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J. Bacteriol. doi:10.1128/JB.01501-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

On Torque and Tumbling in Swimming Escherichia coli

Rowland Institute at Harvard, Cambridge, Massachusetts 02142, and Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138

^* To whom correspondence should be addressed. Email: hberg{at}mcb.harvard.edu.

	Abstract

Bacteria swim by rotating long thin helical filaments, each driven at its base by a reversible rotary motor. When the motors of peritrichously flagellated 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 CCD 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 calculate approximate values for bundle torque and thrust and body torque and drag and we estimate the filament stiffness. In both immobilized and swimming cells, the motor torque, as estimated from resistive force theory, is significantly lower than previously reported. Also, a bundle of several flagella produces little more torque than a single flagellum. Motors driving individual filaments frequently change directions of rotation. Usually, but not always, this leads to a change in handedness of the filament, which goes through a sequence of polymorphic transformations, from normal to semi-coiled to curly 1 and then, when the motor again spins CCW, back to normal. Motor reversals are necessary, although not always sufficient, to cause changes in filament chirality. Polymorphic transformations among helices of the same handedness occur without changes in sign of the applied torque.

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