Abstract
The locomotion of swimming bacteria in simple Newtonian fluids can successfully be described within the framework of low-Reynolds-number hydrodynamics1. 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, experiments have shown that bacterial speeds can increase in polymeric fluids2,3,4,5. Whereas, for example, artificial helical microswimmers in shear-thinning fluids6 or swimming Caenorhabditis elegans worms in wet granular media7,8 increase their speeds substantially, swimming Escherichia coli bacteria in polymeric fluids show just a small increase in speed at low polymer concentrations, followed by a decrease at higher concentrations2,4. The mechanisms behind this behaviour are currently unclear, and therefore we perform extensive coarse-grained simulations of a bacterium swimming in explicitly modelled 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 non-uniform distribution of polymers in the vicinity of the bacterium, leading to an apparent slip. However, this in itself cannot predict the large increase in swimming velocity: coupling to the chirality of the bacterial flagellum is also necessary.
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Data availability
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon request.
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Acknowledgements
We are grateful to T. Shendruk for helpful discussions and comments on the manuscript. We acknowledge discussions with W. Poon, V. Martinez, A. Morozov, A. Balin and A. Doostmohammadi. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 653284. The authors would like to acknowledge the use of the University of Oxford Advanced Research Computing (ARC) facility in carrying out this work.
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A.Z. and J.M.Y. designed the research. A.Z. developed the MPCD simulation code, performed the simulations, analysed the data and developed the theory. A.Z. and J.M.Y. developed the results and wrote the paper.
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Supplementary Figures 1–10.
Supplementary Video 1
Dynamics and swimming performance of a bacterium in different fluids. Swimming in a Newtonian fluid in the absence of polymers (top), and in a semi-dilute (density ρ = 0.05, centre) and dense (density ρ = 0.2, bottom) polymer solution consisting of macromolecular semiflexible polymers (length N = 12, stiffness kb = 12kBT). Left: side view; middle: front view; right: back view. The colours of individual polymers are to aid visualization.
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Zöttl, A., Yeomans, J.M. Enhanced bacterial swimming speeds in macromolecular polymer solutions. Nat. Phys. 15, 554–558 (2019). https://doi.org/10.1038/s41567-019-0454-3
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DOI: https://doi.org/10.1038/s41567-019-0454-3
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