Formation of self-assembled asters in growing bacterial colonies

During colony growth, complex biomechanical interactions regulate the bacterial orientation, leading to the formation of large-scale ordered structures including topological defects. These structures may benefit bacterial strains, providing invasive advantages during colonization. Thus far, the emergence of 1/2 topological defects has been observed and extensively assessed. Here, we experimentally and numerically investigate aster formation in bacterial colonies during inward growth when crowded populations invade a closed area. Herein, the velocity field and torque balance on the rod-shaped bacteria significantly differed, resulting in new stable orientational equilibrium in the radial direction and to the formation of +1 topological defects. The dynamics of these 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 defect formation and provide mechanistic insights into the dynamics of bacterial growth on complex surfaces.