Dynamics of holes in dense monolayers of M. xanthus

Myxococcus xanthus is a rod-shaped bacterium capable of tuning colony-scale behavior through individual gliding motility. In dense colonies, a combination of excluded volume effects, cell-cell adhesion forces and periodic reversals in cell polarity gives rise to nematic order and nematohydrodynamic flows in the bulk. The density profile of a single layer of cells is not uniform, with holes in a monolayer spontaneously forming and closing. Despite consisting of the absence of cells, these holes can interact with the bulk nematic by absorbing and emitting topological defects. We show that while the average velocities at the boundary of a hole cohere with an active nematic model, fluctuations are anisotropic and enhanced along the alignment axis, which we hypothesize is an effect of polarity fluctuations on short time scales. We show qualitative agreement between the dependence of velocities on the local radius of curvature of a boundary and a model of capillary attraction between wetted cells due to water extracted from the gel surface. This builds a picture of hole formation as an emergent effect of fluctuations in active forces and finite-range attraction between cells, such that the free boundary is stabilized when the former overcomes the latter.