Published online 19 June 2008 | Nature | doi:10.1038/news.2008.903

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Bacterial engines have their own clutch

Microbes don't hit the brakes to stop; they just pump the clutch.

Bacillus subtilisLike a lazy driver, Bacillus subtilis can coast to a halt by riding the clutch.ANDREW SYRED / SCIENCE PHOTO LIBRARY

When a bacterium wants to settle down, it uses a molecular 'clutch' to disengage its propulsion system, researchers have found.

No one had previously worked out how bacteria turn off their flagella, the spinning tails that they use to move around. The discovery could lead to a way to keep bacteria moving in places we’d rather they didn’t linger, such as medical equipment.

Many bacteria have two modes — either free-living and swimming, or settled down as part of a stationary 'biofilm'. These films are slimy bacterial cities only a fraction of a millimetre thick that contain vast numbers of cells and often many species.

The gene for the molecular clutch, which is called epsE, is part of a group of 15 genes that, when activated, send the soil bacterium Bacillus subtilis into biofilm mode. The other genes control things such as slime production.

Molecular motor

Daniel Kearns at Indiana University in Bloomington and his colleagues identified epsE by studying cells with mutations that should have triggered biofilm formation, but didn’t. It turned out that they had been unable to disengage their flagella, thanks to mutations in epsE.

epsE attaches to a rotor protein at the flagellum’s base that is driven by protons flowing into the cell. To stop cells moving, it bends the rotor molecule so that it no longer touches the proton motor, the researchers report in Science1.

The flagellum is still free to turn — hence the description of epsE as a clutch, and not a brake — but friction rapidly brings the cell to a halt. Kearn’s team is now looking for a protein that disengages the clutch and reconnects the motor.

It had been thought that bacteria slowed down by switching off the genes that make flagella, says Richard Berry, a physicist at the University of Oxford, UK, who studies molecular motors.

“This is a completely unknown thing,” he says. “The previous wisdom was that flagella would spin for ever.”

Some nanotechnologists have dreamed of harnessing bacterial rotors as molecular pumps, he adds. If that ever happened, epsE could be a useful regulator.

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Horror films

B. subtilis is harmless. But biofilms can be deadly — for example, when they form in the lungs of people with cystic fibrosis, or when MRSA cells settle down on hospital equipment.

"If we could target the clutching mechanism, we might trick cells into staying mobile," says Kearns. "It could be very destabilizing." B. subtilis cells that can’t switch off still form groups, he says, but these are not cohesive. "They writhe around."

“It could be a target,” agrees Berry. “But it’s not the softest target – there are probably more vital things you could shoot for.” 

  • References

    1. Blair, K. M., Turner, L., Winkelman, J. T., Berg, H. C. & Kearns, D. B. Science 320, 1636–1638 (2008).
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