The bacterial flagellum is a right-hand helical fiber, composed almost entirely of the fibrous protein flagellin. That is, the flagellum does not contain microtubules, actin, myosin (see here), or any contractile system. The flagellum itself rotates as a means of propelling the bacterium (Figure 8.29).

The driving force for rotation of the flagellum is a gradient of protons moving across the bacterial inner membrane. Thus, the flagellum behaves like an electric motor, in a sense. The motor runs at about 100 revolutions per second and requires the passage of about 1000 protons per revolution. Marine bacteria have similar rotary motors, but they use a sodium gradient, not a proton gradient, to power the motor.

The flagellar motor can be reversed, so it can operate either clockwise or counterclockwise. When the flagella are all turning counterclockwise, they pull together and propel the bacterium in a straight line (called running). When the flagella rotate clockwise, they fly out in a variety of directions, causing the bacterium to tumble randomly.

Bacterial chemotaxis is the phenomenon of moving in response to external chemical stimuli. Chemotactic bacteria (such as E. coli) move preferentially towards attractants, such as nutrients, and away from repellants, such as poisons. These motions can be accomplished by a combination of running and tumbling (Figure 8.31). In a neutral and uniform environment, brief periods (a few seconds) of running alternate with brief periods of tumbling, so the bacterium wanders randomly. In the presence of a gradient of either attractant or repellant, however, the bacterium delays tumbling, resulting in net motion towards the attractant or away from the repellant.