People with aching joints and arthritis might be surprised to learn that their pain might start partly in the gut. Singapore-based researchers have shown that there is a link between autoimmune arthritis and the gut microbiome (Nat. Microbiol. 2019, DOI: 10.1038/s41564-019-0381-1). The human body is home to trillions of bacteria collectively known as microbiota. These bugs coexist with us and can release small molecules that circulate throughout the body, including bacterial peptidoglycans. These fragments of the bacterial cell wall are released when bacteria remodel their cells. Some are involved in immunostimulation, sleep promotion, and fever. Yue Wang’s team at the Institute of Molecular and Cell Biology decided to investigate the role of the circulating peptidoglycans in autoimmune arthritis. They found that increasing the amounts of peptidoglycans in the blood speeds up disease development in mice that are bred to have arthritis. But a single injection of a peptidoglycan antibody that the group developed stops symptoms progressing. Treated mice stayed free of joint swelling and pain. The researchers also found that the same antibody halts another autoimmune disease, autoimmune encephalomyelitis, in mice. The researchers suggest that peptidoglycan levels in the blood could serve as a biomarker for autoimmune diseases and could even be a drug target for diseases like arthritis.