Reaction-diffusion model of chiral growth accurately describes the behavior of sector boundaries in compact microbial colonies.

Population dynamics are visualized by the spatial pattern produced during the growth of two neutral strains expressing different fluorescent proteins. The growth is largely limited to the colony edge, so the patterns behind the front do not change over time. Although initially the strains are well-mixed, strong genetic drift leads to local extinctions of one of the strains, which manifests as a characteristic pattern of sectors in both experiments (A) and simulations (B). The boundaries between the sectors fluctuate due to genetic drift and twist counterclockwise due to a chiral bias in cell motion. This bias is quantified in (C) for experiments and in (D) for simulations by plotting the polar angle θ, averaged over many sector boundaries, vs. the radius r. A constant boundary velocity along the colony edge should result in a linear increase of θ with lnr [35]. Consistent with this expectation, both plots show that sector boundaries are logarithmic spirals. The excellent agreement between experiments and simulations indicates that our reaction-diffusion model is suitable for the study of competition between chiral strains in compact microbial colonies. The experimental data was obtained from the Dryad digital data repository associated with Ref. [18]. Here, m0 = ms = mb = md = 0, g = 0.03, N = 100, ml = 0.045, mr = 0.005 for both strains. Radius of initial circle was 30 on a lattice of 700x700 sites.