The conundrum of breast cancer and microbiome - A comprehensive review of the current evidence

Disturbance of the microbial balance of a habitat can have detrimental effects on the health of the individual and, in addition, polymorphic microbiomes were recently suggested as emerging cancer hallmarks. Modern sequencing and metagenomics techniques have allowed characterization of intratumoral microbiome composition even in tissues such as the breast. We conducted a comprehensive literature review on different aspects related to the microbial landscape of the breast tissue and breast tumors, as well as its relation to systemic therapy. Emerging data suggest varying microbiome composition intratumorally compared to the normal breast tissue and other tumor types. Differences in the microbes present in normal breast and cancerous lesions of the breast have also been described, as well as potential correlation between microbiome composition and breast cancer subtype and stage. The interplay between gut and breast microbiome is not well understood although bacterial allocation through mesenteric lymph nodes has been suggested as a possible pathway. Moreover, gut bacteria with estrogen metabolizing properties are of special interest in the context of breast cancer and available knowledge and reported studies are hereby described. The relationship of gut microbiome and cancer therapy is another aspect of interest and available data are presented. Notwithstanding, the field of microbiome in the context of breast cancer is starting to evolve and a number of questions arise, with the gut-breast-cancer therapy axis in the center.