Switching and Torque Generation in Swarming E. coli

Abstract

Escherichia coli swarm on semi-solid surfaces with the aid of flagella. It has been hypothesized that swarmer cells 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. 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.

Asymmetric random walks reveal that the chemotaxis network modulates flagellar rotational bias in Helicobacter pylori

“…The distribution of the bias is shown in Figure 3A . The bias was similar to that observed in E. coli ( Block et al, 1983 ; Segall et al, 1986 ; Ford et al, 2018 ; Sagawa et al, 2014 ; Block et al, 1982 ; Stock et al, 1985 ), suggesting that the basal chemotactic output in the two species is similar. As evident, most cells tended to rotate their motors CCW for a higher fraction of time.…”

“…A fraction of the data is shown in Figure 3B and in Appendix 1—figure 1 . This suggested that the default direction of flagellar rotation is CCW, similar to E. coli ( Ford et al, 2018 ; Liu et al, 2020 ). Considering that the bias is zero in the absence and ~0.35 in the presence of CheY, CheY-P binding likely promotes CW rotation in an otherwise CCW rotating motor in H. pylori .…”

Escherichia coli Remodels the Chemotaxis Pathway for Swarming

“…For example, S. enterica swarmers are reported to upregulate tricarboxylic acid (TCA) cycle enzymes ( 42 ). A recent study measuring single-motor behavior of E. coli swarmers reported a CCW bias but not increased speeds ( 67 ). However, the speed measurements were performed on strains whose flagellar motors were locked in CW or CCW modes.…”

“…However, the speed measurements were performed on strains whose flagellar motors were locked in CW or CCW modes. Such strains do not swarm unless excess moisture is added to the surface ( 11 , 57 ), so the cells were lifted from a wet-agar surface ( 67 ). It is not surprising that Ford et al did not see a change in speed/torque because, as reported in our study, non-swarming cells taken from an agar surface do not show this behavior ( Fig.…”

Swarming Motility Without Flagellar Motor Switching by Reversal of Swimming Direction in E. coli

“…Under general swarm assay conditions for wildtype E. coli , any strain with defective che gene cannot swarm on the surface ( Harshey and Matsuyama, 1994 ), unless sufficient wetness was provided to the surface. To increase surface wetness, one way was to spray water on the surface directly, which was proved to be a way to partially restore swarming motility of S. typhimurium and E. coli with Δ cheY ( Wang et al, 2005 ; Ford et al, 2018 ). Another way was to add surfactant, such as surfactin or Tween 80.…”