Enhanced bacterial swimming speeds in macromolecular polymer solutions

TL;DR

This study uses simulations to reveal that bacteria swim faster in polymer solutions due to local polymer depletion and flagellar chirality, challenging traditional expectations of slowed movement in viscous fluids.

Contribution

The paper demonstrates that bacterial speeds increase in polymeric fluids because of local depletion effects and flagellar chirality coupling, providing a new understanding of bacterial motility in complex fluids.

Findings

Bacterial speeds can increase by up to 60% in polymer solutions.

Polymer depletion near bacteria creates an effective slip enhancing speed.

Coupling with flagellar chirality is essential for the observed speed increase.

Abstract

The locomotion of swimming bacteria in simple Newtonian fluids can successfully be described within the framework of low Reynolds number hydrodynamics. The presence of polymers in biofluids generally increases the viscosity, which is expected to lead to slower swimming for a constant bacterial motor torque. Surprisingly, however, several experiments have shown that bacterial speeds increase in polymeric fluids, and there is no clear understanding why. Therefore we perform extensive coarse-grained simulations of a bacterium swimming in explicitly modeled solutions of macromolecular polymers of different lengths and densities. We observe an increase of up to 60% in swimming speed with polymer density and demonstrate that this is due to a depletion of polymers in the vicinity of the bacterium leading to an effective slip. However this in itself cannot predict the large increase in swimming velocity: coupling to the chirality of the bacterial flagellum is also necessary.