(A) CIA was induced in 6-week-old male DBA/1 mice. On days 21–35, mice were treated with antibiotics with or without 0.1 mg/mL indole in the drinking water, and arthritis scores were assessed every other day. n = 10 (Abx+Indole); n = 7 (CIA+Abx). (B) Six-week-old male DBA/1 mice were fed a TL or TS diet starting on day –1 and through the duration of the experiment. Following induction of CIA, mice were treated with indole (200 μL of a 10 mM solution) or vehicle control (0.33% methanol) by oral gavage every other day starting on day 0. Arthritis scores were assessed every other day from days 21–35. n = 39 (TL+Vehicle), n = 24 (TL+Indole), and n = 25 (TS+Vehicle), pooled from 5 independent experiments. Red asterisks: TL+Indole versus TL+Vehicle; black asterisks: TS+Vehicle versus TL+Vehicle. TS+Vehicle versus TL+Indole was not statistically significant. (C) The sum of the inflammation, pannus, and bone erosion score of H&E-stained paws is plotted as the total histology score (maximum score of 15). n = 10–20, pooled from 2 independent experiments. (D) Schematic of Trp breakdown into indole by bacterial Tryptophanase A and Trp synthesis from indole by bacterial Tryptophan synthase. (E) HPLC analysis of Trp in cecal contents from mice with CIA at day 35, plotted as the AUC, normalized to the weight (mg) of the cecal contents. n = 5–10, from 1 experiment. (F) Six-week-old male germ-free DBA/1 mice were colonized with E. coli BW25113 mutants (ΔtnaA or ΔBcsQ) with 10⁸ CFU by oral gavage on day –7 before CIA induction. n = 7 per group. (G) HPLC analysis of indole in cecal contents from CIA mice colonized with either ΔtnaA or ΔBcsQ at CIA day 35. Indole levels were plotted a the AUC per milligram of cecal content weight. Data are reported as the mean ± SEM. *P < 0.05, **P < <0.01, ***P < 0.001, and ****P < 0.0001, by 2-way ANOVA with Bonferroni’s correction for multiple comparisons (A, B, and F), 1-way ANOVA with Bonferroni’s correction for multiple comparisons (C and E), and unpaired 2-tailed Student’s t test (G).