A study found that gut-derived Bifidobacterium mediated tryptophan metabolism to attenuate atopic dermatitis via the gut-skin axis; thus, microbial metabolites from tryptophan may be the means by which Bifidobacteria alleviate atopic dermatitis via the aryl hydrocarbon receptor signaling pathway [13]. The gene discussed is AHR; the disease is atopic eczema.