H-NS is a conserved repressor of the type VI secretion system in Vibrio fischeri

The type VI secretion system (T6SS) is a broadly distributed interbacterial weapon found in both beneficial and pathogenic bacteria and can enhance a microbes ability to colonize a host. Vibrio fischeri is a beneficial symbiont of fish and squid and a model organism for T6SS function, which is activated in high-viscosity conditions. Previously, we isolated an hns mutant in a transposon screen to identify regulators of the T6SS in the fish symbiont V. fischeri MJ11. The hns gene encodes the DNA-binding protein, H-NS, a conserved global regulator of gene expression that aids in adaptation to changing environments. Quantitative transcriptomes of the hns mutant and parent strains grown in liquid or hydrogel media revealed hns is required for the global transcriptional changes that occur during transition from lower to higher viscosity conditions. Furthermore, T6SS gene transcripts are more abundant in the hns mutant in both conditions, suggesting H-NS represses T6SS in the parent. Single-cell fluorescence microscopy confirmed hns mutant cells make more T6SS weapons in both liquid and hydrogel medium, where the hns mutant is more proficient at killing a competitor strain, compared to the wild-type parent. Finally, disrupting the hns gene in additional light organ isolates resulted in a similar derepression of T6SS, indicating H-NS is a conserved repressor of this interbacterial weapon. This work furthers our understanding of the role of H-NS as a global regulator during environmental shifts in a host-associated bacterial symbiont and expands the list of species where H-NS represses T6SS to include V. fischeri.