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Dancing bacteria? UW chemical and biological engineers explain choreography of bacteria


12-MB QuickTime movie. Larger image

Birds fly together in flocks. Fish swim together in schools. Everyone has seen the beautiful, seemingly choreographed motions these collections of organisms can exhibit. But surely bacteria, which have no eyes or brain, cannot behave in such a coordinated way. In fact they do, and researchers are beginning to learn how.

Bacteria are well known to interact with one another through chemical signals — they can "smell" one another, and their behavior and growth may change if they have many neighbors. Chemical signaling between bacteria enables cooperative behavior of a bacterial population; one example of this is "cell swarming," where colonies of bacteria grown in a petri dish form complex and beautiful patterns. More recently, experiments in several laboratories have shown that bacteria swimming in a drop of water also form patterns — jets and whirls that are much larger than the bacteria themselves, and that stir the fluid in ways that an individual swimming bacterium cannot. A key question then is: How do the swimming bacteria interact to form these patterns?

Reporting in the November 11 issue of Physical Review Letters, Chemical and Biological Engineering Professor Mike Graham's group at the University of Wisconsin-Madison developed a model that offers a partial answer to this question. In this model, each bacterium pushes fluid around as it swims, and simultaneously is buffeted like a boat on a wavy sea by the fluid motions generated by all the other swimming bacteria. There are no other interactions between the bacteria in this model: They cannot smell, feel or see one another. This model might be expected to just predict random, uncoordinated swimming of the bacteria, and indeed at low concentrations this is exactly what occurs. As concentration increases, however, there suddenly comes a point where the swimmers no longer move randomly, but organize into large whirls containing tens to hundreds of bacteria that all move together for a while before swimming off to join another whirl, and so on. The model thus demonstrates that not only chemical interactions, but also fluid-mechanical interactions between bacteria can lead to large-scale coordinated motions and patterns. These results, as well as the experimental observations that motivated them, have potentially important implications for understanding how bacteria feed and sample their environment. They may also suggest a novel strategy for mixing fluids, by the addition of living or artificial swimming micromachines, in microscale devices where conventional approaches to mixing are ineffective.

Simulated velocity field generated in a swimming suspension of

Simulated velocity field generated in a swimming suspension of bacteria. Each line indicates the magnitude and direction of the fluid velocity at that point in the fluid. (large image)