The microbiota is composed of commensal bacteria and other microorganisms that live on the epithelial barriers of the host. The commensal microbiota is important for the health and survival of the organism. Microbiota influences physiological functions from the maintenance of barrier homeostasis locally to the regulation of metabolism, haematopoiesis, inflammation, immunity and other functions systemically. The microbiota is also involved in the initiation, progression and dissemination of cancer both at epithelial barriers and in sterile tissues. Recently, it has become evident that microbiota, and particularly the gut microbiota, modulates the response to cancer therapy and susceptibility to toxic side effects. In this Review, we discuss the evidence for the ability of the microbiota to modulate chemotherapy, radiotherapy and immunotherapy with a focus on the microbial species involved, their mechanism of action and the possibility of targeting the microbiota to improve anticancer efficacy while preventing toxicity.

Commensal microorganisms (the microbiota) live on all the surface barriers of our body and are particularly abundant and diverse in the distal gut. The microbiota and its larger host represent a metaorganism in which the cross talk between microbes and host cells is necessary for health, survival, and regulation of physiological functions locally, at the barrier level, and systemically. The ancestral molecular and cellular mechanisms stemming from the earliest interactions between prokaryotes and eukaryotes have evolved to mediate microbe-dependent host physiology and tissue homeostasis, including innate and adaptive resistance to infections and tissue repair. Mostly because of its effects on metabolism, cellular proliferation, inflammation, and immunity, the microbiota regulates cancer at the level of predisposing conditions, initiation, genetic instability, susceptibility to host immune response, progression, comorbidity, and response to therapy. Here, we review the mechanisms underlying the interaction of the microbiota with cancer and the evidence suggesting that the microbiota could be targeted to improve therapy while attenuating adverse reactions.