The oral and gut microbiome in rheumatoid arthritis patients: a systematic review

Background. Recently, researchers have proposed a possible relationship between RA and the microbiome of the oral cavity and gut. However, this relation has not been systematically established. Herein, we conducted a comprehensive review of the pertinent literature to describe this possible association. Methods. We systematically performed searches in databases, namely EMBASE, the Cochrane Library, and PubMed, from inception to 7 June 2020 to identify case-control studies that compared the oral and gut microbiome in adult RA patients with those of controls. The primary outcome was specific bacterial changes between RA and controls. The secondary outcome was microbial diversity changes between RA and controls. Results. In total, 26 articles were considered eligible for inclusion and reported some differences. Therein, >= 3 articles reported decreased Faecalibacterium in the gut of early-RA (ERA)/RA patients compared with healthy controls (HCs). Also, >= 3 articles reported decreased Streptococcus and Haemophilus and increased Prevotella in the oral cavity of ERA/RA patients compared with HCs. In addition, some Prevotella species, including P. histicola and P. oulorum, showed increased trends in RA patients' oral cavity, compared with HCs. The alpha-diversity of the microbiome was either increased or not changed in the oral cavity of RA patients, but it was more commonly either decreased or not changed in the gut of RA patients. Conclusions. In this systematic review, we identified the microbiome associated with RA patients in comparison with controls. More research is needed in the future to find the deep relationship between RA and the microbiome.