Morphological instability of growing competing bacterial colonies*

In nature, bacteria are frequently found in colonies, a communal lifestyle that is known to provide advantages to the group, such as resistance to stressors, adhesion to surfaces, and collective access and processing of resources. Bacterial colonies can consist of cells with different heritable phenotypes sharing and competing for space and essential resources. Recent experiments on 2D growing multi-strain colonies have shown that cells become segregated into single-strain sectors, developing a morphological instability in which the interface between two sectors forms a dented, rough shape. To understand the mechanisms underlying such instability, we consider a minimal continuum model that incorporates cell growth and cell-substrate friction, both of which can vary between single-strain sectors. Stability analysis and numerical simulations suggest that a segregated multi-strain colony becomes morphologically unstable when sectors grow at different rates and exhibit different cell-substrate friction forces. Our model recapitulates the experimental observations, which suggests that a minimal mechanistic description captures the morphodynamics of multi-strain growing bacterial colonies. Moreover, our theoretical framework is not restricted to bacterial colonies, and can be extended to other growth-driven processes in living matter and ecological systems, such as developmental processes, the expansion of heterogeneous tumors, or engineered living materials.