Cross-kingdom signals: microbial, metabolites, and immune cells on the gut-breast axis.

The mammary gland is a dynamic mucosal organ whose structure and function are continually reshaped across pregnancy, lactation, and involution. Emerging evidence places it within a broader gut-breast axis in which intestinal microbes and their products influence mammary immunity, epithelial programs, and the composition of human milk. Three mechanistic lanes organize this crosstalk. First, microbial signals-ranging from structural ligands to extracellular vesicles-can reach the breast and tune innate sensing. Second, gut-derived metabolites-notably short-chain fatty acids (SCFA), secondary bile acids, and tryptophan catabolites-circulate to the gland and act on host receptors to regulate barrier integrity, inflammation, and tumor biology. Third, gut-imprinted immune cells traffic to the lactating breast, exporting protective secretory immunoglobulin A into milk. These routes have tangible clinical implications: selective probiotics reduce mastitis risk and aid abscess recovery; SCFA- and bile-acid-linked pathways protect mammary barriers; and specific commensals or pathobionts can shape breast cancer progression and therapy response. Because milk and mammary tissue are low-biomass niches, rigorous contamination controls are essential for reproducibility. This review synthesizes cross-kingdom signals in the gut-breast axis and outlines translational opportunities-from dietary fiber and targeted postbiotics to bile-acid pathway modulation and antibiotic stewardship-to improve lactation health and inform oncology.