Unfriendly bacteria in the gut might bring on inflammation in the joints.

Dan Littman at the New York University School of Medicine and his colleagues collected stool samples from people with arthritis. Sequencing experiments on gut bacteria showed that individuals with newly diagnosed arthritis were more likely to harbour the bacterium Prevotella copri than were individuals without the condition.

High levels of P. copri and related species correlated with low levels of beneficial microbes that are thought to suppress the immune system and metabolize vitamins into forms that are absorbed by the bloodstream. When inoculated with P. copri, mice became more sensitive to colitis, and the levels of beneficial microbes in their guts dropped.

Gut bacteria have long been implicated in arthritis, but this is the first time that researchers have correlated the onset of human disease with a particular microbial species.

eLife 2, e01202 (2013)