Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A7: Bacterial Flagellar Dynamics, Polymorphism, and Conformational Spread |
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Sponsoring Units: DBP Chair: Phil Nelson, University of Pennsylvania Room: Baltimore Convention Center 307 |
Monday, March 13, 2006 8:00AM - 8:36AM |
A7.00001: Theory of polymorphic transformations of flagella Invited Speaker: Bacterial flagellar filaments can abruptly change shape in response to mechanical load or changes in solution pH or ionic strength. These polymorphic transformations are an instance of a ubiquitous phenomenon, the spread of conformational change in large macromolecular assemblies. We propose a new theory for polymorphism, whose essential elements are two molecular switches, an elastic mismatch strain between the inner and outer cores of the filament, and cooperative interactions between neighborhing subunits on the same protofilament. We calculate the phase diagram for helical and straight states, and the response of a helical filament to an external moment. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 9:12AM |
A7.00002: Synchronization of rotating flagella by hydrodynamic interactions Invited Speaker: Rotating bacterical flagella form bundles, which means that their rotations have to be synchronized. The aim of our study [1] is to show that hydrodynamic interactions, i.e., interactions mediated by the flow field the helical flagella create, can be at the origin of such a synchronization. \par We consider two stiff helices that are modeled by rigidly connected beads, neglecting any elastic deformations in a first approach [1]. The helices are driven by constant and equal torques. They are fixed in space by anchoring their terminal beads in harmonic traps so that they can jiggle around. We observe that, for finite trap strength, hydrodynamic interactions, treated in the low-Reynolds-number regime, do indeed synchronize the helix rotations to a phase difference zero. The speed of phase synchronization decreases with increasing trap stiffness and becomes zero in infinitely stiff traps. So strictly parallel helices do not synchronize. This limit is consistent with recent work based on slender-body theory [2]. We furhermore show that phase synchronization is stable against fluctuations in the torques driving the helices. \par Our results clearly indicate that some kind of flexibility is essential to allow for phase synchronization. In reality, this flexibility might have its origin in the proximal hook connecting the flagellum to the rotatory motor or in elastic deformations of the rotating flagella. Indeed, when we extend our model by implementing the elasticity of a helical worm-like chain, synchronization occurs much faster even for relatively stiff helices. \par [1] M.\ Reichert and H.\ Stark, Eur.\ Phys.\ J.\ E\ \textbf{17}, 493 (2005). \par [2] M.J.\ Kim and T.R.\ Powers, Phys.\ Rev.\ E\ \textbf{69}, 061910 (2004). [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:48AM |
A7.00003: Bacterial Flagellar Transformations Invited Speaker: In many of the bacteria that swim by rotating helical flagella, the flagellum itself is not a simple, passive propeller. Flagella can adopt several helical shapes of varying pitch, radius and handedness in response to changing conditions such as temperature, pH, and load. In \textit{Escherichia coli}, in particular, at least 5 (out of 12 predicted) helical forms are observed during normal swimming. Polymorphic changes commonly occur during tumbling, appear to aid in the reorientation of swimming direction, and are induced by torque- changing variations in motor speed. Measurements on individual, isolated flagellar filaments are revealing the forces required to cause polymorphic transformations. These data will be necessary for a quantitative understanding of the connections between motor reversal, polymorphic change, and tumbling behavior. Since the filament is a uniform polymer of flagellin protein, whose structure is known, it provides a simple, macroscopically visible model of highly cooperative conformational changes in a biological polymer. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:24AM |
A7.00004: Spiroplasma swim by a processive change in body helicity. Invited Speaker: Microscopic organisms must rely on very different strategies than their macroscopic counterparts to swim through liquid. To date, the best understood method for prokaryotic swimming employs the rotation of flagella. I will present data that Spiroplasma, tiny helical bacteria that infect plants and insects, use a very different approach. By measuring cell kinematics during free swimming, we find that propulsion is generated by the propagation of kink pairs down the length of the cell body. A processive change in the helicity of the body creates these waves and enables directional movement. Unlike the motion of other helical swimmers such as Spirochetes, Spiroplasma swimming velocity increases with increasing viscosity. In addition, cell morphological parameters such as helical pitch and cell length influence swimming velocity. [Preview Abstract] |
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