"Dynamics of bacterial propulsion: Behavior of the flagellar rotary motor near zero load" by Junhua Yuan
February 28, 2011 - 10:00am
Flagellated bacteria swim by rotating thin helical filaments, each driven at its base by a reversible rotary motor, powered by an ion flux. Studies of the physiology of the bacterial flagellar rotary motor have been limited to the regime of relatively high load due to technical limitations. Here, we developed a new technique that allows systematic study of the motor near zero load. Sixty-nanometer-diameter gold spheres were attached to motors lacking flagellar filaments, and a novel laser darkfield setup was used to monitor the sphere rotation. Resurrection experiments were carried out near zero load: paralyzed motors without torque generating units were resurrected by adding torque generating units to the motor one at a time. In contrast to the incremental increase in rotation rate during resurrection at high load, the rotation rate for motors near zero load jumped to the maximum value upon addition of the first torque-generating unit. Switching properties of the flagellar motor near zero load also were investigated, and the switching rates showed a linear dependence on motor torque. Rotation in either direction (clockwise: CW or counterclockwise: CCW) has been thought to be symmetric, exhibiting the same torques and speeds. Here, we measured the torque-speed relationship across all load regimes for CW rotation, and found that the torque decreases linearly with speed, a result remarkably different from that for CCW rotation. This work provides further insights into the torque-generating mechanism, helps to better understand the motor switching mechanism, and places tighter constraints on possible motor models.