Fingering instability of growing multi-species microbial communities

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 different species and cells with different heritable phenotypes sharing and competing for space and essential resources. However, the mechanisms that set the shape of a growing multi-species microbial community remain poorly understood. To address this gap of knowledge we perform experiments on growing 2D bacterial colonies comprised by two different species. In agreement with previous works, we find that cells often segregate into single-strain concentric domains as the colony expands. After segregation, the outer expanding front remains smooth but the inner expanding front can develop an instability in which the boundary separating the two species forms a wavy, 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 domains. Stability analysis and numerical simulations suggest that a segregated multi-strain colony becomes morphologically unstable when domains grow at different rates and exhibit different cell-substrate friction forces. Our model recapitulates the experimental observations, suggesting that a minimal mechanistic description captures the morphodynamics of growing multi-strain 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.