Competitions Between Fibrobacter Succinogenes, Ruminococcus Flavefaciens, and Ruminoccus Albus in a Gnotobiotic Sheep Model Revealed by Multi-Omic Analyses

, , and are the three predominant cellulolytic bacterial species found in the rumen. studies have shown that these species compete for adherence to, and growth upon, cellulosic biomass. Yet their molecular interactions have not heretofore been examined. Gnotobiotically raised lambs harboring a 17-h-old immature microbiota devoid of culturable cellulolytic bacteria and methanogens were inoculated first with S85 and sp. strain 87.7, and 5 months later, the lambs were inoculated with 8 and FD-1. Longitudinal samples were collected and profiled for population dynamics, gene expression, fibrolytic enzyme activity, fibrolysis, and metabolite profiling. Quantitative PCR, metagenome and metatranscriptome data show that establishes at high levels initially but is gradually outcompeted following the introduction of the ruminococci. This shift resulted in an increase in carboxymethyl cellulase (CMCase) and xylanase activities but not in greater fibrolysis, suggesting that and ruminococci deploy different but equally effective means to degrade plant cell walls. Expression profiles showed that relied upon outer membrane vesicles and a diverse repertoire of CAZymes, while and preferred type IV pili and either CBM37-harboring or cellulosomal carbohydrate-active enzymes (CAZymes), respectively. The changes in cellulolytics also affected the rumen metabolome, including an increase in acetate and butyrate at the expense of propionate. In conclusion, this study provides the first demonstration of competition between the three predominant cellulolytic bacteria and provides insight on the influence of these ecological interactions on rumen fibrolytic function and metabolomic response. Ruminant animals, including cattle and sheep, depend on their rumen microbiota to digest plant biomass and convert it into absorbable energy. Considering that the extent of meat and milk production depends on the efficiency of the microbiota to deconstruct plant cell walls, the functionality of predominant rumen cellulolytic bacteria, , , and , has been extensively studied to obtain a better knowledge of how they operate to hydrolyze polysaccharides and ultimately find ways to enhance animal production. This study provides the first evidence of competitions between and the two species. It shows that a simple disequilibrium within the cellulolytic community has repercussions on the rumen metabolome and fermentation end products. This finding will have to be considered in the future when determining strategies aiming at directing rumen fermentations for animal production.