An intriguing interplay between skin bacteria and UV radiation reported in the Journal of Investigative Dermatology paves the way for microbiome-aware sun protection

Philadelphia, May 13, 2025 – The skin microbiome plays an important role in health and disease. Researchers have now substantiated that certain skin bacteria can protect us from the sun’s ultraviolet (UV) radiation specifically by metabolizing cis-urocanic acid using an enzyme called urocanase. This enables the skin's ability to fine-tune how it responds to UV radiation The findings of the study in the Journal of Investigative Dermatology, published by Elsevier, provide a striking case study that demonstrates the ability of the skin microbiome to remodel host immune functions.

The skin hosts a vast ecosystem composed of millions of microorganisms, including bacteria, fungi, and viruses. The composition of the cutaneous microbiome is highly unique, complex, and varies greatly depending on the anatomical location. Commensal microbes, also known as normal microbiota or indigenous microbiota that live in a beneficial or neutral relationship without causing harm, adapt their metabolism to the resources available in their microenvironment, feeding on our skin's nutrients and producing various molecules that affect their environment and interact with our skin cells.

Lead investigator VijayKumar Patra, PhD, Centre International de Recherche en Infectiologie; Lyon, France, and Research Unit for Photodermatology, Medical University of Graz, Austria, explains, "To date, many internal and external factors have been identified that influence the composition of the skin microbiome. These include various individual parameters such as race, gender, age, hormone levels, diet, and hygiene, but environmental factors and the effects of occupation, pollution, and climate also have a major influence. We have known for a long time that UV radiation modulates immune responses directed against environmental antigens on the surface of the skin and, more recently, that the skin microbiome also plays a role in regulating these responses. What intrigued us was the idea that certain microbes could be actively involved in or even interfere with UV effects. The overlap between microbial metabolism and host immunity became the focus of our investigation."

Researchers used a combination of microbiome sequencing, immunological assays, in vitro cultures, and gnotobiotic mouse models, in which all microorganisms present are defined, to study how skin bacteria respond to UVB radiation, the type of UV radiation that typically causes sunburn. They discovered that certain skin bacteria specifically metabolize cis-urocanic acid, a photoproduct of a major UV-absorbing chromophore of the stratum corneum, trans-urocanic acid, using an enzyme called urocanase. Compared to trans-urocanic acid, cis-urocanic acid is endowed with potent immunomodulatory properties. This microbial metabolism then limits the ability of cis-urocanic acid to inhibit immune responses, which means that skin bacteria fine-tune our skin's response to UV radiation.

The researchers point out the intriguing interplay between sunscreens, cis-urocanic acid, and the microbiome with each other, competing in and on the stratum corneum, as the most superficial layer of the skin.

Co-investigator Marc Vocanson, PhD, Centre International de Recherche en Infectiologie, Lyon, France, notes, "This is the first time we have demonstrated a direct metabolic link between UV radiation, a host-derived molecule, and bacterial behavior that affects immune function. As interest grows in both microbiome research and personalized medicine, understanding these microbe-host interactions could reshape the way we think about sun protection, immune diseases, skin cancer, or even treatments like phototherapy."

Co-investigator Peter Wolf, MD, Research Unit for Photodermatology, Medical University of Graz, Austria, concludes, "These findings open the door to microbiome-aware sun protection, where we not only protect the skin from UV radiation, but also consider how resident microbes can alter the immune landscape after exposure. In the future, topical treatments that modulate microbial metabolism could be used to minimize, maintain, or enhance UV-induced immunosuppression when clinically beneficial, such as with phototherapy."

Commenting on the findings, noted expert in the field Anna Di Nardo, MD, PhD, University of California San Diego, and San Gallicano Dermatological Institute IRCCS, Rome, says, “This pivotal study shows that microbial communities are not passive victims of environmental stress but dynamic regulators of immune responses, capable of metabolizing UV-induced skin products such as cis-urocanic acid. This newly uncovered role of microbial metabolism in modulating UV tolerance reshapes our understanding of the skin barrier — not just as a structural shield but as a metabolically active, microbially regulated interface. With increasing concerns about UV exposure, skin aging, and cancer, a deeper understanding of this axis offers promising avenues for therapy and prevention.”