Figure 1.

Bacterial flagella control distinct motility. The flagellar motor is a complex nanomachine that drives filament rotation. (A) Cartoon model of the flagellar motor. (B) In the two-step model used by many species, such as E. coli and Salmonella, the cell body is propelled forward, or runs, during counterclockwise (looking from the motor to the filament, CCW) rotation, and the filaments form an organized bundle. To change direction, the cell tumbles by rotating the filament in the clockwise (CW) direction, unwinding the bundle. (C) Vibrio spp. use a three-step method, with CCW rotation moving the cell body forward, CW rotation moving the cell body in reverse, and a flicking motion when CW-to-CCW randomly change direction. (D) Spirochetes, with periplasmic flagella at both poles, require a unique two-step method. During the run, the flagella rotate CCW and CW at opposite poles, such that one pole “pulls” while the other “pushes”. Both poles rotate in the CW direction while the cell tumbles to change direction.