  Emergence of regular spatial patterns is a hallmark in living matter ranging
from subcellular organelles to developing embryos and to ecosystems. Mechanisms
for the formation of ordered spatial patterns in biology often require chemical
signaling that coordinates cellular behavior and differentiation. Here we
discovered a novel route to large-scale regular pattern formation in living
matter mediated by purely physical interactions. We found that dense bacterial
living matter spontaneously developed an ordered lattice of mesoscale,
fast-spinning vortices each consisting of ~10^4-10^5 motile cells; these
mesoscale vortices were arranged in space over centimeter scale with apparent
hexagonal order, while individual cells in the vortices moved in coordinated
directions with strong polar and vortical order. Single-cell tracking and
numerical simulations suggest that the phenomenon is enabled by self-enhanced
mobility of individual cells in the system. Our findings demonstrate a simple
physical mechanism for self-organized pattern formation in living systems and
more generally, in other active matter systems near the boundary of fluidlike
and solidlike behaviors.
