The tumor microenvironment (TME) contains a diverse intratumoral microbiota (ITM) governing cancer onset, progression, and therapeutic response. This review compares ITM profiles of colorectal and lung cancer, two anatomically distinct malignancies linked via the gut-lung axis, and summarizes microbial detection strategies ranging from cultivation and sequencing to spatial and single-cell technologies. Oral pathobionts such as Fusobacterium nucleatum, Streptococcus, and Veillonella accumulate in both cancers, promoting tumorigenesis through chronic inflammation, immune evasion, genotoxic insults, metabolic reprogramming, and facilitating colorectal cancer lung metastasis via venous drainage and systemic dissemination. Microbial signatures in stool, saliva, blood, and tumor tissue offer non-invasive tools for detection, prognosis, and prediction of immunotherapy resistance, while antibiotics, fecal microbiota transplantation, and engineered live biotherapeutics offer strategies to remodel the TME. This synthesis positions the gut-lung axis as a systemic driver of metastasis and outlines a route toward precision microbiome-based diagnostics and therapeutics.

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.