A macroscopic scale model of bacterial flagellar bundling

TL;DR

This study presents a macroscopic model mimicking bacterial flagellar bundling, revealing how motor frequency and helix properties influence the bundling process in viscous fluids.

Contribution

The paper introduces a scaled-up physical model that replicates bacterial flagellar bundling, enabling detailed analysis of the dynamics involved.

Findings

Bundling rate is proportional to motor frequency.

Bundling depends on helix handedness and rotation sense.

Initial bundling rate is independent of filament relaxation time.

Abstract

Escherichia coli and other bacteria use rotating helical filaments to swim. Each cell typically has about four filaments, which bundle or disperse depending on the sense of motor rotation. To study the bundling process, we built a macroscopic scale model consisting of stepper-motor-driven polymer helices in a tank filled with a high-viscosity silicone oil. The Reynolds number, the ratio of viscous to elastic stresses, and the helix geometry of our experimental model approximately match the corresponding quantities of the full scale E. coli cells. We analyze digital video images of the rotating helices to show that the initial rate of bundling is proportional to the motor frequency and is independent of the characteristic relaxation time of the filament. We also determine which combinations of helix handedness and sense of motor rotation lead to bundling.