Do active nematic self-mixing dynamics help growing bacterial colonies to maintain local genetic diversity?

TL;DR

This study investigates whether growth-driven active nematic physics in bacterial colonies can promote genetic mixing, finding that while defect self-propulsion occurs, radial expansion limits chaotic mixing, but rod-like cell colonies exhibit enhanced mixing over isotropic ones.

Contribution

It introduces agent-based simulations of growing bacterial colonies modeled as active nematics, revealing how defect dynamics influence genetic mixing during colony expansion.

Findings

Defect self-propulsion is measurable in growth-driven active nematics.

Radial expansion flow prevents chaotic mixing despite defect activity.

Rod-like cells show more effective self-mixing compared to round cells.

Abstract

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 neighboring sectors. To examine whether growth-driven active nematic physics could influence this genetic demixing process, we conduct agent-based simulations of growing, dividing, and sterically repelling rod-like bacteria of various aspect ratios, and we analyze 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.