Microbiota-Nourishing Immunity: A Guide to Understanding Our Microbial Self

In ecological terms, the microbiome is defined as the microbiota and its environment, a definition that encompasses the human host. The size, species composition, and biogeography of microbial communities is shaped by host interactions, and, in turn, the microbiota influences many aspects of human health. Here we discuss the concept of microbiota-nourishing immunity, a host-microbe chimera composed of the microbiota and host factors that shape the microbial ecosystem, which functions in conferring colonization resistance against pathogens. We propose that dysbiosis is a biomarker of a weakening in microbiota-nourishing immunity and that homeostasis can be defined as a state of immune competence. Microbiota-nourishing immunity thus provides a conceptual framework to further examine the mechanisms that preserve a healthy microbiome and the drivers and consequences of dysbiosis. Litvak and Baumler discuss how the immune functions that shape the microbial ecosystem form a functional unit with our microbial communities, thereby assembling into a host-microbe chimera, termed microbiota-nourishing immunity, that confers colonization resistance against pathogens.