Morphodynamics of mixed bacterial communities proliferating in three dimension

In nature, bacteria often proliferate as communities in three-dimensional (3D) environments, with different cell types cooperating or competing for resources. While many studies have investigated how proliferation drives the spatial organization of multi-strain/species communities in two dimensions, little is known about the morphology of these communities in three dimensions. Here, we address this question by directly visualizing the 3D proliferation of mixtures of two different strains of E. coli suspended in transparent jammed packings of microgel particles. Unexpectedly, even though the strains are initially well-mixed, we find that they proliferate into spatially segregated microcolonies within the overall community, with the size and shape of each microcolony determined by the initial cell density and colony width. We rationalize these results by considering the interplay between proliferation, competition for space, and competition for nutrients. Our results help to shed new light on the morphodynamics of mixed microbial communities, and provide design principles for engineering microbial communities with specific spatial architectures.