Homeostasis and its disruption in the lung microbiome.

The disciplines of physiology and ecology are united by the shared centrality of the concept of homeostasis: the stability of a complex system via internal mechanisms of self-regulation, resilient to external perturbation. In the past decade, these fields of study have been bridged by the discovery of the lung microbiome. The respiratory tract, long considered sterile, is in fact a dynamic ecosystem of microbiota, intimately associated with the host inflammatory response, altered in disease states. If the microbiome is a "newly discovered organ," ecology is the language we use to explain how it establishes, maintains, and loses homeostasis. In this essay, we review recent insights into the feedback mechanisms by which the lung microbiome and the host response are regulated in health and dysregulated in acute and chronic lung disease. We propose three explanatory models supported by recent studies: the adapted island model of lung biogeography, nutritional homeostasis at the host-microbiome interface, and interkingdom signaling and the community stress response.