The Key Role of Human Microbiota in Breast Cancer

Sound evidence recognizes the microbiota as one of the major players in human health and disease, including cancer. Every human being is an holobiont, a shared human and microbial ecosystem, in which microbial composition is individually set by behaviours and environmental factors during the first years of life. Thereafter it is modulated by diet, physical activity, emotions and drugs (in particularly antibiotics and chemotherapeutics). As a consequence, a shift in medicine is needed toward a more comprehensive practice that takes into account every individual's genoma and, in addition, his or her metagenome, known as microbiome: a "microbiota revolution". As regards breast cancer (BC), a clear link between microbiota and oncogenesis is still to be confirmed. Specific microbes display unique features regulating their host niche in a number of body sites, which can result in an increased risk of cancer; in addition, gut microbiota composition plays a role in immune modulation within the intestinal barrier, affecting local and systemic inflammation, recognized drivers of cancer. Moreover, part of the bacterial gene mass inside the gut, constituting the so called “estrobolome”, influences the sexual hormonal balance and subsequentely may impact on the onset, progression and treatment of hormonal dependent cancers. Microbiota is also clearly involved in modulating the response to anticancer treatments, and above all to the emerging immunotherapy. Based on these premises, the microbiome is becoming a potential target, in order to enhance efficacy of antitumoral treatments as well as to lower their toxicity. The complex scenario that links microbiome composition to oncogenesis and response to anticancer treatments defines the frames of a new “oncobiotic” perspective.