O11 Distinct tryptophan metabolite profiles underpin species-specific effects of skin commensals on epidermal barrier protein expression

Introduction and aims The skin microbiome regulates epidermal barrier integrity, largely via the production of tryptophan metabolites that activate the aryl hydrocarbon receptor (AhR). Roles of individual species remain unclear, although we have shown that AhR activation is species-dependent. Our previous work identified Staphylococcus hominis and Micrococcus luteus as regulators of keratinocyte barrier function. Here, we extend these findings to ex vivo human skin and include Staphylococcus saprophyticus – another potent AhR activator – to examine the effects of skin commensal secretomes on the epidermal barrier. Methods Bacteria secretomes, cultured with or without tryptophan, were profiled for tryptophan metabolites using liquid chromatography–mass spectrometry and applied to skin explants for 24 h. Expression of filaggrin, involucrin, occludin and claudin-1 was evaluated by immunofluorescence and cytokines in the tissue culture medium were measured using 48-plex immunoassay. Results S. hominis secretome enhanced filaggrin, involucrin, and occludin expression in a tryptophan-dependent manner, while S. saprophyticus enhanced occludin irrespective of tryptophan. Multivariate analysis revealed species-specific metabolite–protein correlations. The defining feature of S. hominis secretome was tryptophol abundance, which showed the strongest positive association with filaggrin expression, followed by indole-3-lactate and indole-3-acetamide; the strongest correlates of involucrin were indole-3-carboxaldehyde, indole-3-acetaldehyde, tryp­tophol and indole-3-lactate, whereas occludin was primarily ­associated with indole-3-lactate, indole-3-acetamide, and indole-3-acetaldehyde. The 48-plex immunoassay revealed no significant differences between secretome treated skin and controls. This may reflect intrinsic tolerance to commensal-derived signals, low immunogenic potential of the secretomes or that they did not elicit a detectable response under these experimental conditions. Conclusions These results demonstrate that commensal bacteria influence the epidermal barrier through species-specific metabolite profiles, with tryptophol and other indole derivatives emerging as key drivers of barrier function. The absence of inflammatory cytokine cascades suggests that these effects could occur without triggering an adverse immune reaction. Collectively, this work highlights metabolite-mediated host–microbe interactions as a promising avenue for targeted strategies to improve skin health.