Before revision:  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 frequencyand 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 levelwithout complex feedback and chemical communication between cells .
After revision:  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 captures cell motility through speed and reversal frequency. Our work suggests that M. xanthus evolved to take advantage of an active-matter phase transition that can be controlled through changes in motility at the individual cell levelwithout complex feedback and chemical communication between cells .