How does sun exposure affect skin microbiota composition and diversity?

By Neha MathurAug 10 2023Reviewed by Sophia Coveney

In a recent study published in Frontiers in Aging, researchers recruited 21 healthy North European volunteers who agreed to go on a holiday in a sunny destination for a minimum of seven days to study how ultraviolet radiation (UVR) exposure and other sun-seeking behaviors impacted their skin microbiota composition, diversity, and overall skin health.

Background

Human skin is a dry and desiccating surface where most microbes do not permanently reside except for some niche microbial species depending on skin type, sebaceous (oily), dry, or moist.

These microbial communities are subject to internal and external pressures, e.g., UVR. High doses of UVR damage skin DNA and also cause inflammation and photoaging. Studies have shown that UVR also disrupts the activity of skin-residing microbial communities and their phylogenetic and genotypic composition. 

Skin-residing microbes protect the human skin from UVR damage by releasing cytokines, such as tumor necrosis factor α and interleukin-6. So, their disruption likely activates the host immune system.

However, fewer studies have examined the in vivo effects of sun- or UVR exposure on the skin microbiota. Also, any damage to skin microbiota by UVR or other sun-seeking, pro-tanning behaviors might disturb local and systemic immunity and epidermal homeostasis.

Thus, dermatologists recommend sun-protection practices, e.g., applying sunscreen to reduce UVR exposure, a well-recognized risk factor for skin cancer, especially among people with lightly pigmented skin

About the study

In the present study, researchers examined the effect of the sun-seeking behavior of all participants on their epidermal melanin levels during a high sun exposure holiday. 

They analyzed their microbiota composition before going on the holiday and on days 1, 28, and 84 after coming back from the holiday. Specifically, the team used polymerase chain reaction (PCR)-based 16s ribosomal ribonucleic acid (rRNA) gene sequencing to quantify microbial diversity and changes in the abundance of skin bacterial taxa in skin swabs collected from all participants.

The researchers also made colorimetric measurements of skin and evaluated their individual typology angle (ITA) status and any disease-related shifts in the skin microbiota.

Results

According to the authors, it is one of the first studies on the effects of various sun-related behaviors during a 'real-world' high sun exposure holiday setting on the composition and diversity of the skin microbiota and skin health. The researchers remarkably showed that the development of a tan was markedly associated with a lower relative abundance of Proteobacteria post-holidays (day 1), with its community structure recovering by day 28 after the holidays ended. Also, the observed differences no longer seemed significant on days 28- and 86 post-holiday. 

Proteobacteria, a large Gram-negative bacteria phylum, is typically found in abundance on human skin. Many previous studies have associated a perturbed Proteobacteria microbiota with diseases, such as psoriasis, eczema, and diabetic foot ulcers, and its greater diversity with immune responses protective against allergic inflammation.

The effect of UV exposure on the skin microbiome during the holiday was acute, and recovery was quick after ceasing the high UVR exposure, likely because Proteobacteria dominate the skin microbiota. It helped the affected people re-establish their epidermal homeostasis. 

Many previous studies have described the ill effects of disruptions in microbiome populations and components in different bodily sites. For instance, Hanski et al. found an association of reduced skin bacterial richness with atopic dermatitis. It raises the concern that the observed loss of bacterial diversity with increased facultative tanning might also impact skin health.

However, in this study, the authors noted no profound differences in the populations of any bacterial genera due to UVR-induced tanning but only a decrease in Proteobacteria one day post-holiday at the phylum level. 

Conclusions

To summarize, sun exposure in “real-world” holidaymakers substantially reduced the abundance of Proteobacteria and skin microbiota diversity; however, it recovered by day 28 after the holidays. People planning sun-seeking holidays could consider topically applying Proteobacteria before going out for protection and limiting the skin microbiome alterations. 

The current study findings could help further explore UVR-induced inflammation, damage, and skin disorders associated with changes in the skin microbiota diversity. In particular, future studies should explore the long-term effects of changes in the microbial diversity of Proteobacteria phyla and skin health upon higher UVR exposure. 

More importantly, more data on the protective effects of skin microbiota could help inform the design of novel topical pre- and probiotics to promote dermatological health in response to higher UV and sun exposure.