Microbiome—Stealth Regulator of Breast Homeostasis and Cancer Metastasis

Cumulative evidence attests to the essential roles of commensal microbes in the physiology of the hosts. Although microbiome has been a major research subject since the time of Luis Pasteur and William Russell over 140 years ago, recent findings that certain intracellular bacteria contribute to the pathophysiology of healthy or diseased tissues have brought the field of microbiome to a new era of investigation. In particular, in the field of breast cancer research, breast-tumor resident bacteria are deemed to be the essential players of tumor initiation and progression. This is a resurrection of Russel’s bacterial cause of cancer theory abandoned over 100 years ago. This review will introduce some of the recent findings that exemplify the roles of breast tumor-resident microbes in breast carcinogenesis and metastasis and provide mechanistic explanations of these phenomena. Such information would be able to justify the utility of breast-tumor resident microbes as biomarkers for disease progression and therapeutic targets.