Biomechanics of pili/fimbriae

Force measurements using optical tweezers

Optical tweezers allow for sub-pN force spectroscopy measurements on bacterial pili and fimbriae. We have developed a simple protocol that allows for rapid measurements, which procedure in short is described in figure A) and B).

A bacterium is mounted using optical tweezers on a poly-l-lysine coated 10 µm bead that is immobilized to a coverslip. A 3 µm bead is then trapped and attached to the pilus/fimbria.

The coverslip is moved using a piezo-stage and the pilus will then move the trapped bead of equilibrium. By probing the position of the trapped bead we can measure the force.

How to measure biomechanical properties of biopolymers using optical tweezers

This video illustrates how to measure the biomechanical properties of a bacterial pili, that is, the surface organelles that bacteria such as E. coli use for attachment to host surfaces. In short, 10 µm polystyrene beads are immobilized to a coverslip and coated with poly-L-lysine to make them sticky. A bacterium can be trapped and mounted on this bead. A small 2 µm bead is thereafter trapped, the stiffness of the trap is calibrated, and subsequently the bead is brought in close proximity of the bacterium so the bead attaches to a pilus. By separating the bacterium and bead, the biomechanical properties of the pilus can be measured.

Force-extension of a single bacterial fimbria

The left figure shows the force response when extending a single fimbria using optical tweezers. We clearly see three distinct regions during these phases. I) Force increase linearly (stretching of the fimbrial shaft). II) Force is constant (sequential unwinding of subunits). III) Force is non-elastic (conformational change of individual subunits).

Simulation showing unwinding of a pilus

This simulation shows a rigid-body physical simulation of an adhesion pili. Video pubhlished in : https://link.springer.com/article/10.1007%2Fs00249-015-1021-1

Zakrisson, J., Wiklund, K., Servin, M., Axner, O., Lacoursière, C. & Andersson, M. (2015). Eur. Biophys. J. 44, 291–300.

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