Bacterial propeller rotates in steps
DOI: 10.1063/1.4796812
Some bacteria swim in liquids with the help of a molecular machine that spins a corkscrew-like filament hundreds of times per second. The roughly 45-nm-diameter machine typically runs on energy from an electrochemical potential that drives either protons or sodium ions into the cell through a channel in the cell membrane. Within the channel are rings of stator molecules surrounding a rotor. Ion flow causes the stators to move or change shape, thereby imparting a torque to the rotor. The filament is attached to the rotor, and the entire assemblage is called a flagellum. (For more on how bacteria move, see Physics Today, January 2000, page 24 .) Researchers have long thought that the flagellar motor rotates in discrete angular steps rather than smoothly, but until now such motion has been indiscernible. A team of scientists from Japan and the UK attached a submicron-sized bead to a filament and monitored its motion as they slowed the bacterial propeller to about 1 Hz by lowering the concentration of sodium ions and reducing the number of stator molecules. They directly observed 26 steps per revolution, which is consistent with the periodic arrangement of the rotor. (
