like a pathogen under certain conditions. This broadens our understanding of how could interact with different hosts and offers new insights into the dual roles bacteria can play in nature.