In mouse models of arthritis, the introduction of a single species of intestinal bacteria is sufficient to induce joint inflammation in otherwise healthy animals. Littman and colleagues now report that patients with new-onset rheumatoid arthritis (NORA) have an increased abundance of the bacterial species Prevotella copri in their faeces compared with patients with chronic, treated rheumatoid arthritis (CRA), patients with psoriasis or healthy controls. P. copri was present in the microbiota of 75% of patients with NORA, but only in 21.4% of healthy controls. Sequencing experiments showed that P. copri strains vary among individuals, and the authors could associate particular open-reading frames (ORFs) within the P. copri genome with strains isolated from either healthy individuals or patients with NORA. These ORFs could be useful biomarkers for distinguishing healthy microbiota from disease-associated microbiota, although it remains to be determined whether expansion of P. copri is a causative factor in the development of rheumatoid arthritis.