Host–microbiome interactions in breast cancer progression and treatment response

Breast cancer (BC) is a biologically heterogeneous disease in which tumor progression and therapeutic response vary substantially across patients and molecular subtypes. Alongside genetic, endocrine, and immunological determinants, microbial ecosystems have been proposed as components of the host environment that interact with tumor biology. Microorganisms detected in breast tissue, the gastrointestinal tract, and the oral cavity coexist with epithelial and immune compartments and participate in metabolic and inflammatory processes relevant to mammary physiology. Differences in microbial composition have been reported between non-malignant and malignant breast tissue, while intestinal microbial metabolism generates bioactive compounds capable of interacting with immune regulation and systemic endocrine signaling. Microbial enzymatic activity involved in estrogen deconjugation further connects intestinal ecology with hormone-responsive disease. Microbiome-related variation has also been examined in relation to systemic therapies, where differences in microbial composition have been observed alongside variability in therapeutic outcomes. This review examines current knowledge on host-microbiome interactions across breast, gut, and oral environments and discusses how microbial ecology intersects with inflammatory signaling, metabolic regulation, and endocrine pathways relevant to breast cancer progression and treatment response. Methodological challenges and future research directions for microbiome-informed oncology are also considered.