# The human skin microbiome remains unchanged after 24 h of sunscreen application

## TL;DR

This study shows that applying sunscreen for 24 hours does not harm or change the skin's natural bacteria, suggesting it is safe for long-term use.

## Contribution

The study is the first to show that common sunscreens do not disrupt the skin microbiome's diversity or viability.

## Key findings

- Sunscreen application for 24 hours did not alter skin microbiome diversity or viability.
- Core skin bacteria like Staphylococcus and Corynebacterium remained stable.
- Minor fluctuations in low-abundance genera were observed but deemed transient.

## Abstract

To ensure safe, long-term use, topical products should be investigated to understand how they interact with the resident skin microbiota to mitigate potential risk. Sunscreens are essential for protecting skin from UV damage, but their effects on skin-resident microbes have not been well characterized. We examined the impact of two sunscreen formulations (containing titanium dioxide or zinc oxide) on both cultured skin bacteria and the skin microbiomes of human volunteers. No loss of viability was observed after a 2 h exposure to either sunscreen in cultures of Staphylococcus epidermidis, Staphylococcus capitis, Staphylococcus hominis, Micrococcus luteus, and Corynebacterium tuberculostearicum. The effects of the sunscreens were then studied across the skin microbiomes of 20 human participants. Skin swabs were collected before application and at 1, 6, and 24 h afterward. DNA was extracted and sequenced at the 16S rRNA V4 region, and sequences were denoised and taxonomically assigned using the nf-core/ampliseq pipeline. Across all time points, alpha diversity (Shannon index, Friedman test) and beta diversity (permutational multivariate analysis of variance) remained stable, with no significant differences in beta dispersion. Differential abundance analysis revealed minor fluctuations in some low-abundance genera, identified as likely transient due to their low prevalence, but overall resident community composition was not significantly altered. These findings suggest that short-term sunscreen application does not disrupt the skin microbiome, supporting their safe use from a microbial standpoint. Outcomes from both in vitro and in vivo experimentation point to the compositional resilience of the skin microbiota to sunscreens.

Understanding how sunscreens affect the skin microbiome is important, given their widespread use and the role of the microbiome in skin health. This study demonstrates that common sunscreens do not significantly alter skin microbiome diversity or viability, including that of the core skin microbiome genera, Staphylococcus, Micrococcus, Kocuria, Cutibacterium, and Corynebacterium. This highlights the resilience of the skin microbiota and supports the microbiome-safe profile of these products.