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7th Annual T. Francis Ogilvie Young Investigator Lecture PDF Flyer Professor Thomas R. Powers, James R. Rice Assistant Professor of Solid Mechanics, Division of Engineering, Brown University.
ABSTRACT:At the micron scale of a bacterial cell, viscous effects completely dominate inertial effects, and Brownian motion is important. These distinctive features of the small scale have led bacteria to adopt strategies for locomotion and sensing which are qualitatively different from macroscopic strategies. This talk will address the mechanical aspects of bacterial chemotaxis, the means by which E. coli moves towards higher concentrations of favorable chemicals. These cells swim using several slender helical propellers, or flagella, driven by rotary motors embedded at random points in the cell wall. In the presence of a chemical gradient, a bacterium drifts up the gradient by following a directed random walk. The random walk is not due to thermal fluctuations, but is part of the bacterial behavior. At the beginning of each step of a walk, the rotating helical flagella form a bundle, and the cell moves in a directed manner. At the end of each step, the bundle flies apart, and the cell randomizes its direction for the next step. A clear understanding of the elements of the mechanics of the bundling and unbundling processes is only now emerging. These elements are (1) the interplay of elastic and viscous stresses in slender filaments, (2) the polymorphism of the flagella, and (3) the hydrodynamic interactions among the flagella. I will use slender-body theory to show that the counter-rotation of the cell body necessary for torque balance is sufficient to wrap the flagella into a bundle, even in the absence of the swirling flows produced by each individual flagellum. Then I will briefly review recent work by R.E. Goldstein and collaborators on polymorphism and hydrodynamics. Finally, I will present results of our macroscopic experiments on rotating flexible helices in viscous fluids. - This work is supported by a National Science Foundation CAREER Award. - Thomas R. Powers is the James R. Rice Assistant Professor of Solid Mechanics in the Division of Engineering at Brown University. He received baccalaureate degrees in Physics and Mathematics at MIT, and earned a Ph.D. in theoretical soft condensed matter physics at the University of Pennsylvania in 1995. For his dissertation, he studied the statistical mechanics of lipid bilayer membranes with P. Nelson. Before arriving at Brown University in July 2000, Prof. Powers held postdoctoral appointments at Princeton University and the University of Arizona with R. Goldstein, and Harvard University with H. Stone. He has also taught physics at Dartmouth College. His research interests include the physics of soft matter, biological physics, and nonlinear dynamics; he is particularly interested in problems of a geometric nature. Current work largely consists of applying mechanics to biological systems at the micron scale and below, such as biopolymers, membranes, and bacteria. |
