Does sunscreen alter the skin microbiome?

Researchers have recently questioned how sunscreen can affect the skin's microbiome. Indeed, a study led by Christel LEFRANCOIS and her team showed that exposure to commercial sunscreen formulations altered the skin microbiome of fish. The relative abundances of bacteria Bacteroidota, Actinobacteroidota and Proteobacteria were significantly altered. Furthermore, the proliferation of certain genera, such as Mycobacterium, Tenacibaculum and Nocardia, was observed in the skin microbiome. Thus, sunscreens could potentially disrupt the first biological barrier against bacteria.

Most tested sunscreen formulas contain zinc oxide nanoparticles (ZnO) and titanium dioxide (TiO2) as mineral UV filters. However, it has been demonstrated that TiO2 and ZnO nanoparticles have anti-microbial properties in vitro. More specifically, Fernand FIÉVET and his team found that at concentrations higher than 1.3 x 103 M, ZnO nanoparticles had bactericidal effects on Escherichia coli. A subsequent study delved into this point, reporting that concentrations higher than 1.6 x 102 M of ZnO would increase the permeability of bacterial membranes, leading to cell death.

Direct research on this subject is, however, still limited and some of these conclusions are theoretical. Studies on human skin are necessary to take a stance on these results.

Mechanisms are suspected to be responsible for these actions.

• Generation of Reactive Oxygen Species (ROS).

  When exposed to UV light, nanoparticles of TiO2 and ZnO can generate free radicals, such as hydroxyl radicals and superoxide radicals. These free radicals possess oxidative properties that can damage the cellular membranes of microorganisms, leading to their death.

• Release of metallic ions.

  ZnO nanoparticles can release zinc ions (Zn2+) into their environment. These ions have anti-microbial properties by disrupting the function of enzymes essential to microorganisms and by causing damage to their DNA, which results in their death.

At Typology, we have made the decision to exclude nanoparticles of ZnO and TiO2 from our sunscreen products, due to the potential risks they pose for skin penetration and ecotoxicity.