A fluid-dynamic interpretation for the asymmetrical motion

Fluid dynamic interactions between a surface and a cell

 When the motor rotates in one direction, the cell body rotates in the opposite direction, as shown in Fig. 7. If there are no interfering surfaces, these rota-tions of the flagellum and the cell body do not induce any lateral motion. If there is a surface, however, the rotational motions of the cell body and the flagellar filament are both affected by the surface. This explains the difference between bacterial motion close to a rigid surface and its motion in free space. The resistive force exerted on the side facing the surface and the force on the opposite side are different. As a result, a lateral force perpendicular to the rotational axis and parallel to the surface is exerted on the filament. Another lateral force in the opposite direction is exerted on the cell body. These two forces produce a torque around an axis perpendicular to the surface. When the motor rotates CCW, a clockwise torque is generated as seen from above (from the plus to minus direction in the z-axis) in the configuration shown in Fig. 7. Consequently, the bacterium swims along a clockwise trajectory because it is swimming forward. When the motor rotates CW, the torque is counterclock-wise, and the bacterium swims backward drawing a counterclockwise trajec-tory. The surface affects the drag of the cell body and the propulsive force of the flagellum along the rotational axis of the motor, as well as the lateral forces that produce the torque perpendicular to the axis of the motor. The closer a bacterium is to the surface, the greater the drag and the propulsive force. It can be assumed that the interaction of the cell body with the surface will reduce the swimming speed and that the interaction of the filament with the surface will increase the speed. The attitude of a cell relative to a rigid surface is another factor that influ-