Ultraviolet Radiation and the Skin Microbiome: Dose-Dependent Effects and Consequences for Epidermal Barrier Integrity

The skin microbiome is continuously exposed to solar ultraviolet radiation (UVR), but the magnitude, direction and clinical meaning of the associated microbial changes remain uncertain. This narrative review critically evaluates evidence from controlled human, environmental, phototherapy, animal and in vitro studies linking UVA/UVB exposure to changes in skin microbial composition or function and to epidermal barrier outcomes. The available evidence indicates that UVA and UVB can alter community composition within minutes to days. However, these effects vary according to dose, wavelength, body site, host and sampling time, and may partly recover. The evidence does not support a universal pattern of decreased Cutibacterium acnes and increased Staphylococcus aureus. Dose is central to this relationship: repeated very-low or suberythemal exposure can induce antimicrobial peptides and adaptive responses, whereas single high exposures are more likely to cause inflammation, barrier injury and immunosuppression. Microbiome-dependent mechanisms include changes in tryptophan metabolites and AhR signaling, short-chain fatty acids, extracellular DNA and modulation of cytokine responses. Evidence from phototherapy studies shows that microbial remodeling can accompany clinical improvement, while sunscreen studies suggest that some formulations preserve microbial features without measurably disrupting dominant resident taxa. Because the available studies differ substantially in exposure, site, platform and endpoint, quantitative pooling is not currently defensible. Numerical results are therefore summarized in a structured evidence table Broad-spectrum photoprotection remains foundational. Microbiome-compatible formulations and microbial or metabolite adjuncts are promising but still require randomized, dose-resolved, multi-omic clinical validation.