Supplementation of Bacillus cereus Enhances Anti-Colitis Effects of Triple Probiotics in Mice: Correlative Evidence for a Gut Microbiota–Spermidine-Related Axis in Quadruple-Probiotic Protection

Background: Ulcerative colitis (UC) is a chronic relapsing inflammatory bowel disease associated with gut microbiota dysbiosis. The triple-probiotic mixture BLE (Bifidobacterium infantis, Lactobacillus acidophilus, Enterococcus faecalis) contains highly oxygen-sensitive strains prone to viability loss during gastrointestinal transit. Adding facultative-aerobic Bacillus cereus generates a four-strain probiotic formulation BLEB. This study aimed to determine whether adding B. cereus improves the anti-colitis capacity of BLE and explore the underlying microbiota–spermidine-related regulatory mechanism. Methods: Acute dextran sulfate sodium (DSS)-induced colitis mouse models were applied to compare the therapeutic effects of BLE and BLEB. We evaluated disease phenotype, intestinal epithelial barrier integrity, and splenic Treg/Th17 immune homeostasis. Dynamic 16S rRNA sequencing and untargeted intestinal metabolomics characterized gut microbial and metabolic alterations, with Spearman correlation for microbiota–metabolite association analysis. Transcriptomic profiling was performed on DSS-injured Caco-2 cells to examine molecular responses to spermidine. Results: BLEB exerted superior dose-dependent anti-colitis effects compared with BLE. BLEB enriched beneficial Lactobacillaceae and restored luminal polyamine metabolites including spermidine. Spearman correlation revealed significant positive correlations between Lactobacillaceae-affiliated genera and polyamine levels. In vitro, spermidine reversed DSS-provoked transcriptional perturbations and upregulated the transcription of glutathione-centered antioxidant-related genes in Caco-2 cells. Conclusions: Ecological synergy introduced by B. cereus may reshape gut microbiota and increase microbiota-related spermidine, partially contributing to the enhanced mucosal protection conferred by BLEB. This work provides pre-clinical evidence supporting the development of aerobic–anaerobic combined probiotic cocktails for auxiliary UC therapy.