On Torque and Tumbling in Swimming Escherichia coli

Abstract

Bacteria swim by rotating long thin helical filaments, each driven at its base by a reversible rotary motor. When the motors of peritrichous cells turn counterclockwise (CCW), their filaments form bundles that drive the cells forward. We imaged fluorescently labeled cells of Escherichia coli with a high-speed charge-coupleddevice camera (500 frames/s) and measured swimming speeds, rotation rates of cell bodies, and rotation rates of flagellar bundles. Using cells stuck to glass, we studied individual filaments, stopping their rotation by exposing the cells to high-intensity light. From these measurements we calculated approximate values for bundle torque and thrust and body torque and drag, and we estimated the filament stiffness. For both immobilized and swimming cells, the motor torque, as estimated using resistive force theory, was significantly lower than the motor torque reported previously. Also, a bundle of several flagella produced little more torque than a single flagellum produced. Motors driving individual filaments frequently changed directions of rotation. Usually, but not always, this led to a change in the handedness of the filament, which went through a sequence of polymorphic transformations, from normal to semicoiled to curly 1 and then, when the motor again spun CCW, back to normal. Motor reversals were necessary, although not always sufficient, to cause changes in filament chirality. Polymorphic transformations among helices having the same handedness occurred without changes in the sign of the applied torque.The peritrichous bacterium Escherichia coli executes a random walk: an alternating sequence of runs (relatively long intervals during which the cell swims smoothly) and tumbles (relatively short intervals during which the cell changes course) (8). A cell is propelled by several helical flagellar filaments, each attached by a hook (a universal joint) to a reversible rotary motor (7). During runs, the filaments coalesce into a bundle that pushes the cell forward (24). When viewed from behind the cell, the bundle rotates counterclockwise (CCW), and, to balance the torque, the cell body rotates clockwise (CW). Tumbles are initiated by CW motor rotation (21). Based on studies of Salmonella using dark-field microscopy, it was thought that the motors change direction synchronously, causing the bundle to fly apart (24,25). Based on studies using fluorescence microscopy, it became apparent that different filaments can change directions at different times and that a tumble can result from a change in direction of as few as one filament (30). During a tumble, the reversed filament comes out of the bundle and transforms from normal (a left-handed helix with a pitch of 2.3 m and a diameter of 0.4 m) to semicoiled (a right-handed helix with half the normal pitch but normal amplitude) and then to curly 1 (a right-handed helix with half the normal pitch and half the normal amplitude). The change in direction of the cell's track generated by the tumble occurs during the transformation from normal to semi...

Modelling bacterial behaviour close to a no-slip plane boundary: the influence of bacterial geometry

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A 3D Motile Rod-Shaped Monotrichous Bacterial Model

“…The study of peritrichously flagellated bacteria such as E. coli has received considerable attention [31,12,68,13,24,14,15,74,16,76,79,29]. However, many bacterial strains such as Pseudomonas aeruginosa have a single flagellum at one end of a rodshaped body or a pair of flagella, one at each end [91,39].…”

Bacteria can exploit a flagellar buckling instability to change direction

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