Lactate cross-feeding between Bifidobacterium species and Megasphaera indica contributes to butyrate formation in the human colonic environment

Butyrate, a physiologically active molecule, can be synthesized through metabolic interactions among colonic microorganisms. Previously, in a fermenting trial of human fecal microbiota, we observed that the butyrogenic effect positively correlated with the increasing population and an unidentified species. Therefore, we hypothesized that a cross-feeding phenomenon exists between and , where is the butyrate producer, and its growth relies on the metabolites generated by . To validate this hypothesis, three bacterial species ( , , and ) were isolated from fecal cultures fermenting hydrolyzed xylan; pairwise cocultures were conducted between the and isolates; the microbial interactions were determined based on bacterial genome information, cell growth, substrate consumption, metabolite quantification, and metatranscriptomics. The results indicated that two isolates contained distinct gene clusters for xylan utilization and expressed varying substrate preferences. In contrast, alone scarcely grew on the xylose-based substrates. The growth of was significantly elevated by coculturing it with bifidobacteria, while the two species responded differently in the kinetics of cell growth and substrate consumption. Coculturing led to the depletion of lactate and increased the formation of butyrate. An RNA-seq analysis further revealed the upregulation of genes involved in the lactate utilization and butyrate formation pathways. We concluded that lactate generated by through catabolizing xylose fueled the growth of and triggered the synthesis of butyrate. Our findings demonstrated a novel cross-feeding mechanism to generate butyrate in the human colon.IMPORTANCEButyrate is an important short-chain fatty acid that is produced in the human colon through microbial fermentation. Although many butyrate-producing bacteria exhibit a limited capacity to degrade nondigestible food materials, butyrate can be formed through cross-feeding microbial metabolites, such as acetate or lactate. Previously, the literature has explicated the butyrate-forming links between and and between and . In this study, we provided an alternative butyrate synthetic pathway through the interaction between and is a species named in 2014 and is indigenous to the human intestinal tract. Scientific studies explaining the function of in the human colon are still limited. Our results show that proliferated based on the lactate generated by bifidobacteria and produced butyrate as its end metabolic product. Th