Gut segments outweigh the diet in shaping the intestinal microbiota composition in grass carp Ctenopharyngodon idellus

Although dynamics of the complex microbial ecosystem populating the gastrointestinal tract of animals has profound and multifaceted impacts on host's metabolism and health, it remains unclear whether it is the intrinsic or extrinsic factors that play a more dominant role in mediating the composition of intestinal microbiota. To address this, we studied the impacts of two strikingly different diets on a herbivorous fish, grass carp (Ctenopharyngodon idellus): a high-protein, low-fiber formula feed, and low-protein, high-fiber Sudan grass. After a 16-week feeding trial, microbial profiles of midgut and hindgut segments in both groups were compared. Bacterial composition was significantly different between the midguts of both groups, but not between the hindguts of two groups. Both PerMANOVA and VPA analyses suggested that gut segments explain a higher proportion of variation in intestinal microbiota than diet. Overall, our results suggest that intestinal compartments are a stronger determinant than diet in shaping the intestinal microbiota. Specifically, whereas diet has a strong impact on the composition of microbiota in proximal gut compartments, this impact is much less pronounced distally, which is likely to be a reflection of a limited ability of some microbial taxa to thrive in the anoxic environment in distal segments.