For 1.3 million American adults, rheumatoid arthritis (RA) is a debilitating autoimmune disease that greatly affects their day-to-day lives. RA usually impacts joints in the hands, wrists, and feet, and can cause extreme pain that gets in the way of tasks like writing, opening doors, and climbing stairs.

But to find the cause of joint stiffness and discomfort, it’s time to look elsewhere in the body.

Researchers from the New York University (NYU) School of Medicine have linked the prevalence of Prevotella copri, an intestinal bacteria, to the onset of RA by examining patients' fecal matter. 

P. copri is linked to an inflammatory response that may be related to the onset of RA. While a causal relationship hasn’t been established, the correlation between P. copri and RA onset could clarify the link between the gut and pain in other parts of the body.

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The Truth Is in the Gut

The researchers built on past studies in animals. In a previous study of arthritis published in The Journal of Clinical Investigation, mice that were healthy when raised in germ-free conditions began to develop joint inflammation after the introduction of specific gut bacteria.

“Studies in rodent models have clearly shown that the intestinal microbiota contribute significantly to the causation of systemic autoimmune diseases,” said study author Dan Littman, M.D., Ph.D., a professor of pathology and immunology, in a press release.

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The human body is full of different microorganisms—there’s an estimated 10 times as many microbial cells as human cells in the body. These microbes often work alongside the body, helping to break down food and fight off dangerous organisms.

Occasionally, however, as seems to be the case with P. copri, a microorganism can trigger an autoimmune response that causes inflammation. 

More Bacteria in the Newly Diagnosed

Researchers analyzed 144 stool samples from RA patients and healthy controls. Using DNA analysis to compare gut bacteria, researchers found that P. copri was more abundant in patients with newly diagnosed RA than in healthy individuals or patients with an established condition.

To make matters worse, too much P. copri meant fewer beneficial gut bacteria in RA patients. 

That gut bacteria can profoundly impact immune disorders not related to the gut, including arthritis, comes as a surprise, says Diane Mathis, Ph.D., in an eLife editorial.

“[These researchers] have taken an important step in extending these findings to humans: They analyzed the microbiota of healthy individuals and of various cohorts of patients with inflammatory arthritis,” Mathis wrote.

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Moving forward, researchers hope to expand their sample group outside of New York because in a different environment, they may well find new types of gut flora.