How do bacteria swim? Brown physicis...( PROVIDENCE R.I. [Brown University] ...)

PROVIDENCE, R.I. [Brown University] Imagine yourself swimming in a pool: It's the movement of your arms and legs, not the viscosity of the water, that mostly dictates the speed and direction that you swim.

For tiny organisms, the situation is different. Microbes' speed and direction are subjected more to the physical vagaries of the fluid around them.

"For bacteria to swim in water," explained Jay Tang, associate professor of physics at Brown University, "it's like us trying to swim through honey. The drag is dominant."

Tang and his team at Brown have just completed the most detailed study of the swimming patterns of one particular bacterium, Caulobacter crescentus. In a paper published online this week in the Proceedings of the National Academy of Sciences (in print Nov. 25), the researchers show how this microbe's movement is affected by drag and a phenomenon called Brownian motion. The observations would appear to hold true for many other bacteria, Tang said, and shed light on how these organisms scavenge for food and how they approach surfaces and "stick" to them.

Caulobacter is a single-celled organism with a filament-like tail called a flagellum. As it swims, its rounded cellular head rotates in one direction, while the tail rotates in the opposite direction. This creates torque, which helps explain the bacterium's nonlinear movement through a fluid. What Tang and his team found, however, is that Caulobacter also is influenced by Brownian motion, which is the zigzagging motion that occurs when immersed particles are buffeted by the actions of the molecules of the surrounding medium. What that means, in effect, is that Caulobacter is being pinballed by the water molecules surrounding it as it swims.

This twin effect of hydrodynamic interaction and Brownian motion governs the circular swimming patterns of Caulobacter and many other microorganisms, the scientists found.

"Random forces are alwa
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