  During colony growth, complex interactions regulate the bacterial
orientation, leading to the formation of large-scale ordered structures,
including topological defects and microdomains. These structures may benefit
bacterial strains, providing invasive advantages during colonization. Active
matter dynamics of growing colonies drives the emergence of these ordered
structures. However, additional biomechanical factors also play a significant
role during this process. Here we show that the velocity profile of growing
colonies creates strong radial orientation during inward growth when crowded
populations invade a closed area. During this process, growth geometry sets
virtual confinement and dictates the velocity profile. Herein, flow-induced
alignment and torque balance on the rod-shaped bacteria significantly differed,
resulting in new stable orientational equilibrium in the radial direction. Our
analysis revealed that the dynamics of these orientational defects depend on
bacterial length and can promote the survival of the longest bacteria around
localized nutritional hot spots. The present results indicate a new mechanism
underlying structural order and provide mechanistic insights into the dynamics
of bacterial growth on complex surfaces.
