Figure 5.

Model for the twitching motility. (a) Pili, which are attached to the surface and retract, propel the cell forward (shown by inward arrows and circles around their tips). (Outward arrows) Growing pili. A pilus that is trying to attach to the surfaces is shown (dashed circle around its tip). Those pili that failed to attach retract without generating any force acting on the cell (inward arrows; no circle at the tips). (b) Pili are pulling against each other and trap the cell. (c) A new coming pilus can attach and break the force balance, thus pulling the cell out of the trap. (d and e) When most of pili are trapped, some of them may randomly detach from the surface (dashed circles), giving an advantage to their opponents and releasing the cell from a longer trap.