Organisms exert effects on each other as they evolve. Similarly, microorganisms have interacted with other organisms over the course of their survival. For example, the gut microbiome, which is composed of a diverse array of microbes, coexists with humans and virtually all other animals, and is involved in the biological activity of the host. Human microbiome is also greatly influenced by the host human. Accordingly, factors that are the root cause of dysbiosis —— changes in the gut microbiome —— have come under the spotlight. Shedding light on the mechanism would lead to more effective preventive measures and treatment.