Do active nematic self-mixing dynamics offer evolutionary benefits to growing bacterial colonies?

Recent works have shown that diverse types of cell packings, including eukaryotic cellular tissues and growing or swarming bacterial colonies, are well-described by hydrodynamic models of active nematic liquid crystals. A key property of volume-conserving active nematic dynamics is chaotic self-mixing characterized by motile topological defects. However, for active nematics driven by growth rather than motility, less is understood about mixing and defect motion.  Mixing could provide evolutionary benefits to bacterial colonies by counteracting the tendency to spatially segregate into monoclonal sectors, which reduces the local genetic diversity. To study whether growth-driven active nematic physics could benefit bacteria evolutionarily, we conduct agent-based simulations of growing, dividing, and sterically repelling rod-like bacteria of various aspect ratios, and we analyze the results with tools from both active matter and population genetics. Our results demonstrate important differences in defect dynamics between growth-driven and motility-driven active nematics. We also show that at biologically relevant aspect ratios, self-mixing is more effective in growing active nematics of rod-like cells compared to growing isotropic colonies of round cells, predicting measurable differences in local genetic diversity.