PlzD modifies Vibrio vulnificus foraging behavior and virulence in response to elevated c-di-GMP

Some marine bacteria can swim at speeds exceeding 50× their body length per second as they actively seek out fleeting microscale nutrient patches. Yet, even in vast marine environments, most micro-organisms live in surface-associated biofilm communities. Such astonishing speeds can pose a serious challenge to initiating biofilm formation, and our understanding of the mechanisms that regulate deceleration is lacking. Calcium, a major constituent of marine ecosystems, promotes biofilm formation in the marine bacterium and opportunistic human pathogen Vibrio vulnificus by increasing cellular levels of the key secondary signaling molecule bis-(3´-5´)-cyclic dimeric guanosine monophosphate (c-di-GMP). Here, we show that elevated c-di-GMP levels inhibit V. vulnificus swimming motility and identify a PilZ domain protein, PlzD, that mediates this effect. PlzD is localized at the flagellar pole in slow-moving, initially adherent, and mature biofilm cells. This positioning was partly dependent on the flagellar stator protein PomA and was regulated by c-di-GMP. Mutating the conserved RXXXR c-di-GMP binding motif of the PilZ domain abrogated PlzD activity. Single-cell tracking of bacterial swimming trajectories revealed that PlzD altered the foraging behavior of cells by slowing swimming speed and by decreasing the number of directional changes, ultimately limiting exploration of the surrounding 3D space. The cumulative effects were increased biofilm formation, aggregation, oyster colonization, and attenuated virulence in mice, phenotypes underpinning the evolution of V. vulnificus as a resilient environmental organism and potent human pathogen. IMPORTANCE: Many free-swimming bacteria propel themselves through liquid using rotary flagella, and mounting evidence suggests that the inhibition of flagellar rotation initiates biofilm formation, a sessile lifestyle that is a nearly universal surface colonization paradigm in bacteria. In general, motility and biofilm formation are inversely regulated by the intracellular second messenger bis-(3´-5´)-cyclic dimeric guanosine monophosphate (c-di-GMP). Here, we identify a protein, PlzD, bearing a conserved c-di-GMP binding PilZ domain that localizes to the flagellar pole in a c-di-GMP-dependent manner and alters the foraging behavior, biofilm, and virulence characteristics of the opportunistic human pathogen, Vibrio vulnificus. Our data suggest that PlzD interacts with components of the flagellar stator to decrease bacterial swimming speed and changes in swimming direction, and these activities are enhanced when cellular c-di-GMP levels are elevated. These results reveal a physical link between a second messenger (c-di-GMP) and an effector (PlzD) that promotes transition from a motile to a sessile state in V. vulnificus.