| dc.description.abstract | Escherichia coli swarm on semi-solid agar surfaces with the aid of flagella. One hypothesis suggests swarmer cells may overcome the increased viscous drag near surfaces by developing higher flagellar thrust and by promoting surface wetness with the aid of a flagellar switch. The switch enables reversals between clockwise (CW) and counterclockwise (CCW) directions of rotation of the flagellar motor. Here, we measured the behavior of flagellar motors in swarmer cells in response to semi-solid surfaces. Results indicated that although the torque was similar to that in planktonic cells, the tendency to rotate CCW was higher in swarmer cells. This suggested that swarmers likely have a smaller pool of phosphorylated CheY. Results further indicated that the upregulation of the flagellin gene was not critical for flagellar thrust or swarming. Consistent with earlier reports, moisture added to the swarm surface restored swarming in a CCW-only mutant, but not in a FliG mutant that rotated motors CW-only (FliGCW). Fluorescence assays revealed that FliGCW cells grown on agar surfaces carried fewer flagella than planktonic FliGCW cells. The surface-dependent reduction in flagella correlated with a reduction in the number of putative flagellar preassemblies. These results hint toward a possibility that the conformational dynamics of switch proteins play a role in the proper assembly of flagellar complexes and flagellar export, thereby aiding bacterial swarming.
Next, we utilized blinking optical tweezers and colloidal probes to indirectly measure the viscous drag associated with the semi-solid agar surfaces which Escherichia coli swarm on. E. coli are known to swarm over a very fine range of agar concentrations. One hypothesis proposes that this sensitivity to agar concentration is associated with an increasing viscous drag as agar concentration increases. Moreover, there are numerous studies that suggest increased viscous load may play a role in the induction of swarming.
We found that semi-solid agar surfaces behave remarkably similarly to no-slip glass surfaces; hence, the viscous drag experienced by swarming cells is likely very high. Finally, our findings suggest that viscous drag does not increase with agar concentration and is not responsible for E. coli’s inability to swarm at higher agar concentrations. | en |
