Circles, slaloms, figure eights, and loop-the-loops , biologists studying the motion of Listeria monocytogenes sensed that these paths were related, but they didn’t have a good way to define what fit in and what didn’t.

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.

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