PROVIDENCE, R.I. [Brown University] -- Circles, slaloms, figure eights, and loop-the-loops biologists studying the motion of Listeria monocytogenes sensed that these paths were related, but they didnt have a good way to define what fit in and what didnt. A remarkably simple new mathematical description, published online in the Proceedings of the National Academy of Sciences [Abstract], reproduces all these shapes with just one pair of equations and only two key variables. Besides helping to identify bacterial mutants, the equations suggest which mechanisms could be driving the motion.

Last winter, Vivek Shenoy, an associate professor of engineering at Brown University, was matched with Julie Theriot, an associate professor at the Stanford School of Medicine, at a biophysics "boot camp" run by Rob Phillips at the California Institute of Technology. Theriot studies Listeria, a disease-causing bacterium that hijacks the actin network of an infected cell to propel itself. Embedded in a network of actin fibers, the bacterium keeps adding actin molecules at its back end, pushing itself forward and leaving behind an actin tail tracing its path.

Those paths intrigued Shenoy as soon as he saw Theriots movies of Listeria traveling in the two-dimensional world of a microscope slide. Some bacteria spun in circles, others followed a sine curve, some followed a path like the cloverleaf exchange on a highway. The circles, he thought, were easy to explain. If an actin filament pushed just a bit off center, the bacterium would go in circles, like a swimmer who paddles harder on one side.

With a bit more thought, he cracked the sine curve. What if that off-center point rotated around the axis of motion" When it pushed harder on the right, the bacterium would move to the left; when it pushed to the left of center, the cell would move to the right. If the bacterium moved forward fas
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Contact: Martha Downs
martha_downs@brown.edu
401-863-2752
Brown University
15-May-2007