SwrD (YlzI) Promotes Swarming in Bacillus subtilis by Increasing Power to Flagellar Motors.

The bacterium is capable of two kinds of flagellum-mediated motility: swimming, which occurs in liquid, and swarming, which occurs on a surface. Swarming is distinct from swimming in that it requires secretion of a surfactant, an increase in flagellar density, and perhaps additional factors. Here we report a new gene, , located within the 32 gene operon dedicated to flagellar biosynthesis and chemotaxis, which when mutated abolished swarming motility. SwrD was not required for surfactant production, flagellar gene expression, or an increase in flagellar number. Instead, SwrD was required to increase flagellar power. Mutation of reduced swimming speed and torque of tethered flagella, and all -related phenotypes were restored when the stator subunits MotA and MotB were overexpressed either by spontaneous suppressor mutations or by artificial induction. We conclude that swarming motility requires flagellar power in excess of that which is needed to swim. Bacteria swim in liquid and swarm over surfaces by rotating flagella, but the difference between swimming and swarming is poorly understood. Here we report that SwrD of is necessary for swarming because it increases flagellar torque and cells mutated for swim with reduced speed. How flagellar motors generate power is primarily studied in , and SwrD likely increases power in other organisms, like the , , , and the .