The interplay of the microbiome and breast cancer: beyond the gut: a narrative review

Breast cancer remains a leading cause of morbidity and mortality among women worldwide, with emerging evidence underscoring the microbiota's pivotal role in its etiology, progression, and therapeutic response. This narrative review synthesizes the intricate interplay between the breast tissue, skin, and lung microbiomes in breast cancer pathogenesis, with particular emphasis on inflammatory breast cancer (IBC) and metastatic dissemination. The healthy breast microbiome, dominated by Firmicutes, Proteobacteria, Actinobacteria, and Bacteroidetes, maintains tissue homeostasis through pH regulation, metabolite production, and immune modulation. Dysbiosis disrupts this equilibrium, fostering carcinogenesis via chronic inflammation, estrogen deconjugation, and DNA damage-induced genomic instability, with subtype-specific microbial signatures influencing tumor growth and therapy resistance. In IBC, skin microbiome alterations characterized by overgrowth of pathogens like Pseudomonas aeruginosa and Staphylococcus aureus exacerbate inflammation, epithelial-mesenchymal transition (EMT), and lymphatic invasion, while promoting a pro-tumorigenic microenvironment enriched in regulatory T cells and M2 macrophages. Concurrently, lung microbiota dysbiosis impairs immune surveillance, remodels the extracellular matrix, and facilitates metastatic seeding through neutrophil extracellular traps and cytokine storms. Cross-talk among these microbiomes amplifies systemic effects, highlighting their synergistic contributions to disease aggressiveness. Advanced analytical techniques, including 16S rRNA sequencing, metagenomics, and metabolomics, offer promising microbial biomarkers for early detection and risk stratification. By elucidating these host-microbe dynamics, this review advocates for microbiome-centric interventionssuch as probiotics, fecal microbiota transplantation, and targeted antimicrobials to enhance precision diagnostics and therapies, ultimately improving outcomes in breast cancer management.