For example, colonization of gnotobiotic mice with a consortium of human skin commensals enriches indole-related metabolites compared to germ-free skin, improving barrier repair and function in AD-like models, while topical phytic acid increases tryptophan-metabolizing bacteria such as S. epidermidis, upregulates indole derivatives and activates AHR to promote barrier formation and ameliorate AD (27, 42) (54). The gene discussed is AHR; the disease is Alzheimer disease.