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Description
Liquid crystals serve as model systems for structured environments and represent a broader class of anisotropic, non-Newtonian fluids encountered by bacteria, such as host mucus [1] and extracellular polymeric substances involved in biofilm formation [2]. In this study, we investigated the collective swimming of fluorescently labelled Escherichia coli in nematic liquid crystals and observed the emergence of long-lived chains of bacteria swimming along the nematic director, similar to those observed for Proteus mirabilis by Mushenheim et al. [3]. Remarkably, we found that longer chains swim faster, contrary to predictions from fundamental force-balance models and observations of merging bacterial pairs. To explain this counterintuitive behaviour and identify the physical mechanism driving bacterial aggregation in liquid crystals, we combined experiments with agent-based simulations and minimal theoretical modeling. By incorporating the intrinsic speed distribution of individual bacteria, our simulations revealed a positive correlation between chain length and swimming speed, consistent with experimental observations. A minimal aggregation model, based on encounter probabilities between a bacterium and its two nearest neighbours, further supports our interpretation that longer chains swim faster because they are more likely to contain faster-swimming individuals, which meet and merge with their neighbours in less time.
[1] N. Figueroa-Morales, L. Dominguez-Rubio, T. L. Ott, and I. S. Aranson. Mechanical shear controls bacterial penetration in mucus. Sci. Rep. 9: 9713, 2019.
[2] A. Repula, E. Abraham, V. Cherpak, and I. I. Smalyukh. Biotropic liquid crystal phase transformations in cellulose-producing bacterial communities. Proc. Natl. Acad. Sci. U.S.A. 119: e2200930119, 2022.
[3] P. C. Mushenheim, R. R. Trivedi, H. H. Tuson, D. B. Weibel, and N. L. Abbott. Dynamic self-assembly of motile bacteria in liquid crystals. Soft Matter 10:88–95, 2014.
