Requirements for swarming ability by lateral flagella on an agar surface in marine Vibrio cells

The marine bacterium Vibrio alginolyticus possesses two types of flagella in one cell: proton-driven lateral flagella extending from the periphery of the cell body and sodium ion-driven polar flagella extending from a cell pole. It is believed that, for swimming in seawater, they use polar flagella, suppressing the expression of lateral flagella, and when they attach to the body surface of a fish or the intestinal tract, lateral flagella are induced, allowing them to crawl along the surface or through mucus. The dynamometer hypothesis, which proposes that polar flagella sense rotation and control the expression of lateral flagellar genes, has been presented. However, how rotation is sensed and how expression is controlled remain unclear. Although swarming has recently been analyzed by physical, biological, or biochemical perspectives, it remains unclear how this motility is controlled, or which substances and conditions are necessary for swarming ability. In this study, we discovered that adding gelatin to agar medium promotes swarming on the agar surface by the lateral flagella of Vibrio. Based on our data, we speculate that surfactants or viscous polysaccharides secreted extracellularly are important for promoting swarming on the agar surface, and we identified that swarming is likely to be driven by S (social)-motility, in which bacteria move by interacting with each other, and A (adventure)-motility, in which bacteria move by interacting with the agar surface. Our study provides clues that help clarify the mechanism of bacterial swarming.IMPORTANCEWe discovered that adding gelatin to hard agar medium promoted swarming on agar surfaces by the lateral flagella of Vibrio cells. The surfactants or viscous polysaccharides secreted extracellularly seem to be important for swarming ability on agar surfaces. We proposed that the swarming is thought to occur through S (social)-motility, where cells move by interacting with cell bodies each other, and A (adventure)-motility, where cells move by interacting with the agar surface and cell body. The present study should provide clues to clarify the mechanism of bacterial swarming and how to move in a viscous environment.