a, Quantification of isoDCA production by engineered and reference strains in vitro. Bacteria were grown to exponential phase and transferred to medium containing DCA. Following incubation for 24 h, medium was extracted with methanol and supernatants were analysed by LC–MS. AUC, area under curve. b, c, GF mice were colonized with consortia containing either the engineered strain of B. frag capable of producing isoDCA or the catalytically dead mutant in combination with C. scindens (C. scindens plus B. frageWT and C. scindens plus B. frageCD, respectively). Recipients of an FMT and noncolonized mice (PBS) served as references. Immune-cell composition and isoDCA quantification were performed 10 days post-colonization. b, FACS analysis of the frequency of RORγt+ Foxp3+ CD4+ T cells in the LILP. c, Quantification of isoDCA in caecal contents. Faecal material was weighed, homogenized and extracted with methanol for LC–MS analysis. In a, c, the AUC is normalized by the weight of the input material. Shown are means ± s.d. (a, n = 3; b, n = 10; c, n = 5). Data in a, c are representative of two independent experiments. Data in b are pooled from two independent experiments. Statistical significance determined by a one-way ANOVA followed by Tukey’s multiple comparison’s test. *P < 0.05; ND, not detected; ns, not significant. Source Data