How does the sun impact the skin microbiome?

It is well known that high doses of ultraviolet rays are associated with acute and chronic impacts on skin health, including inflammation and photoaging. However, many holidaymakers expose themselves to high doses of UV in pursuit of the sun. Human skin is home to a vast range of bacteria, fungi, and viruses that make up the skin microbiota. These microbes play essential roles in skin homeostasis. Therefore, it is worth questioning whether sunlight can impact this microbiota.

A study conducted by Nabiha YUSUF and her colleagues aimed to test the effect of UVA and UVB rays on the human skin microbiome. To do this, participants were exposed to doses of UVA (22 - 47 J/cm2) or UVB (100 - 350 mJ/cm2), and samples were taken. The DNA was isolated and sequenced to identify the microbial composition of each sample. The results are reported in the following table.

Overall, the composition of the microbiota after UV exposure has been altered. The disruption of the skin's microbial components can impact the host's health. Proteobacteria, which include cyanobacteria and bacteria of the genus Pseudomonas, constitute a vast phylum of Gram-negative bacteria of healthy human skin. However, a disturbed microbiota of Proteobacteria has been associated with psoriasis, eczema, and diabetic foot ulcers, a greater diversity of Proteobacteria being correlated with protective anti-inflammatory immune responses. Thus, an imbalance in proteobacteria, and consequently of the skin microbiota, may suggest a deterioration of skin health in sun-seeking individuals.

The way in which microorganisms respond and cope with UV rays can vary significantly. It has been suggested that Gram-positive bacteria are better adapted to the oxidative stress associated with UV than Gram-negative bacteria, because their cell walls filter out a significant portion of UV rays. The results of the previously mentioned study may be attributed to various unique adaptation mechanisms inherent to these bacteria.

Although the precise mechanisms by which the sun alters the skin microbiome are not well understood, assumptions have been made. It has been demonstrated that UV exposure has altered the skin metabolome in mice, leading to an increase in amino acid metabolism and a decrease in fatty acid, sphingolipid, and histidine metabolism. These results indicate that UV exposure results in changes in the metabolites present on the skin, upon which microbes depend.

Furthermore, UV rays also suppress adaptive immunity and activate innate immunity. The immunosuppression caused by UV rays is primarily due to their impact on the persistence, phenotype, and specificity of resident memory T lymphocytes. In vivo, it has been demonstrated that an exposure of 50 mJ/cm2 of UVB to psoriatic skin lesions causes the death of T cells. Changes in the skin immune system following UV exposure can potentially alter the immune selective pressures to which the microbiome is exposed, and thus affect the skin microbiota.

These results should be approached with caution, as the number of participants is small (n = 6), which could affect the statistical significance. Furthermore, they may be influenced by other factors, such as temperature and lifestyle, making it challenging to determine whether the microbial variation is directly associated with UV exposure.

Contrary to expectations, studies have shown that sun exposure could be beneficial to the skin microbiome. Indeed, Edvard S. FALK and his team aimed to demonstrate in 2007 the effect of UVB radiation treatment on patients suffering from atopic dermatitis on the presence of Staphylococcus aureus and Staphylococcus epidermidis, pathogenic bacteria involved in atopic dermatitis. 20 patients and 20 healthy controls received UVB treatment. Bacterial samples were taken before treatment, after four weeks of treatment, and after two weeks of follow-up.

The number of Staphylococcus aureus decreased non-significantly in lesions, unblemished skin, and the forehead after four weeks of treatment, and the number of Staphylococcus aureus was slightly higher after two weeks of follow-up. No difference was observed for Staphylococcus epidermidis. In fact, S. epidermidis is primarily found in the hair follicles of the skin, while S. aureus is found on the skin surface, making the latter easily accessible to UV radiation.

These results, although not significant, suggest a potentially positive effect of sunlight on the microbiome, by reducing the bacteria responsible for skin conditions. Studies have shown that UV rays induce the production of antimicrobial peptides (AMPs), triggers of the innate immune system, such as human beta-defensin 2 (hBD2), hBD3, ribonuclease 7 (RNase7), S100A12 and elafin in the epidermis in vitro and in vivo. These peptides help to combat pathogenic microorganisms, and therefore consequently improve the state of the skin microbiome. This difference should be taken into account in the context of possible UVB therapy for atopic dermatitis.

• DOUKI T. & al. Effect of the GC content of DNA on the distribution of UVB-induced bipyrimidine photoproducts. Photochemical & Photobiological Sciences (2008).

• FALK E. S. & al. The effect of UVB radiation on skin microbiota in patients with atopic dermatitis and healthy controls. International Journal of Circumpolar Health (2008).

• WOLF P. & al. The Skin Microbiome: Is It Affected by UV-induced Immune Suppression? Frontiers in Microbiology (2016).

• YUSUF N. & al. Ultraviolet radiation, both UVA and UVB, influences the composition of the skin microbiome. Experimental Dermatology (2018).

• LANGTON A. K. & al. Behaviour and sun exposure in holidaymakers alters skin microbiota composition and diversity. Frontiers in Aging (2023).

• TASNEEM F.M. & al. A narrative review of the impact of ultraviolet radiation and sunscreen on the skin microbiome. Photodermatology, Photoimmunology & Photomedicine (2023).