LAUSR

Simple Summary Human gut is colonized by a wide variety of microorganisms, which collectively form the so-called microbiota. A healthy microbiota interacts with our organism as a symbiont positively participating in the regulation of many physiological activities of the host, detoxification of xenobiotics, and prevention of the development of specific pathologies. However, various conditions may change the status of microbiota from “healthy” to “unhealthy”; an unhealthy status may negatively impact the host health contributing to the onset of maladies. Cancer is among those that microbiota can prevent or promote according to its conditions. The knowledge of the situations and of the mechanisms underlying this bifaceted role of microbiota is of great importance in cancer prevention, epidemiology, diagnosis, prognosis, and therapy. Here we will illustrate and discuss how and when microbiota interferes with development and progression of human cancer, with a particular emphasis on colorectal cancer. Abstract Human gut microbiota physiologically and actively participates as a symbiont to a wide number of fundamental biological processes, such as absorption and metabolism of nutrients, regulation of immune response and inflammation; gut microbiota plays also an antitumor role. However, dysbiosis, resulting from a number of different situations—dysmicrobism, infections, drug intake, age, diet—as well as from their multiple combinations, may lead to tumorigenesis and is associated with approximately 20% of all cancers. In a diagnostic, prognostic, therapeutic, and epidemiological perspective, it is clear that the bifaceted role of microbiota needs to be thoroughly studied and better understood. Here, we discuss the anti- and pro-tumorigenic potential of gut and other microbiota districts along with the causes that may change commensal bacteria from friend to foes.