Bacterial Flagella: Flagellar Motor

Abstract

The bacterial flagellar motor is a complex biological rotary molecular motor, which is situated in the cell envelopes of bacteria. Whereas most biological motors use adenosine triphosphate as their energy source, the rotation of the flagellar motor is driven by a flow of charged ions across the bacterial plasma membrane. The motor powers the rotation of helical flagellar filaments at speeds of up to several hundred hertz. These rotating filaments act like propellers, pushing the cells through their environment. The motors are regulated by one of the best-characterised biological signalling pathways, the chemotaxis pathway. This pathway changes the swimming pattern of the bacteria in response to changes in the concentration of external chemicals so that they move into environments, which are optimal for their growth. Other pathways can regulate the flagellar motor and the motor itself can respond to changing conditions by adapting parts of its structure.

Key Concepts:

• Many bacteria swim using a small biological rotary motor which is powered by the movement of ions (H⁺ or Na⁺) across the plasma membrane.

• The bacterial flagellar motor consists of a rotor which rotates against stator units that are anchored to the peptidoglycan cell wall.

• Torque is generated by the interaction of the stator units, MotA and MotB (or PomA and PomB for Na⁺-driven motors), with FliG in the rotor.

• Despite the fact that the driving ions always flow in one direction through the stator units, many flagellar motors can switch between clockwise and counterclockwise rotation.

• The structure of the flagellar motor is highly dynamic, some of its components undergo rapid turnover while the motor is functioning in response to changing conditions.

• A complex signalling pathway regulates the motor output in response to environmental signals ensuring that bacteria swim towards nutrient rich environments.