Long-term feeding of a high-concentrate diet can induce subacute ruminal acidosis (SARA) in ruminants, which further leads to systemic inflammatory response. However, few studies have examined the effects of feeding a high-concentrate diet on the hindgut of ruminants. The purpose of this study was to investigate the effects of a high-concentrate diet on the composition of gut microbiota in colonic contents, inflammatory response, and barrier damage in the colon tissue of ruminants. A total of 12 healthy multiparous lactating Hu sheep were randomly allotted into the following 2 groups: a high-concentrate (HC) group (concentrate:forage = 7:3) and a low-concentrate (LC) group (concentrate:forage = 3:7). All sheep were fitted with ruminal fistulas. The formal feeding experiment lasted for 8 wk. After the feeding experiment, rumen fluid, portal vein blood, hepatic vein blood, colonic contents, and colon tissue samples were collected. The results showed that feeding the HC diet induced SARA in Hu sheep and significantly reduced pH in the colonic contents. The abundances of Firmicutes, Verrucomicrobiota, and Actinobacteriota decreased significantly, whereas those of Bacteroidota, Spirochaetota, and Fibrobacterota significantly increased in colonic contents. At the genus level, the relative abundances of 29 genera were significantly altered depending on the different type of diets. Analysis of the 10 bacterial genera with high relative abundance revealed that feeding the HC diet significantly reduced the abundance of UCG-005, Christensenellaceae R-7 group, UCG-010-norank, Monoglobus, [Eubacterium] coprostanoligenes group_norank, and Alistipes, whereas the abundances of Rikenellaceae RC9 gut group, Treponema, Bacteroides, and Prevotella increased. Compared with the LC group, feeding the HC diet significantly increased the concentration of LPS in rumen fluid, portal vein blood, hepatic vein blood, and colonic contents, and significantly upregulated the mRNA expression levels of proinflammatory cytokines in colon tissue, including TNF-α, IL-1β, IL-6, and IL-8, indicating the occurrence of inflammatory response in the colon tissue. In addition, the structure of colonic epithelial cells was loose, the intercellular space became larger, epithelial cells were exfoliated, and the mRNA and protein abundances of ZO-1, occludin, claudin-1, claudin-3, and claudin-4 were significantly decreased in the HC group, which was consistent with the results of immunohistochemistry. Furthermore, feeding the HC diet increased the ratios of DNA methylation and chromatin compaction in the promoter regions of occludin and claudin-1, which in turn inhibited their transcriptional expression. Therefore, the present study demonstrated that feeding an HC diet induced SARA in Hu sheep, altered the composition and structure of the microbial community in the colonic contents, induced an inflammatory response, and disrupted the intestinal mucosal barrier in the colonic tissue.
