A motility-induced phase transition drives Myxococcus xanthus aggregation

A hallmark of living systems is their ability to generate complex spatial patterns at the molecular, cellular, and multicellular levels. Many such systems rely on coupled biochemical and genetic signaling mechanisms that can produce large-scale organization. Long-range order and patterning can also emerge, however, through purely mechanical interactions. Here, we study the starvation-induced aggregation of gliding Myxococcus xanthus bacteria and show that these cells phase separate by tuning their motility over time. By experimentally varying the density and speed of gliding cells, tracking individual cells in large populations, and comparing to simulations of a model of reversing Active Brownian Particles (ABPs), we show that cell aggregation can be understood with a single phase diagram in terms of density and a dimensionless inverse rotational Peclet number that characterizes cell motility. We further track changes in motility of the wild-type during starvation and show that a reduction of the reversal frequency and an increase in gliding speed change the rotational Peclet number to drive aggregation. Thus, M. xanthus evolved to take advantage of an active-matter phase transition that can be controlled through changes in motility at the individual cell level without complex feedback and chemical communication between cells.