For the first time in nearly 30 years, the US is getting a new active ingredient for sunscreen. The FDA just approved the broad spectrum UV filter, bemotrizinol, after beginning the approval process over 20 years ago. The UV filter has been used in Europe for decades but because the FDA regulates sunscreen as a medicine rather than as a cosmetic, things took longer during clinical trials. Bemotrizinol is more stable than other UV filters in sunscreen and its larger molecule size means that it is less likely to enter the bloodstream.
Sunscreen has been on my mind quite a bit lately, after finally seeing the dermatologist for a long overdue skin check and becoming a parent. I started to wonder how sunscreen could affect the skin microbiome. What I found was that research in this area is relatively new, but I’m intrigued by the possibilities of how what we learn about sunscreen and the skin microbiome can be applied to the development of cosmetics and other products to make our skin healthier (think: probiotics for the skin).
How sun exposure changes the skin microbiome
Exposure to solar radiation can change the microbiome composition and diversity, which could lead to dysbiosis and immune dysfunction. Solar radiation could directly damage microbial cells and create reactive oxygen species and cause oxidative stress to the bacteria. A study from the University of Manchester found that the length of a sun-soaked week-long vacation can change the skin microbiome. The team did note that the microbiome composition returned to normal in a few weeks.
Effect of 24 hours of sunscreen on the skin microbiome
A study published earlier this year in Applied and Environmental Microbiology looked at how sunscreen changed the skin microbiome after 24 hours. This study tested two mineral sunscreens on cultured bacteria from the skin and the skin microbiome of 20 human volunteers. In cultured skin microbiome bacteria, they found that the sunscreen didn’t affect their viability after two hours of sunscreen exposure. In humans, the researchers collected skin swab samples before adding sunscreen, added the sunscreen, and then collected more skin swab samples one, six, and 24 hours later. They sequenced the DNA to identify microbes on the skin and reported that the microbial community on the whole wasn’t significantly changed even after 24 hours of sunscreen exposure.
I did find this study interesting but I had so many questions. I’m curious about whether there will be an effect over longer timelines such as daily use for an entire summer. Additionally, sunscreen manufacturers recommend reapplying every 2 hours, while this study only applied the sunscreen once. I’d assume in this experiment, the amount of sunscreen left on the skin decreased over the 24 hours. I’d be interested to see if this more frequent application would change the microbiome.
Another interesting point was that this study did mention that they did not control for sun exposure. This could mean that some participants of the study had more sun exposure than others. This brings us to the next study where they did add in UV exposure as a study element.
Adding UV exposure to the mix
This study looked at how a chemical sunscreen affected the skin microbiome when exposed to UV radiation. Participants were treated with the sunscreen or a placebo, which had the same formulation as the sunscreen but without the UV filters. Some of this skin was exposed to 2 hours of UV exposure at levels meant to simulate sunlight. The researchers swabbed for microbes from four skin samples: (1) unexposed to UV, (2) exposed to UV, (3) sunscreen treated and exposed to UV, and (4) placebo treated and exposed to UV. They sequenced these samples to identify the bacteria present
In those treated with sunscreen, they saw a reduction of Cutibacterium acnes, which, as the name suggests, is a bacterium linked to acne. The researchers also found that sunscreen could benefit certain microbes like Lactobacillus crispatus, the most abundant lactobacillus species in this study. This bacterium was decreased in the placebo treatment when exposed to UV light, but was increased with sunscreen treatment despite UV exposure. Recently this bacterium has been the focus of in antiaging probiotic skin cream and the authors of this study suggest that this bacterium might build beneficial biofilms to protect host tissue and contribute to innate immune response.
Towards a growing number of skin probiotics?
While probably the most common association between probiotics and our bodies remains the gut microbiome, I’m excited to see work on probiotics extend to other niches, like the possibility of Lactobacillus crispatus as a skin probiotic. Other research finds that certain skin microbiome bacteria can metabolize a “photoproduct” which arises with light exposure. This photoproduct affects the immune response so a skin probiotic could help modulate the immune response as well. As the largest organ of the human body, the skin offers a compelling target for new microbiome–based approaches.