Gut Microbiome-Driven Strategies to Overcome Immunotherapy Resistance in Microsatellite-Stable Colorectal Cancer

Background/Objectives: Microsatellite-stable colorectal cancer (MSS CRC) accounts for the vast majority of CRC cases and remains largely resistant to immune checkpoint inhibitors. Emerging evidence suggests that the gut microbiome is an important regulator of antitumor immunity and may contribute to immunotherapy resistance through multiple mechanisms involving the tumor microenvironment. This review aims to summarize current knowledge of the microbiome–immunity–therapy axis in MSS CRC and to explore microbiome-based strategies to enhance immunotherapy responsiveness. Methods: A narrative review of the recent literature was conducted, focusing on studies published within the last five years that investigated gut microbiota composition, microbial metabolites, tumor immune regulation, immunotherapy response, and microbiome-targeted therapeutic interventions in CRC. Evidence from mechanistic studies, translational research, clinical investigations, and multi-omics analyses was integrated. Results: Current evidence indicates that gut dysbiosis contributes to immune resistance in MSS CRC through immune exclusion, myeloid-driven immuno-suppression, T-cell dysfunction, chronic inflammation, and altered microbial metabolite signaling. Specific microorganisms, including Fusobacterium nucleatum, enterotoxigenic Bacteroides fragilis, pks-positive Escherichia coli, and other CRC-associated pathobionts, have been implicated in tumor progression and modulation of antitumor immunity. Microbial metabolites such as short-chain fatty acids, tryptophan-derived compounds, bile acids, succinate, and inosine represent key functional mediators linking microbial communities to host immune responses. Emerging microbiome-targeted interventions, including fecal microbiota transplantation, next-generation probiotics, postbiotics, selective microbial depletion, and engineered bacterial therapeutics, show potential to restore antitumor immunity and improve immunotherapy efficacy. In parallel, advances in metagenomics, metabolomics, spatial transcriptomics, and artificial intelligence are facilitating the development of precision immuno-microbiome oncology approaches. Conclusions: The gut microbiome functions as a critical regulator of immune resistance in MSS CRC through coordinated effects on microbial composition, metabolite production, and tumor immune remodeling. Microbiome-targeted interventions, combined with multi-omics-based patient stratification, may provide new opportunities to overcome immunotherapy resistance and expand the clinical benefits of immune checkpoint blockade in this traditionally refractory disease.