3D-induced polar order and topological defects in growing bacterial populations

Colonies of rod-shaped bacteria, such as Escherichia coli, initially expand two-dimensionally but eventually show 3D growth. While it was recently reported for motile bacteria that the switch is promoted by influx of cells toward +1/2 topological defects, how cellular alignment plays a role in non-motile cases is largely unknown. Here, we study the role of cellular alignment and topological defects in colony formation of non-motile E. coli populations. We show that, while initially only +1/2 defects attract cells, cells gradually flow toward -1/2 defects as well, leading to vertical growth around both types of defects. This is in contrast to the current knowledge on 2D active nematics, which indicates repulsion of cells from -1/2 defects. We reveal that the key is in 3D tilting of cells around defects, leading to the formation of tilt-induced polar order and resulting polarity-induced force. We included this to the 2D active nematics theory and successfully accounted for the influx to -1/2 defects. We argue that 3D cell orientations may result in qualitative changes in properties of 2D active nematics, which may be characteristic of such non-motile but growing active matter.