  Bacteria commonly live in structured communities that affect human health and
influence ecological systems. Heterogeneous populations, such as motile and
non-motile populations, often coexist in bacteria communities. Motile
subpopulations in microbial communities are believed to be important to
dispersal, quest for food, and material transport. However, except in
circumstances where motile cells drive colony expansion (e.g. bacterial
swarming), the physiological functions of motile subpopulations in bacterial
communities are largely unclear. Here we discovered that motile cells in
routinely cultured sessile colonies of peritrichously flagellated bacteria can
self-organize into two adjacent, centimeter-scale motile rings surrounding the
entire colony. The motile rings arise from spontaneous segregation of a
homogeneous swimmer suspension that mimics a phase separation; the process is
mediated by intercellular interactions and shear-induced depletion. As a result
of this self-organization, cells drive fluid flows to circulate around the
colony at a constant peak speed of approximately 30 microns per second,
providing a stable and high-speed avenue for directed material transport at the
macroscopic scale. These findings present a unique form of bacterial
self-organization that influences population structure and material
distribution in bacterial communities.
