Certain gut bacteria have been linked to rheumatoid arthritis, which could suggest new ways to delay or prevent the onset of this disease.

We share our bodies with a diverse set of microorganisms, known collectively as the human microbiome. Indeed, estimates suggest that our bodies contain 10 times as many microbial cells as human cells. Our stomach and intestines alone are home to many hundreds and possibly thousands of microbial species that break down indigestible compounds and help to prevent the growth of harmful bacteria. The immune system must therefore learn to tolerate these microorganisms, while retaining the ability to launch attacks against microorganisms that cause harm. Failure of this process may increase the risk of autoimmune diseases in which the body mistakenly attacks its own cells and tissues.

Rheumatoid arthritis is a chronic autoimmune disease marked by inflammation of the joints. Although the causes of rheumatoid arthritis are unknown, mice with mutations that increase the risk of the disease remain healthy if they are kept under sterile conditions. However, if these mice are exposed to certain species of bacteria sometimes found in the gut, they begin to show signs of joint inflammation.

Jose Scher, Andrew Sczesnak, Randy Longman and co-workers used genome sequencing to compare gut bacteria from patients with rheumatoid arthritis and healthy controls. A bacterial species called Prevotella copri was more abundant in patients suffering from untreated rheumatoid arthritis than in healthy individuals. Moreover, the presence of P. copri corresponded to a reduction in the abundance of other bacterial groups — including a number of beneficial microbes. In a mouse model of gut inflammation, animals colonized with P. copri had more severe disease than controls, consistent with a pro-inflammatory function of this organism.

Current treatments for rheumatoid arthritis target symptoms. However, by highlighting the role played by gut bacteria, this work suggests that new treatment options focused on curbing the spread of P. copri in the gut could delay or prevent the onset of this disease.

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