Bacterial Flagella: Flagellar Motor


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 (ATP) 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‐characterized 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.

Key concepts:

  • Bacteria swim using a small biological rotary motor which is powered by the movement of charged ions across the plasma membrane.

  • The 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, 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.

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

Keywords: bacterial motility; molecular motors; protein machines; chemotaxis; signalling

Figure 1.

The bacterial flagellum. (a) The bacterial flagellar motor is a rotary motor that sits in the cell envelope of bacteria. It is driven by the flow of ions across the cytoplasmic (plasma) membrane, and its purpose is to rotate the long helical filaments that protrude from the cell and propel swimming bacteria. The diagram depicts a Gram‐negative envelope, typifying E. coli; the L and P rings are associated with the outer membrane and thin peptidoglycan layer. In Gram‐positive bacteria, flagellar basal bodies lack the L and P rings because of their thick cell walls. (b) Two models showing how the motor might work. The model on the left is like an electrostatic proton turbine, whereas that on the right is more like a turnstile.

Figure 2.

Swimming and flagellar rotation in E. coli. (a) Cells of the bacterium E. coli swim in a series of ‘runs’ and ‘tumbles’. In a run, flagella rotate counterclockwise (CCW) and form a bundle that propels the cell. In a tumble, flagella rotate clockwise (CW) and the bundle flies apart, causing the cell to jiggle on the spot. The rotation of individual flagellar motors can be measured using (b) tethered cells, (c) laser dark‐field microscopy or (d) beads attached to flagella.

Figure 3.

The chemotaxis pathway in E. coli. Bacteria modulate the probability direction of rotation of their flagellar motors in response to changes in the concentration of chemicals in the environment. This allows them to swim towards nutrients or away from harmful chemicals, a process known as chemotaxis. The biochemical pathway that controls this response consists of membrane‐bound receptors – also known as methyl‐accepting chemotaxis proteins (MCPs) – and the chemotaxis proteins CheW, A, Y, Z, B and R. The curved arrows represent phosphorylation and dephosphorylation reactions; the thin straight arrows represent transitions between alternative states of the MCPs or of the flagellar motor and the wide straight arrows represent positive influences of one element in the pathway upon a particular transition. The arrows in red highlight the sequence of events that generates an increase in the probability of clockwise (CW) rotation in response to a reduction in the amount of attractant bound to an MCP; CCW, counterclockwise.



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Further Reading

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Sowa Y and Berry RM (2008) Bacterial flagellar motor. Quarterly Reviews of Biophysics 41: 103–132.

Stock AM and Mowbray SL (1995) Bacterial chemotaxis: a field in motion. Current Opinion in Structural Biology 5: 744–751.

Wadhams GH and Armitage JP (2004) Making sense of it all: bacterial chemotaxis. Nature Reviews of Molecular Cell Biology 5: 1024–1037.

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How to Cite close
Wadhams, George H, and Sowa, Yoshiyuki(Sep 2009) Bacterial Flagella: Flagellar Motor. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000744.pub3]