London, June 20 : Scientists at Indiana University in Bloomington have found that a bacterium uses a molecular 'clutch' called epsE to disengage its propulsion system, when it wants to settle down.
The researchers claim that they are the first to study how bacteria run off their flagella, the spinning tails that they use to move around.
According to them, many bacteria have two modes - either free-living and swimming, or settled down as part of a stationary 'biofilm'.
Lead researcher Daniel Kearns describes 'biofilms' as slimy bacterial cities only a fraction of a millimetre thick, which contain vast numbers of cells, and often many species.
He says that 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, he adds.
The researcher revealed that he and his colleagues identified epsE by studying cells with mutations that should have triggered biofilm formation, but did not.
He said that it was due to mutations in epsE that the cells had become unable to disengage their flagella.
Reporting their study in the journal Science, the researchers revealed that 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, they said.
Since the flagellum remains free to turn even after this process, the researchers describe epsE as a clutch and not a brake. They say that it's friction that rapidly brings the cell to a halt.
The researchers are now planning further studies to find out a protein that disengages the clutch and reconnects the motor.
Richard Berry, a physicist at the University of Oxford in the UK, said that it had been thought that bacteria slowed down by switching off the genes that make flagella.
"This is a completely unknown thing. The previous wisdom was that flagella would spin for ever," Nature magazine quoted him as saying.
He even revealed that some nanotechnologists were of the opinion that bacterial rotors could be harnessed as molecular pumps, something that might make epsE a useful regulator.
Kearns said that though B. subtilis is harmless, biofilms could be deadly - like 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. It could be very destabilizing," he said.
Berry agreed: "It could be a target. But it's not the softest target - there are probably more vital things you could shoot for."