Douchi-derived Bacillus subtilis modulates gut microbiota and bile acid metabolism to alleviate metformin-induced diarrhea in type 2 diabetes mellitus

1,1-Dimethylbiguanide hydrochloride (Metformin) effectively lowers blood glucose levels in type 2 diabetes mellitus (T2DM) patients, but often causes gastrointestinal side effects, possibly due to disrupted bile acid metabolism. Bacillus subtilis, a key fermentative strain in traditional Chinese fermented foods like douchi, has been shown to aid in blood glucose control in T2DM mice, although the mechanism remains unclear. We induced T2DM in male C57BL/6J mice using a 60 % high-fat diet (HFD) and 75 mg/kg streptozotocin (STZ). Mice were administered PBS, metformin (200 mg/kg/day), Bacillus subtilis (10(10) CFU/mL), or their combination (Metformin + B. subtilis) via oral gavage for six weeks. The results indicated that B. subtilis significantly increased the quantities and compositions of key bacteria associated with bile acid metabolism, including Firmicutes, Actinobacteria, Bacteroidetes, and Proteobacteria compared to the metformin group (P < 0.05). B. subtilis also altered bile acid composition, promoting the conversion of primary bile acids like cholic acid (CA), muricholic acid (MCA), taurochenodeoxycholic acid (TCDCA), and glycochenodeoxycholic acid (GCDCA) into secondary bile acids, including lithocholic acid (LCA) and deoxycholic acid (DCA) (P < 0.05), thereby enhancing bile acid metabolism. Furthermore, B. subtilis significantly reduced bile acid accumulation and colon tissue damage induced by metformin, while enhancing its blood glucose-lowering effect and effectively mitigating gastrointestinal side effects (P < 0.05). These findings propose a novel therapeutic approach for T2DM.