The microbiome and the hallmarks of cancer

Microbes have long been linked to cancer: from studies on Epstein–Barr virus (EBV) and lymphoma to Helicobacter pylori and gastric cancer. Although single infectious agents were initially associated with carcinogenesis, technological advances have broadened our knowledge of microbial communities that may impact carcinogenesis, namely, the trillions of microorganisms that live in symbiosis with humans. Commensal microbes (microbiota) live in close association with their human hosts and impact host health, immunity, and homeostasis. Disruption to the composition of these microbial communities can dysregulate host cellular processes and promote the development of various diseases, including cancer. The “hallmarks of cancer” is an important framework for understanding the processes of how normal cells turn cancerous. This framework can also be applied to the mechanisms underlying how microbes and microbial communities influence carcinogenesis and cancer development in their human hosts. This chapter uses the hallmarks of cancer as a framework to discuss mechanisms for how microbiota promote tumorigenesis through crosstalk with the host, interactions between other microbes, and the role of microbial localization in relation to carcinogenesis.