Morphodynamics of bacterial communities proliferating in three dimensions

In nature, bacteria often grow as communities in three-dimensional (3D) environments, with multiple 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 3D. Here, we use two different strains of E. coli suspended in a transparent jammed packing of microgel particles to investigate the morphodynamics of communities with multiple cell types proliferating in 3D. Unexpectedly, even though the strains are initially well-mixed, we find that they proliferate into single-strain 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 nutrients. Taken together, our results help to shed new light on the morphodynamics of mixed microbial communities, as well as other forms of proliferating active matter, in 3D.