Arthritis

The Gut Bacterium Found in Newly Diagnosed Rheumatoid Arthritis

The evidence here is strong and specific, and it names a single organism rather than describing a general imbalance. Rheumatoid arthritis is an autoimmune disease in which the immune system attacks the lining of joints, causing pain, swelling and eventual joint destruction. It's driven by a combination of inherited risk and something environmental. Attention has turned to where the immune system meets the outside world, and the largest of those surfaces is the gut.

Studying people before treatment starts

Most studies of gut bacteria in disease face the same problem. By the time people are recruited, they've been on medication for years, and those drugs change gut bacteria on their own.

Researchers avoided that by recruiting people with new-onset rheumatoid arthritis who hadn't yet been treated.

They sequenced 114 stool samples, including these patients, people with long-standing treated disease, people with psoriatic arthritis, and healthy volunteers. A subset of 44 had their bacteria sequenced in full genetic detail.

One organism separated the untreated new-onset group from everyone else. Prevotella copri was strongly overrepresented, in many cases dominating the community.

Where it expanded, other bacteria contracted. Bacteroides fell, along with several groups generally regarded as beneficial.

Source: Expansion of intestinal Prevotella copri correlates with enhanced susceptibility to arthritis

Testing whether it does anything

An overrepresented bacterium could easily be a passenger, so the team took it to animals.

They colonised mice with Prevotella copri and then induced gut inflammation. Those animals developed more severe disease than controls.

That's consistent with the bacterium having a genuinely inflammatory effect rather than simply thriving in an inflamed gut.

The researchers also identified particular Prevotella genes present in patients and absent elsewhere, which suggests specific strains rather than the species as a whole.

The immune system recognising it

The most persuasive human evidence came later, and it looked at immune responses rather than abundance.

Researchers tested whether people with rheumatoid arthritis had immune reactivity to Prevotella copri specifically.

About half did. Some produced IgA antibodies against it, the antibody class made at mucosal surfaces like the gut lining, suggesting a local response. Others produced IgG antibodies, which circulate throughout the body, suggesting a systemic one. T cell responses were present too.

Those responses were rarely found in people with other rheumatic diseases, which argues against them being a general feature of joint inflammation.

So the immune system in these patients isn't merely coexisting with this bacterium. It's reacting to it.

Source: Gut Microbe, Prevotella copri, Implicated in RA Pathogenesis

What remains unresolved

Two limits deserve stating plainly.

Direction is unsettled. An expansion of this bacterium could contribute to the autoimmunity, or early gut inflammation could create conditions favouring it. The animal work supports the first, but mice given a bacterium aren't the same as people who developed a disease.

Prevotella copri is also not straightforwardly a villain. It's common in healthy people, particularly on diets high in plant fibre, and in those contexts it's associated with better outcomes rather than worse. The finding appears to concern particular strains in particular immune contexts, not the species as such.

That nuance matters, because it rules out the simple response of trying to eliminate it.

What this means for you

Joint pain with morning stiffness lasting more than half an hour, especially with swelling, needs prompt assessment, since early treatment substantially changes long-term joint damage in rheumatoid arthritis. In addition, the research above found a single bacterium dominating the guts of newly diagnosed untreated patients, with around half of people mounting a specific immune response against it. A varied, plant-rich diet supports the same gut bacteria this research ties to a balanced community rather than one organism taking over. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.