Fabian Schwarzendahl - Growth, mixing, and nematic dynamics of growing bacterial colonies

Recent studies have shown that packings of cells, both eukaryotic cellular tissues and growing or swarming bacterial colonies, can often be understood as active nematic fluids. A key property of volume-conserving active nematic model systems 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 affect evolutionary outcomes in bacterial colonies by counteracting the tendency to spatially segregate into monoclonal sectors, which reduces the local genetic diversity and confines competition between subpopulations to the boundaries between neighbouring sectors. To examine whether growth-driven active nematic physics could influence this genetic de-mixing process, we conduct agent-based simulations of growing, dividing, and sterically repelling rod-like bacteria of various aspect ratios, and we analyse colony morphology using tools from both soft matter physics and population genetics. We find that despite measurable defect self-propulsion in growth-driven active nematics, the radial expansion flow prevents chaotic mixing. Even so, at biologically relevant cell aspect ratios, self-mixing is more effective in growing active nematics of rod-like cells compared to growing isotropic colonies of round cells. This suggests potential evolutionary consequences associated with active nematic dynamics. Further, bacteria are able to react to steric forces exerted on them, a mechanism called mechano-sensing. We develop a dynamical density functional theory for growing bacterial colonies, in which mechano-sensing arises naturally. By analysing the length distributions of bacteria, we identified regimes in which interactions or diffusion dominate. These results can have implications for the formation of biofilms.