Pathogens, Commensal Symbionts, and Pathobionts: Discovery and Functional Effects on the Host

During the last decade, we have witnessed a stunning increase in information on the composition of the microbiota; its influence on a variety of host functions; and associations with the susceptibility to inflammatory, metabolic, and autoimmune diseases. We have thus obtained insight into the potentially harmful consequences of an altered microbiota and also learned about the many beneficial functions of commensal bacteria. The present review aims at summarizing for the reader the general concept of pathogenic and commensal bacteria and their particular features. It also discusses the more recently defined pathobionts, members of the microbiota that exert specific effects on the host's mucosal immune system associated with the development of clinical disease.