Mechanics limits ecological diversity and promotes heterogeneity in confined bacterial communitie

Multispecies bacterial populations often inhabit confined and densely packed environments where spatial competition determines the ecological diversity of the community. However, the role of mechanical interactions in shaping the ecology is still poorly understood. Here, we study a model system consisting of two populations of non-motile E. coli bacteria competing within open, monolayer microchannels. After seeding, either one strain rapidly fixates or both strains orient into spatially stratified, quasi-stable communities. We find that mechanical interactions with other cells and the confining environment influence the resulting community ecology in unexpected ways, severely limiting the overall diversity of the communities while simultaneously allowing for the establishment of stable, heterogeneous populations of bacteria displaying disparate growth rates. Surprisingly, the populations have a high probability of coexisting even when one strain has a significant growth advantage. We then go on to study the competitive advantage of cellular morphology by affecting the structural protein MreB. Our observations are well supported by agent based simulations and qualitatively reproduced by a simple Pólya urn model,. These results provide fundamental insights into the determinants of community diversity in dense confined ecosystems such as organized biofilms or intestinal crypts.