Nutritional and host environments determine community ecology and keystone species in a synthetic gut bacterial community

Microbe-microbe interactions are critical for gut microbiome function. A challenging task to understand health and disease-related microbiome signatures is to move beyond descriptive community-level profiling towards disentangling microbial interaction networks. Here, we aimed to determine members taking on a keystone role in shaping community ecology of a widely used synthetic bacterial community (OMM12). Using single-species dropout communities and metabolomic profiling, we identified Bacteroides caecimuris I48, Blautia coccoides YL58 and Enterococcus faecalis KB1 as major drivers of in vitro community assembly and elucidated underlying mechanisms of these keystone functions. Importantly, keystone species and bacterial strain relationships were found to strongly vary across different nutritional conditions, depending on the strains potential to modify the corresponding environment. Further, gnotobiotic mice transplanted with communities lacking B. caecimuris I48 and B. coccoides YL58 exhibited morphological anomalies and altered intestinal metabolomic profiles, exposing physiologically relevant functions of these keystone community members. In summary, the presented study experimentally confirms the strong interdependency between bacterial community ecology and the biotic and abiotic environment, underlining the context-dependency and conditionality of bacterial interaction networks.