Flagellated bacterial motility in polymer solutions

TL;DR

This study investigates how the swimming speed of bacteria varies with polymer concentration, revealing that impurities cause non-monotonic behavior and that high molecular weight polymers exhibit non-Newtonian effects detectable through bacterial flagella.

Contribution

The paper provides high-throughput measurements of bacterial motility in polymer solutions and demonstrates that non-monotonic speed curves are mainly due to impurities, with new insights into non-Newtonian effects at high molecular weights.

Findings

Impurities cause non-monotonic v(c) curves in bacterial motility.

Newtonian hydrodynamics explains v(c) and Ω(c) for most polymers.

High molecular weight PVP shows non-Newtonian behavior detectable via flagella.

Abstract

It is widely believed that the swimming speed, $v$, of many flagellated bacteria is a non-monotonic function of the concentration, $c$, of high-molecular-weight linear polymers in aqueous solution, showing peaked $v(c)$ curves. Pores in the polymer solution were suggested as the explanation. Quantifying this picture led to a theory that predicted peaked $v(c)$ curves. Using new, high-throughput methods for characterising motility, we have measured $v$, and the angular frequency of cell-body rotation, $\Omega$, of motile Escherichia coli as a function of polymer concentration in polyvinylpyrrolidone (PVP) and Ficoll solutions of different molecular weights. We find that non-monotonic $v(c)$ curves are typically due to low-molecular weight impurities. After purification by dialysis, the measured $v(c)$ and $\Omega(c)$ relations for all but the highest molecular weight PVP can be described in detail by Newtonian hydrodynamics. There is clear evidence for non-Newtonian effects in the highest molecular weight PVP solution. Calculations suggest that this is due to the fast-rotating flagella `seeing' a lower viscosity than the cell body, so that flagella can be seen as nano-rheometers for probing the non-Newtonian behavior of high polymer solutions on a molecular scale.