Tryptophan is an essential amino acid found in a variety of foods such as red meat, fish and eggs. Commensal bacteria expressing tryptophanase catabolize tryptophan to indole, a quorum-sensing compound for bacteria. Lactobacillus spp. convert tryptophan to indole-3-aldehyde (I3A) through unidentified enzymes. Indole-3-aldehyde (I3A) acts on the aryl hydrocarbon receptor (AhR) found on intestinal immune cells and increases interleukin-22 (IL-22) production. Activation of AhR plays a crucial role in gut immunity, such as in maintaining the epithelial barrier function and promoting immune tolerance to promote microbial commensalism while protecting against pathogenic infections.