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Abstract: Colorectal cancer (CRC) arises within a complex microbial ecosystem that may influence carcinogenesis, tumor immunity, and therapeutic response. This is particularly relevant in metastatic disease, where most tumors are microsatellite-stable/mismatch repair-proficient (MSS/pMMR) and derive little meaningful benefit from immune checkpoint inhibitor (ICI) monotherapy. Specific pathobionts, including Fusobacterium nucleatum, enterotoxigenic Bacteroides fragilis, and colibactin-producing Escherichia coli, have been implicated in genotoxic, inflammatory, and checkpoint-related mechanisms that may contribute to tumor progression and immune evasion, whereas selected commensals and microbiota-derived metabolites may support antitumor immunity in a context-dependent manner. This review integrates translational evidence linking microbial taxa and functions, metabolites, neutrophil biology, bacterial extracellular vesicles, and T-cell dysfunction with CRC progression and immunotherapy response. This review critically examines emerging microbiome-directed strategies, including fecal microbiota transplantation, next-generation probiotics, defined bacterial consortia, dietary and perioperative interventions, postbiotics, and bacteriophage-based approaches, as potential means of modifying immune resistance in MSS/pMMR CRC. Although early clinical signals from microbiota-modulating combinations are encouraging, the available evidence remains limited by small cohorts, heterogeneous interventions, and the absence of randomized CRC-specific studies. Microbiome-based biomarkers and therapeutics therefore remain investigational, and standardized sampling, prospective validation, and carefully designed trials will be required before microbiome modulation enters routine CRC care.