An Ecological Network of Polysaccharide Utilization among Human Intestinal Symbionts

BACKGROUND The human intestine is colonized with trillions of microorganisms important to health and disease. There has been an intensive effort to catalog the species and genetic content of this microbial ecosystem. However, little is known of the ecological interactions between these microbes, a prerequisite to understanding the dynamics and stability of this host-associated microbial community. Here we perform a systematic investigation of public goods-based syntrophic interactions among the abundant human gut bacteria, the Bacteroidales. RESULTS We find evidence for a rich interaction network based on the breakdown and use of polysaccharides. Species that utilize a particular polysaccharide (producers) liberate polysaccharide breakdown products (PBPs) that are consumed by other species unable to grow on the polysaccharide alone (recipients). Cross-species gene addition experiments demonstrate that recipients can grow on a polysaccharide if the producer-derived glycoside hydrolase, responsible for PBP generation, is provided. These producer-derived glycoside hydrolases are public goods transported extracellularly in outer membrane vesicles allowing for the creation of PBP and concomitant recipient growth spatially distant from the producer. Recipients can exploit these ecological interactions and conditionally outgrow producers. Finally, we show that these public goods-based interactions occur among Bacteroidales species coresident within a natural human intestinal community. CONCLUSIONS This study examines public goods-based syntrophic interactions between bacterial members of the human gut microbial ecosystem. This polysaccharide-based network likely represents foundational relationships creating organized ecological units within the intestinal microbiota, knowledge of which can be applied to impact human health.