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Y. Jiao M. Hasegawa and N. Inohara (2014)
Direct attack is not the only way that microbes cause disease. Previous studies on infectious diseases have focused extensively on pathogenic microbes that directly damage tissues in the host. However, evidence is accumulating that another set of microbes can also induce disease or contribute critically to disease development. These microbes live as normal residents on the skin, and internal cavities, particularly the intestines of animals, including humans, are called commensals (Chow et al., 2011). Among them, a particular group of commensals can cause or promote disease, and these commensals are often called pathobionts. The concept “pathobiont” includes some opportunistic pathogens that
live as commensals in healthy hosts but can cause disease in susceptible hosts
(e.g., immunodeficient individuals). The overgrowth of pathobionts is often triggered by immunodeficiency, pathogen infection, and treatment with antibiotics and host-damaging drugs (Chow et al., 2011) (Fig. 1A). In addition, the overgrowth of some pathobionts can result in secondary infection or infection by opportunistic pathogens, but little is known about the roles of pathobionts in oral diseases including periodontitis, a common dental disease.