Narrative review examines microorganisms in breast cancer with limited evidence from cross-sectional studies and modelsReview of microorganisms and breast cancer evidence remains uncertain

Breast cancer is a complex pathological process involving multiple factors and stages, characterized by pronounced molecular and phenotypic heterogeneity. Its global incidence and mortality rates have shown a continuous upward trend. With the advancement of microbiome research, microbial communities have been recognized as key determinants influencing host health and disease states. Increasing evidence suggests a close association between breast tissue–resident and systemic microbiota and the initiation and progression of breast cancer. Specifically, microorganisms may be associated with abnormal proliferation and malignant transformation of mammary epithelial cells through diverse mechanisms, including the modulation of estrogen metabolism, production of bioactive metabolites, induction of chronic inflammation, and remodeling of the tumor microenvironment. In addition, certain microbes may directly interact with host cells, potentially inducing DNA damage and contributing to the transition from normal to malignant phenotypes. This review systematically summarizes the origins and compositional characteristics of the breast microbiota, with a particular focus on current evidence regarding its roles in breast cancer initiation, progression, metastasis, therapeutic response, and prognosis. Currently, the majority of evidence originates from cross-sectional studies and in vitro/in vivo model, to better evaluate the current evidence, the limitations of different research designs and the levels of evidence are summarized in Table 1, aiming to provide new theoretical insights and research perspectives for microbiota-based strategies in breast cancer diagnosis and therapy.