  Bacterial swarming is a rapid mass-migration, in which thousands of cells
spread collectively to colonize a surface. Physically, swarming is a natural
example of active particles that use energy to generate motion. Accordingly,
understanding the constraints physics imposes on the dynamics is essential to
understand the mechanisms underlying the swarming phenomenon. We present new
experiments of swarming Bacillus subtilis mutants with different aspect ratios
and densities. Analyzing the dynamics reveals a rich phase diagram of
qualitatively distinct swarming regimes, describing how the shape and density
of cells govern the global dynamical characteristics of the entire swarm.
Moreover, we show that under standard conditions bacteria inhabit a region of
phase space that is associated with rapid mixing and robust dynamics, with
homogeneous density and no preferred direction of motion. This contrasts
characteristic clustering behavior of self-propelled rods that is recovered
only for very elongated mutant species. Thus, bacteria have adapted their
physics to optimize the principle functions assumed for swarming.
