Experimental determination of the propulsion matrix of the body of helical Magnetospirillum magneticum cells

TL;DR

This study measures the propulsion matrix of helical Magnetospirillum magneticum bacteria, revealing how their shape influences translational and rotational friction, and suggesting rotation may aid propulsion.

Contribution

It provides the first detailed experimental measurement of the full propulsion matrix of these bacteria, including coupling effects, and compares results to cylindrical models.

Findings

Rotational friction coefficients are mainly determined by cell size.

Translational friction is influenced by precise cell shape.

Rotation around the cell's axis may significantly contribute to propulsion.

Abstract

Helical-shaped magnetotactic bacteria provide a rare opportunity to precisely measure both the translational and rotational friction coefficients of micron-sized chiral particles. The possibility to align these cells with a uniform magnetic field allows to clearly separate diffusion along and perpendicular to their longitudinal axis. Meanwhile, their corkscrew shape allows detecting rotations around their longitudinal axis, after which orientation correlation analysis can be used to retrieve rotational diffusion coefficients in the two principal directions. Using light microscopy, we measured the four principal friction coefficients of deflagellated Magnetospirillum magneticum AMB-1 cells, and compared our results to that expected for cylinders of comparable size. We show that for rotational motions, the overall dimensions of the cell body are what matters most, while the exact body shape influences translational motions. To obtain a full characterization of the friction matrix of these elongated chiral particles, we also quantified the coupling between the rotation around and translation along the longitudinal axis of the cell. Our results suggest that for this bacterial species cell body rotation could significantly contribute to cellular propulsion.