One bacterium, working alone, can'it accomplish much. But put a bunch of them together, and they can move mountains. Well, maybe not mountains. But how about a tiny gear? Researchers at Argonne National Laboratory, Northwestern and Princeton have shown that the collective swimming behavior of bacteria can be harnessed for work. While the process is not very efficient, it is a promising step toward the development of hybrid biological and micromechanical machines. In some respects, bacterial swimming resembles Brownian motion, the random movement of particles or molecules in a medium. But Igor S. Aranson, an Argonne researcher who is the senior author of a paper describing the work in The Proceedings of the National Academy of Sciences, said that in equilibrium conditions, it was impossible to extract useful energy from Brownian motion‚ the laws of thermodynamics did not allow it. But bacteria, they don't know about this law,Dr. Aranson said. The researchers used tiny polymer gears with asymmetric teeth floating in a thin film teeming with Bacillus subtilis, a bacterial species known for its swimming ability. Above a concentration of about 10 billion bacteria per cubic centimeter, the gear would rotate. Dr. Aranson said that unlike molecules in Brownian motion, which reflect off whatever they strike, when the bacteria hit a tooth, they just keep pushing. They slide along the edge of the tooth until they reach the, junction where the next tooth starts. Since one edge of each tooth is longer than the other, more bacteria slide along the long edges, transferring more momentum to them and rotating the gear in one direction. One of the limitations of the process, Dr. Aranson said, is that the bacteria eventually run out of nutrients. But they can stop pushing even before that. Bacteria behave too much like people, he said. They start to do something else.