Flagella are required to activate expression of aggregation factors necessary for T6SS-mediated competition in...

Bacteria employ antagonistic strategies to eliminate competitors of an ecological niche. Contact-dependent mechanisms, such as the type VI secretion system (T6SS), are prevalent in host-associated bacteria, yet we know relatively little about how T6SS+ strains make contact with competitors in highly viscous environments, such as host mucus. To better understand how cells respond to and contact one another in such environments, we performed a genome-wide transposon mutant screen of the T6SS-wielding beneficial bacterial symbiont, Vibrio fischeri MJ11, and identified two sets of genes that are conditionally required for killing. We found that surface modification and flagellar-associated genes do not affect T6SS directly and are therefore not required for interbacterial killing when cell contact is forced, yet are necessary for killing in high-viscosity liquid (hydrogel), where cell-cell contact must be biologically mediated. Quantitative transcriptomics revealed that V. fischeri significantly increases expression of both T6SS genes and cell surface modification factors upon transition from low- to high-viscosity media. Consistent with coincubation and fluorescence microscopy data, flagella are not required for T6SS expression in hydrogel. However, flagella were necessary to enhance expression of ~50% of the genome in hydrogel, including the surface modification genes identified in our screen and functional pathways important for host colonization, such as uptake of host-relevant iron and carbon sources, and nitric oxide detoxification enzymes. Our findings suggest that flagella play a key role when V. fischeri cells coordinately activate competitive strategies and host colonization factors, underscoring the significance of the physical environment in directing complex bacterial behaviors.IMPORTANCEThe physical environment has dramatic effects on bacterial behavior, but little is known about how mechanical signals impact antagonistic interactions. Symbiotic bacteria use molecular weapons to eliminate competitors for limited space within highly viscous host tissue and mucus. Previously, we found that a putative lipoprotein adhesin, TasL, and an unknown ligand are required to bring competitor cells within range of the T6SS weapon. Here, we found that mutations in flagella or predicted surface modification genes prevent TasL-mediated adhesion and killing in high viscosity. Transcriptomics revealed the flagella are required to coordinate expression of host colonization factors with the T6SS interbacterial weapon when transitioning from lower to higher viscosity conditions. These findings suggest that flagella may play a role in sensing mechanical signals, such as environmental viscosity, to enhance a symbiont's ability to successfully colonize the host while efficiently eliminating potential competitors from the host niche.