Nematic and polar order in a thin layer of rod-shaped bacteria Myxococcus xanthus*

Myxococcus xanthus is a rod-shaped bacterium that moves on a solid surface. In a nutrient-rich environment, they form a thin cell layer that behaves as an active liquid crystal, with the system's nematic order leading to half-integer defects [1]. However, our direct experimental measurements of the cell polarity show that as a collective of self-propelled rods [2], the system also exhibits instantaneous local polar order, despite the absence of a torque that forces the polarities of neighboring cells to align. Furthermore, this local polar order is controlled by the reversal frequency of the cells. Lowering the reversal frequency results in enhanced local polar order, stronger forces between the colony and the solid substrate, and increased out-of-plane cell motion. Through modulating their reversal frequency, M. xanthus cells regulate the behavior of the population. Our research reveals the biological significance of an interplay between nematic and polar order in a bacterial population.  

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

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