Instantaneous polar order enhances cell aggregation in thin Myxococcus xanthus layers - theory

Populations of self-propelled rods, such as bacteria, are active materials with both polar and nematic characteristics. The motion of cells is polar, while the interactions between cells is nematic. We study the social bacterium Myxococcus xanthus, a gliding species that utilizes its properties as an active material to drive layer formation as part of a developmental cycle [1]. M. xanthus cells reverse direction every few minutes. Reversal frequency drops during starvation, leading to increased layer formation. The directed motion of non-reversing cells results in regions with high polar order, while also displaying half-integer charge defects characteristic of nematics. We aim to understand the interplay of polar order and nematic defects within M. xanthus layers. We track cells and study their motion within monolayers, but cell tracking is nearly impossible during layer formation events. We introduced the essential properties of M. xanthus populations into a customized version of the molecular dynamics simulation program LAMMPS [2], and simulate monolayers and layer formation events to study the role of polarity in active materials with nematic interactions.

[1] Copenhagen, et al. Nature Physics, 17(2):211–215, 2021

[2] A. P. Thompson, et al, Comp Phys Comm, 271 (2022) 10817