Measuring the motility and drag forces acting on biological particles using optical tweezers

Using absolutely calibrated optical tweezers, we make quantitative measurements of the motility force of Escherichia coli (E. coli) by measuring the change in momentum of the deflected beam. By tracking the position of the particle, in addition to the optical force measurements, it should be possible to simultaneously calculate the motility force and drag. In a simple Gaussian beam optical trap away from the sample chamber E. coli tends to align and swim along the beam axis, which can make tracking the particle position and measuring the force difficult. We use a 3-D optical force detection system to measure the absolute force on the particle, allowing us to measure the motility force of E. coli in a simple Gaussian beam. By using a line-shaped trap, it is possible to align E. coli transverse to the beam axis, facilitating easy particle position measurement. The investigated methods are not specific to E. coli and could be applied to other motile organisms, the study of wall effects and bio-films.