Proposing the 'Holobious System': Reframing the Human Microbiome as an Organ System

The human microbiome comprises trillions of bacteria, archaea, fungi, and viruses that contribute to immune regulation, metabolic processing, colonisation resistance against pathogens, and modulation of the gut–brain axis. Its effects extend across nearly every branch of medicine, yet it has no formal status as an organ system, no dedicated clinical field, and no coherent place in undergraduate medical education. This article proposes the holobious system as the distributed physiological system formed by resident, recurrent, and ecologically integrated host-associated microbial communities, their collective metabolic and genetic activities, and the host interfaces that sustain and respond to them. It also proposes holobiontology as the interdisciplinary clinical field concerned with the study, diagnosis, prevention, and management of disorders affecting the holobious system. The microbiome-as-organ concept is not new; what remains absent is a systems-level medical term and a clinical framework built around it. Although the science is not yet sufficiently mature for routine system-level diagnosis or treatment, that limitation strengthens the case for a clearer framework to organise research, education, and future clinical practice.