More than Motility: Salmonella Flagella Contribute to Overriding Friction and Facilitating Colony Hydration during Swarming

We show in this study that Salmonella cells, which do not upregulate flagellar gene expression during swarming, also do not in-crease flagellar numbers permof cell length as determined by systematic counting of both flagellar filaments and hooks. In-stead, doubling of the average length of a swarmer cell by suppression of cell division effectively doubles the number of flagella per cell. The highest agar concentration at which Salmonella cells swarmed increased from the normal 0.5 % to 1%, either when flagella were overproduced or when expression of the FliL protein was enhanced in conjunction with stator proteins MotAB.We surmise that bacteria use the resulting increase in motor power to overcome the higher friction associated with harder agar. Higher flagellar numbers also suppress the swarming defect of mutants with changes in the chemotaxis pathway that were previ-ously shown to be defective in hydrating their colonies. Here we show that the swarming defect of these mutants can also be sup-pressed by application of osmolytes to the surface of swarm agar. The “dry ” colony morphology displayed by chemutants was also observed with other mutants that do not actively rotate their flagella. The flagellum/motor thus participates in two func-tions critical for swarming, enabling hydration and overriding surface friction. We consider some ideas for how the flagellum might help attract water to the agar surface, where there is no free water. Swarming bacteria may be divided into two categories: robustswarmers, which can navigate across a hard agar surface (1.5% agar and above), and temperate swarmers, which can swarm only on a softer agar surface (0.5 to 0.8 % agar) (1–3). Robust swarmer