  Bacterial colonies are abundant on living and nonliving surfaces and are
known to mediate a broad range of processes in ecology, medicine, and industry.
Although extensively researched, from single cells to demographic scales, a
comprehensive biomechanical picture, highlighting the cell-to-colony dynamics,
is still lacking. Here, using molecular dynamics simulations and continuous
modeling, we investigate the geometrical and mechanical properties of a
bacterial colony growing on a substrate with a free boundary and demonstrate
that such an expanding colony self-organizes into a "mosaic" of microdomains
consisting of highly aligned cells. The emergence of microdomains is mediated
by two competing forces: the steric forces between neighboring cells, which
favor cell alignment, and the extensile stresses due to cell growth that tend
to reduce the local orientational order and thereby distort the system. This
interplay results in an exponential distribution of the domain areas and sets a
characteristic length scale proportional to the square root of the ratio
between the system orientational stiffness and the magnitude of the extensile
active stress. Our theoretical predictions are finally compared with
experiments with freely growing E. coli microcolonies, finding quantitative
agreement.
