(222b) The Role of Flagellar Motor Reversals in Swarming in Escherichia coli

Swarming in thin fluid films on top of solid surfaces is enabled by the rotation of flagellar filaments in bacteria. To swarm, cells must overcome an increase in viscous drag due to the proximity to the surface. Previous reports suggest that cells likely compensate for the surface drag by promoting surface wetness through an unknown mechanism with the aid of a flagellar switch. The switch helps the filament erratically reverse the direction of rotation between clockwise (CW) and counterclockwise (CCW). The direction of rotation is dependent on the conformation of a switch-component, FliG. Here, we investigated the role of FliG conformation in facilitating bacterial swarming. We observed that swarming was abolished when FliG was locked in the CW conformation (FliG^CW), but not in the CCW conformation (FliG^CCW). Swimming was not abolished, so the conformation of FliG is likely critical for swarming. We found that torque generation in FliG^CW strains was similar to WT strains. Swimming speed analysis revealed the loss of swarming ability correlated with a reduction in the flagellar thrust in the CW strain, which was likely due to a reduction in the number of filaments per cell. Fluorescence visualization showed that FliG^CW swimmers have one less filament per cell on average compared to WT swimmers. Yet, FliG^CW swarmers tend to lack filaments entirely (~57% of cells visualized carried no filaments, compared to ~10% in the WT). We hypothesized that there was a link between the function of the switch and degradation of the flagellar assembly; however, TIRF imaging revealed no significant degradation. Although the role of the switch in swarming remains unclear, our assays revealed that WT swarmer cells show a significant decrease in the probability of CW rotation (known as CW_bias) suggesting modifications are occurring at the level of the switch, and further pointing to its importance.