Cobamide-based interactions between soil bacteria can be predicted based on monoculture growth.

Interactions between microbes shape the structure and function of microbial communities. While studying interactions is key to understanding microbial communities as a whole, gaining a detailed mechanistic view is challenging due to the number of co-occurring interactions. Here, we focus on a single class of shared nutrients, the cobalamin (vitamin B12) family of enzyme cofactors known as cobamides, to study nutrient interactions in laboratory consortia of increasing complexity. Measuring cobamide-dependent growth of two soil Caulobacter isolates in monoculture revealed that each grows faster within a distinct range of cobamide concentrations. Co-culturing the two isolates demonstrated that the cobamide concentration predictably determines which isolate predominates. These results suggest that, although the two organisms appear functionally redundant, with identical metabolic capabilities predicted in their genomes, they may occupy distinct niches in their environment based on cobamide concentration. We further examined the ability of cobamide-producing Mesorhizobium and Priestia isolates to share cobamides and found that both support the cobamide-dependent growth of the two Caulobacter isolates. The Caulobacter isolates could coexist in tri-cultures with each producer following serial passaging, indicating the producers can support the growth of multiple cobamide-dependent organisms. We analyzed the metabolic capacity encoded in the genomes of the four isolates and found that cobamides are likely the main shared nutrient in our co- and tri-cultures. These results highlight the utility of cobamides as a model nutrient to characterize and predict interactions in bacterial consortia of increasing complexity.IMPORTANCEInteractions between microbes shape their communities and the world around them. Because microbial communities are physically and chemically complex, predicting microbial interactions is often difficult. Using a set of four bacteria isolated from the same soil environment, we generated and tested predictions about the interactions involving a family of model nutrients related to cobalamin (vitamin B12). Understanding and predicting interactions can lead to greater insights into microbiome function.