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Image problem puts bacterium in a spin

By Hamish Johnston

The bacterium in the video above has an image problem that’s forcing it to go round in circles.

I’m not speaking in metaphors; physicists in Italy are the first to notice that certain bacteria move in anti-clockwise loops when they swim close to the surface of a liquid.

Roberto Di Leonardo and colleagues also watched as E. coli swam in a clockwise direction when they were very close to the bottom of a water-filled dish – something that others had already seen.

This behaviour is puzzling because the little creatures manage to swim in straight lines when they are a good distance away from either the top or bottom of a dish.

Now, Di Leonardo and colleagues at the University of Rome Sapienza think they know why, and have published a paper in Physical Review Letters outlining their theory.

The bacterium propels itself by spinning a whip-like flagellum that acts much like a ship’s screw. The presence of a solid interface below – or an air interface above – the bacterium disrupts the movement of fluid around the bacterium. It turns out that the effect of this disruption can be modelled by removing the interface and replacing it with a mirror image of the swimming bacterium.

The effect of a mirror image is a torque on the real bacterium causing it to swim in a clockwise manner if it is close to the bottom. The image has the opposite effect when the bacterium swims close to the surface.

The analysis also suggests that long rod-like bacteria swim in larger circles than stubby bacteria.

Understanding this effect could prove very important in creating bacteria ratchets, which prevent some or all motile bacteria from passing through a barrier. Such ratchets could prove useful in preventing the spread of disease.

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