Rheumatic diseases and the microbiome.

Every human is intimately associated with a large and diverse population of microorganisms living on the skin and mucous membranes. These commensal organisms are known as the microbiome, or microbiota, and are acquired in young childhood. The microbiome is critically important in establishing a fully function immune system. For example, Th17 T helper cells are not present in a germ-free environment. The relationship of the microbiome to autoimmune disease is being explored actively. Mechanisms by which the microbiome may influence these diseases include, but are not limited to, molecular mimicry as well as induction and regulation of both Th17 and regulatory T cells. There are ample data that a specific oral microbe, Porphyromonas gingivalis, the only bacteria with the enzyme peptidylarginine deiminase, is involved in the pathogenesis of rheumatoid arthritis. Connection between other rheumatic autoimmune diseases and the microbiome remains to be made.