  Growing monolayers of rod-shaped bacteria exhibit local alignment similar to
extensile active nematics. When confined in a channel or growing inward from a
ring, the local nematic order of these monolayers changes to a global ordering
with cells throughout the monolayer orienting in the same direction. The
mechanism behind this phenomenon is so far unclear, as previously proposed
mechanisms fail to predict the correct alignment direction in one or more
confinement geometries. We present a strain-based model relating net
deformation of the growing monolayer to the cell-level deformation resulting
from single-cell growth and rotation, producing predictions of cell orientation
behavior based on the velocity field in the monolayer. This model correctly
predicts the direction of preferential alignment in channel-confined,
inward-growing, and unconfined colonies. The model also quantitatively predicts
orientational order when the velocity field has no net negative strain rate in
any direction. We further test our model in simulations of expanding colonies
confined to spherical surfaces. Our model and simulations agree that cells away
from the origin cell orient radially relative to the colony's center.
Additionally, our model's quantitative prediction of the orientational order
agrees with the simulation results in the top half of the sphere but fails in
the lower half where there is a net negative strain rate. The success of our
model bridges the gap between previous works on cell alignment in disparate
confinement geometries and provides insight into the underlying physical
effects responsible for large-scale alignment.
