Resource competition and self-inhibition shape morphology and expansion across isogenic bacterial colonies

Bacterial biofilms rarely exist in isolation - in natural habitats, colonies are embedded within complex communities spanning diverse growth phases, where multiple biofilms interact across scales. While resource competition is clearly a determinant of colony expansion, a growing number of studies have revealed a variety of antagonistic interactions that directly inhibit expansion of neighboring colonies, even when they are genetically identical (“sibling rivalry”). A hallmark of sibling rivalry is the conditional merging of genetically identical, but spatially separated colonies: while colonies initiated at sufficient proximity will eventually fuse, those seeded farther apart remain segregated by a cell-free, inhibitory zone. Although this behavior is increasingly recognized across microbial species, its mechanisms remain unresolved. By combining modeling and experiments, we show that nutrient competition and direct growth inhibition control colony morphology and expansion in spatially extended colony networks of the opportunistic pathogen Enterococcus faecalis. We identify distinct regimes of gap formation, and we find that direct inhibition is necessary and sufficient to capture intra- and inter-colony spatial patterns of isogenic populations across nutrient conditions.