Impacts of multiflagellarity on stability and speed of bacterial locomotion

TL;DR

This study models bacterial locomotion with multiple flagella, revealing how flagellar flexibility influences stability and speed, and showing that multflagellarity significantly expands stable swimming regimes.

Contribution

It introduces a coarse-grained model analyzing the effects of flagellar elasticity on bacterial stability and speed, highlighting differences between uni- and multiflagellar bacteria.

Findings

Flexible hooks enable stable biflagellar swimming by flagellar bundling.

Stability regimes for uni- and quadriflagellar bacteria are nearly inverted.

Swimming speed increases weakly with the number of flagella.

Abstract

Trajectories and conformations of uni- and multiflagellar bacteria are studied with a coarse-grained model of a cell comprised of elastic flagella connected to a cell body. The elasticities of both the hook protein (connecting cell body and flagellum) and flagella are varied. Flexibility plays contrasting roles for uni- and multiflagellar swimmers. For a uniflagellar swimmer, hook and/or flagellar buckling occurs above a critical flexibility relative to the torque exerted by the flagellar motor. Addition of a second flagellum greatly expands the parameter regime of stable locomotion, because flexible hooks that would lead to buckling instability in the uniflagellar case provide the flexibility required for flagellar bundling in the biflagellar case. Similar observations hold for tri- and quadriflagellar swimmers. Indeed the stability regimes for uni- and quadriflagellar swimming are virtually inverted -- to a first approximation what is stable in one case is unstable in the other. Swimming speed is also examined: it increases very weakly with number of flagella and a simple theory is developed that explains this observation.