Scientists don’t know what causes rheumatoid arthritis, but many suspect the bacteria living in our gastrointestinal tracts may be to blame.

A study by Jose Scher, a rheumatologist at New York University, found that people with rheumatoid arthritis were much more likely to have a bug called Prevotella copri in their intestines than people that did not have the disease. P. copri has been linked to an inflammatory response that may be related to the onset of rheumatoid arthritis.

Researchers analyzed 144 stool samples from people both with and without rheumatoid arthritis (RA). P Copri as more abundant in people with newly diagnosed rheumatoid arthritis than healthy people or those with chronic RA.

In another study, Scher found that patients with psoriatic arthritis, another kind of autoimmune joint disease, had significantly lower levels of other types of intestinal bacteria.

The studies are part of a growing effort by researchers around the world to understand how the microbiome, the mass of microbes that live in the gastrointestinal tract, affects overall health. The gut contains up to a thousand different bacteria species, which together weigh between one and three pounds. This mass contains trillions of cells, more than the number of cells that make up our own bodies. Over the past several years, scientists have compiled a growing collection of evidence that many of these bugs may have a major effect on our well-being, with some triggering chronic, non-infectious ailments such as rheumatoid arthritis, and others protecting against such diseases.

“This is frontier stuff,” said Scher, the director of the NYU’s Microbiome Center for Rheumatology and Autoimmunity. “This is a shift in paradigm. By including the microbiome, we’ve added a new player to the game.”

Dr. Daniel Lupash, a rheumatolotist at McKay-Dee Hospital, said infection has long been suspected as a trigger for rheumatoid arthritis, but no specific bacterial infection has been proven to cause it.

“One recent theory has been that an infection of the mouth is the trigger because rheumatoid arthritis patients have more bacterial infection in the mouth than non-rheumatoid arthritis persons,” Lupash said. “But the problem with associations is that you don’t really know what causes what. There are a lot of factors making it difficult to determine causation.”

Lupash said there is a different type of inflammatory arthritis that is similar to rheumatoid that is triggered by an infection, often a gastrointestinal or reproductive and urinary infection that is called reactive arthritis. It typically involves one, two or three joints that are often below the waist and usually occurs one to four weeks after the onset of infection.

Lupash said it used to be thought that reactive arthritis would go away when the infection was cured. However, today doctors have learned that is not always the case. While numerous patients do experience relief, others experience a chronic or episodic arthritis that seems to have been initially triggered by the infection but continues, despite effective treatment of the infection.

Rheumatoid arthritis is an autoimmune disease in which the body turns on itself to attack the joints. Lupash said it typically causes pain, swelling, and stiffness in the joints. Most commonly, it affects the small joints of the hands.

“Symptoms are usually worse at night or in the morning or when the person is inactive. In most types of non-inflammatory arthritis, such as osteoarthritis, it is the opposite. The hands or knees are more painful with use of the joints,” he said. “Many people will have a few minutes a stiffness, often 15 to 30 minutes, but rheumatoid arthritis patients experience an hour or more of stiffness and often have significant joint swelling.”