Geometrical constraints on the tangling of bacterial flagellar filaments

TL;DR

This paper investigates the geometric conditions that prevent bacterial flagellar filaments from tangling, explaining how their shape and spacing promote coherent bundle formation despite passive assembly.

Contribution

It establishes theoretical constraints on filament tangling and compares them with experimental data, revealing that bacterial flagella are geometrically unlikely to tangle.

Findings

Flagella are too straight and spaced too far apart to tangle naturally.

Coherent bundle formation is robust due to geometric constraints.

Passive bundling is facilitated by the intrinsic geometry of flagella.

Abstract

Many species of bacteria swim through viscous environments by rotating multiple helical flagella. The filaments gather behind the cell body and form a close helical bundle, which propels the cell forward during a "run". The filaments inside the bundle cannot be continuously actuated, nor can they easily unbundle, if they are tangled around one another. The fact that bacteria can passively form coherent bundles, i.e. bundles which do not contain tangled pairs of filaments, may appear surprising given that flagella are actuated by uncoordinated motors. In this article, we establish the theoretical conditions under which a pair of rigid helical filaments can form a tangled bundle, and we compare these constraints with experimental data collected from the literature. Our results suggest that bacterial flagella are too straight and too far apart to form tangled bundles based on their intrinsic, undeformed geometry alone. This makes the formation of coherent bundles more robust against the passive nature of the bundling process, where the position of individual filaments cannot be controlled.