Although human mammary glands were traditionally considered sterile, accumulating evidence has established the presence of distinct microbial communities that may have colonized breast tissue primarily via retrograde nipple flow or via hematogenous or lymphatic translocation from other body sites. Comparative studies reveal differences in the microbiota of healthy and diseased breast tissues, with variations in microbial signatures across breast cancer subtypes and in comparison with adjacent normal tissues. This review synthesizes current evidence on the composition of the breast microbiome, the factors shaping its development, and alterations it undergoes in inflammatory and malignant breast diseases. Furthermore, the article discusses mechanistic insights, methodological challenges, and future therapeutic perspectives based on published studies employing culture-independent approaches, such as 16S rRNA gene sequencing and metagenomic analyses. Key host-related factors influencing breast-associated microbial communities, including hormonal regulation, environmental exposure, diet, and therapeutic interventions, are explored. The existing literature is assessed to identify key associations between the breast microbiome and host signaling pathways, as well as the significant challenges that remain unresolved, including low biomass contamination, inter-study variability, limited longitudinal data, and an incomplete understanding of causality. Addressing these limitations is critical for advancing microbiome-based diagnostic and therapeutic strategies for breast disease.

Breast cancer is the most frequently diagnosed malignancy among women worldwide, with the luminal A subtype being the most prevalent. Several studies have reported a complex interplay between breast cancer cells and the local microbiome; however, the mechanisms by which tumor cell-secreted factors influence bacterial biological properties remain insufficiently explored. In this study, we established an in vitro model that partially recapitulates the luminal A breast cancer microenvironment by exposing three breast-associated bacterial species, Pseudomonas aeruginosa, Enterococcus faecalis, and Escherichia coli, to conditioned media (CM) derived from MCF-7 (tumor) or MCF-10A (non-tumor control) cell lines. A combination of complementary approaches, including ultrastructural morphological assessment, biofilm formation assays, antimicrobial susceptibility testing, and virulence gene abundance profiling by genomic qPCR, was employed to reveal distinct tumor-microbiota interactions. Exposure to MCF-7 CM induced dose-dependent structural alterations in P. aeruginosa and E. faecalis, with pronounced membrane blebbing and structural disruption in E. faecalis. Biofilm formation was differentially modulated in a species- and concentration-dependent manner, with a persistent increase observed in E. coli. Antibiotic susceptibility profiles were selectively altered in E. faecalis, which displayed increased sensitivity to vancomycin, penicillin, and imipenem, along with decreased sensitivity to chloramphenicol. P. aeruginosa exhibited increased sensitivity to imipenem along with reduced sensitivity to meropenem and gentamicin, whereas no significant changes were observed in E. coli. qPCR analyses demonstrated that MCF-7 CM was associated with enrichment of multiple virulence-associated genes (e.g., lasB, exoS, pilB, plcH, fsrC, esp, fimH, and papG), reflecting enhanced pathogenic and adhesive potential. Collectively, these findings suggest that luminal A breast cancer-derived factors can reprogram microbial phenotypes in a species-specific manner, providing mechanistic insight into breast tumor-microbiome crosstalk and a platform to explore microbiome-targeted interventions.