The Intratumoral Microbiota in Breast Cancer: Roles in Progression, Immunity, and Therapy.

Breast cancer (BC) remains a leading cause of cancer-related mortality worldwide, and accumulating evidence suggests that tumor-associated microbiota may contribute to disease heterogeneity beyond host genetic and immune determinants. Advances in sequencing and multi-omics technologies have uncovered a reproducible intratumoral microbiome in BC, with distinct compositional patterns associated with molecular subtypes, clinicopathological features, and clinical outcomes. Alterations in specific microbial taxa have also been linked to tumor immune status, metastatic potential, and therapeutic sensitivity, underscoring their potential value in disease stratification and prognostic assessment. Although breast tissue represents a low-biomass environment, multiple studies employing stringent contamination control strategies have confirmed the reliability of these microbial signals. Experimental evidence further demonstrates that intratumoral microbes are functionally active components of the tumor microenvironment (TME), influencing tumor progression and metastasis through immune modulation, inflammatory signaling, and metabolic or hormonal reprogramming, while also shaping responses to chemotherapy and immunotherapy. This review summarizes current knowledge on the compositional features, functional mechanisms, and clinical relevance of the BC intratumoral microbiome, highlights methodological challenges in low-biomass profiling, and discusses future directions for translating these findings into clinically actionable strategies. The aim of this review is to systematically evaluate the role of the intratumoral microbiome in breast cancer pathogenesis and treatment, and to propose a framework for translating current findings into clinical practice.