The human gut has digestion, absorption, barrier, and immune functions. The intestinal barrier mainly keeps the body away from the invasion and destruction of bacteria and harmful substances to maintain whole body homeostasis (Camilleri et al., 2012). Many diseases can cause intestinal barrier injury, the most common of which are short bowel syndrome (SBS) in children and inflammatory bowel disease (IBD) in adults (Allan and Lal, 2018, Mezoff et al., 2019). When the intestinal barrier is severely damaged, pathogenic bacteria and endotoxins can be easily displaced, leading to endogenous infections (Konig et al., 2016) and even intestinal failure, which is life-threatening. Therefore, protection of the intestinal barrier is essential but difficult in the treatment of intestinal dysfunction.

Probiotics are known to protect the intestinal mucosal barrier (Dylag et al., 2014). C. butyricum is a strictly anaerobic Gram-positive bacterium that has the following properties: tolerates gastric acid when entering the intestine; secretes butyric acid, an important nutrient for intestinal mucosa regeneration and repair (Stoeva et al., 2021); regulates the gut microbiota composition within the intestinal lumen (Miao et al., 2018); and stimulates intestinal macrophages through the toll-like receptor 2(TLR2) signaling pathway and induces macrophages to produce IL-10, thereby controlling the progression of inflammation (Hagihara et al., 2018). In addition, C. butyricum has the excellent characteristics of acid resistance and bile resistance, which makes it not susceptible to the influence of human digestive fluids such as stomach acid and bile acid in the process of passing through the digestive tract, and can still maintain high activity after reaching the intestine, laying a good foundation for further exerting its various effects (Bai and Xin, 2020). C. butyricum also has certain resistance to some antibiotics such as penicillin, streptomycin, chloramphenicol, etc., and is considered to be used in combination with antibiotics (Huang, 2024). However, the mechanism underlying the protective effect of C. butyricum on the intestinal barrier especially the small intestine is not fully understood.

Farnesoid X receptor(FXR) also plays a vital role in the protection of intestinal barrier damage. The role of FXR in hepatoenteric diseases has recently received attention (Almeqdadi and Gordon, 2024). FXR is a ligand-activated transcription factor and a member of the nuclear receptor superfamily that is widely expressed in the small intestine, colon, and liver (Chiang and Ferrell, 2022). It has been shown that expression of FXR differs in animals treated with C. butyricum (Wang et al., 2020). In previous studies on gut FXR, we observed that after OCA (FXR-specific activator) was administered to animals, the regulatory FXR was correlated with the trend of Clostridium in intestinal flora (Hou et al., 2024, Yan et al., 2022). This suggests that there may be some adjustment mechanism between C. butyricum and FXR.

C. butyricum regulates the bile acid (BA) profile, which significantly reduces the tauro-α-muricholic acid (TαMCA) content and increases the tauroursodeoxycholic acid (TUDCA) and lithocholic acid (LCA) content. BAs have been shown to be endogenous ligands of FXR. FXR is activated by conjugated and unconjugated BAs (Sun et al., 2021). Chenodesoxycholic acid (CDCA) is the best activator of FXR. However, not all kinds of BAs are FXR agonists and some hydrophilic BAs, such as ursodeoxycholic acid (UDCA) and muricholic acid (MCA) do not activate FXR (Ding et al., 2015). TαMCA is an FXR antagonist, while TUDCA and LCA are FXR agonists (Jiang et al., 2015a). Indeed, it is possible that C. butyricum regulates FXR expression by altering the intestinal bile acid(BA) profile.

The mechanism of action by which FXR protects the intestine has also been extensively studied. Specifically, metabolites downstream of FXR in the ileum are inhibited in rats with biliary obstruction. Obeticholic acid (OCA) activation of the FXR signaling pathway significantly reduces liver fibrosis and intestinal inflammation in BDL rats and improves intestinal microbiota and protects the intestinal mucosa (Yan et al., 2022). OCA activates the FXR signaling pathway, regulates the intestinal flora, inhibits pro-inflammatory factor expression, and improves intestinal barrier damage in a rat SBS model (Hou et al., 2024). In addition, the FXR-FGF15/19 pathway can protect the intestinal mucosal barrier by regulating the size and composition of the BA pool, which counters the local inflammatory response and adjusts the composition of intestinal bacteria (Inagaki et al., 2005, Lundasen et al., 2006).

C. butyricum may alleviate dextran sulfate sodium (DSS)-induced small intestine injury through FXR regulation, but the underlying mechanism has not been established. In the current study we elucidated the role of C. butyricum by constructing an ileal injury model and evaluated the impact of FXR on the intestinal protective effect of C. butyricum. FXR was shown to have an important role in the intestinal protection of C. butyricum, which was confirmed in vivo and in vitro.