Activity-driven emergence of genealogical enclaves in growing bacterial colonies

Bacterial dispersal, the movement of cells spanning diverse physical scales and environments, has been long investigated owing to its far-reaching ramifications in the ecology and evolution of bacterial species and their consortia. A major proportion of bacterial species are surface associated, yet if and how they disperse, specifically during the early stages of biofilm formation, remains to be understood. While physical vectors like fluid flow drive dispersal across large scales, surface-associated cells may benefit from the active biomechanical forces to navigate locally within a colony. Here, by analyzing sessile bacterial colonies, we study how cells disperse over generations due to the growth-induced forces under different conditions. A custom-built label-free algorithm, developed to track the progeny cells as they grow and divide, reveals the emergence of distinct self-similar genealogical enclaves which intermix over time. Biological activity, indicated by the division times, is a key determinant of the intermixing dynamics; while topological defects appearing at the interface of the enclaves mediate the morphology of finger-like interfacial domains. By quantifying the Shannon entropy, we show that dividing bacterial cells have spatial affinity to close relatives, at the cost of the entropically favourable option of intermixing, wherein faster growing colonies show higher drop in the Shannon entropy over time. A coarse-grained lattice modelling of such colonies, combined with insights from the thermodynamics of phase separation, suggest that the emergence of genealogical enclaves results from an interplay of growth-induced cell dispersal within the colony (which promotes intermixing) and stochasticity of cell division, alongwith the cell-cell interactions at a given growth condition. Our study uncovers the evolution of so-far hidden emergent self-organising features within growing bacterial colonies, which while displaying a high degree of self-similarity on a range of phenotypic traits, point at competing roles of growth-induced forces and entropic landscapes which ultimately shape the genealogical distance of cells to their kith and kin within growing colonies.