Abstract
From insects to mammals, a large variety of animals hold in their intestines complex bacterial communities that play an important role in health and disease. However, the complexity of these gut microbiomes and their hosts often constrains our ability to understand how these bacterial communities assemble and function. In order to elucidate basic principles of community assembly in a host intestine, we study the assembly of the microbiome of Caenorhabditis elegans with a bottom-up approach. We colonize the gut of the worm C. elegans with 11 bacterial species individually, in all possible pairs, and in selected trios, and we find an organized mixture of coexistence and competitive exclusion that indicates a hierarchical structure in the bacterial interactions. The capacity of a bacterial species fed in monoculture to colonize the C. elegans intestine correlates with its average fraction in co-culture experiments, yet fails to predict its abundance in many two- and three-species microbiomes. Hence, the bacterial fractional abundances in co-culture experiments—pairwise outcomes—are influenced by interspecies interactions. These pairwise outcomes accurately predict the trio outcomes in the worm intestine, further highlighting the importance of pairwise interactions in determining community composition. We also find that the C. elegans gut environment influences the outcome of co-culture experiments, and demonstrate that the low intestinal pH is one of the causes. These results highlight that a bottom-up approach to microbiome community assembly may provide valuable insight into the structure and composition of complex microbial communities.
