The progression of breast cancer is intimately governed by the tumor microenvironment, with emerging evidence underscoring the critical modulatory roles of intratumoral microbiota. This review comprehensively delineated the compositional profiles, pathogenic mechanisms, therapeutic modulatory effects, and targeted intervention strategies pertaining to the intratumoral microbiota in breast cancer. At the compositional level, neoplastic tissues display pronounced dysbiosis, with microbial signatures stratified by molecular subtypes and specific genera demonstrating prognostic relevance. Mechanistically, intratumoral microbiota synergistically orchestrates tumorigenesis, local invasion, and metastatic dissemination via the induction of DNA breaks, inflammatory cascade amplification and immune landscape remodeling, metabolic rewiring, epigenetic perturbation, and activation of canonical oncogenic signaling axes. With regard to therapeutic outcomes, intratumoral microbiota contributes to chemoresistance, endocrine therapy failure, and anti-HER2 refractoriness, while concurrently potentiating immunotherapeutic responses. On the interventional front, engineered bacterial vectors and nanoplatform-based drug delivery systems exhibited favorable synergistic antitumor efficacy in preclinical settings. Finally, this review addressed prevailing challenges, including assay standardization, integrative multi-omics interrogation, and the translational gap toward clinical implementation.
