Gut Microbiome Dynamics and Their Emerging Role in Breast and Colorectal Cancer: Implications for Diagnosis and Therapy

The gut microbiome, a complex community of microorganisms residing in the gastrointestinal tract, is increasingly recognized as a critical player in cancer pathophysiology. Its influence extends beyond local gastrointestinal disorders to systemic diseases, including breast and colorectal cancers. This review examines current evidence on the interplay between gut microbial dysbiosis and the development, progression, and treatment response in both malignancies. In colorectal cancer (CRC), pathogenic bacteria such as Fusobacterium nucleatum, Escherichia coli, and Bacteroides fragilis have been implicated in tumor initiation and progression through mechanisms involving chronic inflammation, DNA damage, and immune modulation. Meanwhile, in breast cancer, the gut microbiome may contribute indirectly via regulation of estrogen metabolism, immune signaling, and systemic inflammation. Furthermore, the gut microbiota is increasingly being explored as a source of non-invasive biomarkers for early detection and disease monitoring, as well as a modifiable factor that could enhance therapeutic efficacy. Interventions such as probiotics, prebiotics, dietary modulation, and fecal microbiota transplantation (FMT) hold promise in reprogramming the microbiome to support antitumor responses and reduce treatment-related toxicity. Despite the growing body of research, translating these findings into clinical application requires deeper mechanistic understanding and standardized methodologies. This review emphasizes the need for longitudinal and multi-omics studies to unravel causal relationships and identify therapeutic targets. Understanding gut microbiome dynamics offers exciting opportunities for developing microbiota-informed strategies for cancer prevention, diagnosis, and therapy—particularly in breast and colorectal cancers.