Force Measurement of Bacterial Swimming Using Optical Tweezers

Velocity is a physical parameter most commonly used to quantify bacterial swimming. In the steady-state motion at a low Reynolds number, the swimming force can be estimated from the swimming velocity and the drag coefficient based on the assumption that the swimming force balances with the drag force exerted on the bacterium. Though the velocity-force relation provides a significant clue to understand the swimming mechanism, the odd configuration of bacteria could develop problems with the accuracy of the force estimation. This chapter describes the force measurement using optical tweezers. The method uses parameters obtained from the shape and movement of a microsphere attached to the bacteria, improving the quantitativeness of force measurement.