Cross-feeding enables robust coexistence between four bacterial species.

Resource supply is generally considered a dominant driver of microbial diversity. Here, using a four-species bacterial consortium, we asked whether coexistence can emerge from interactions between species rather than being dictated by the external environment. Across 31 simple nutrient conditions, including 16 single-resource environments, all four species survived and coexisted over eight growth-and-dilution cycles. We then chose 28 additional conditions to further probe the boundaries of coexistence by varying resource concentrations, temporal dynamics, nutrient complexity and relief of auxotrophy-driven dependencies, and only observed the extinction of a single species in one of these conditions. Although the community composition in each environment was largely shaped by species' fitness on the supplied resources, experimental assays and consumer-resource modeling showed that the coexistence was explained by cross-feeding and niche partitioning of metabolic byproducts. These metabolic interactions were strong enough to sustain coexistence even for species that failed to exhibit significant growth on the supplied resources in monoculture. Furthermore, robust coexistence across environments appears to be an emergent property of the community itself, ingrained in the members' metabolic byproduct profiles and niche differences. These findings demonstrate that the richness of microbial consortia is ultimately governed by these intrinsic metabolic traits, rather than being strictly dictated by the number or identity of the externally supplied resource.