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

Colonies of the social bacterium Myxococcus xanthus transition from a thin layer of cells to three-dimensional droplet-like fruiting bodies in a nutrient-poor environment. This morphological transition happens as the cells reduce their reversal frequency, but mechanically why this change in motility leads to the formation of 3D layering is not understood. As a system composed of self-propelled rods [1], many features of the densely packed cell layer are well captured when modeled as an active nematic [2]. However, the role of local polar order has not been explored. Using a mutant with fluorescently labeled MglB proteins, we directly measured the polarity of individual cells and revealed a transient local polar order in thin M. xanthus colonies controlled by cell reversal frequency. We find that polar order leads to large fluctuations in surface stress and drives out-of-plane cell motion. Positive defects in the nematic director field that develop high polarity then ultimately drive layering and fruiting body formation. By controlling their reversal frequency, M. xanthus cells control local polarity and regulate the behavior of the population.

[1] Bar, et al. Annual Review of Condensed Matter Physics, 11(1):441–466, 2020.

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