Do Active Nematic Self-Mixing Dynamics Help Growing Bacterial Colonies to Maintain Local Genetic Diversity?

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.

Stress anisotropy in confined populations of growing rods

“…However, as the open domains show, the decoupling seems to be an intrinsic feature of near-perfect orientational order. Therefore, it could also be relevant for the emergent dynamics on other length scales, such as the growth and break-up of microdomains in expanding colonies, which are highly ordered patches at intermediate length scales [ 19 , 21 , 40 ]. Moreover, the decoupling is closely related to the buckling instability, as it allows the build-up of unequal stresses in the first place [ 30 ].…”

Resource limitation and population fluctuation drive spatiotemporal order in microbial communities

“…To this end, it is worth mentioning that, in a similar context of nonmotile bacterial colonies, a recent study by Schwarzendahl et al 55 identified +1/2 and À1/2 topological journal is © The Royal Society of Chemistry 2024 defects. [56][57][58] Their methodology involved computations of the nematic tensorial orientation field, mapping it to continuum fields using a smoothing function and identifying these defects through integration and clustering algorithms.…”

“…For further details regarding calculating the orientational autocorrelation, please refer to the ESI. †To this end, it is worth mentioning that, in a similar context of nonmotile bacterial colonies, a recent study by Schwarzendahl et al55 identified +1/2 and À1/2 topological…”

Dispersion of activity at an active–passive nematic interface

“…47,48 Moreover, simulations of growing bacterial colonies revealed that the cells mix more efficiently when they are elongated. 49 It is known that bacteria can change their aspect ratio depending on the conditions. 8,50 It is also possible to control the aspect ratio of bacteria in experiments, which revealed that the vorticity in swarming bacteria is maximized for an intermediate aspect ratio.…”