Mammary Gland Microbiota in Benign Breast Diseases

The human microbiome is a critical factor in health and disease, including breast pathology. While its association with breast cancer (BC) is increasingly studied, the specificity of the microbiome and its role in benign breast diseases (BBD) remain poorly understood. This review synthesizes current evidence on the origins of the mammary gland (MG) microbiota, which is distinct from the skin and formed via exogenous (e.g., cutaneous, retrograde via ducts) and endogenous (e.g., enteromammary, hematogenous) pathways. We detail the mechanisms of host-microbiota interaction, such as regulation of estrogen metabolism, immunomodulation, and epigenetic modifications, which can influence disease pathogenesis. The analysis reveals that while taxonomic profiles of tissue and gut microbiota share similarities between BBD and BC, key differences exist in the abundance of specific taxa (e.g., Pseudomonadota, Bacillota) and associated metabolic pathways. The review summarizes microbiota alterations associated with specific BBDs, including fibroadenomas, cysts, lactational and non-lactational mastitis (e.g., linked to Corynebacterium kroppenstedtii), and purulent-septic complications. Major limitations in the field are identified, such as the low microbial biomass of breast tissue, a lack of data on the virome and mycobiome, and the inability of current studies to establish causality. We conclude that microbial dysbiosis is implicated in BBD. However, further research is essential to elucidate cause-effect relationships. Understanding the microbiome’s role holds significant promise for developing novel diagnostic, preventive, and personalized therapeutic strategies for benign breast conditions.