2021 FDA Science Forum
The Impact of Nanoscale TiO2 and ZnO Used in Sunscreens on Skin Bacteria
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- Center:
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Contributing OfficeNational Center for Toxicological Research
Abstract
Background:
Nanoscale titanium dioxide (TiO2) and zinc oxide (ZnO) are two major physical ultraviolet (UV) blockers used in sunscreens. These particles at size of nanoscale can protect human skin from harmful UV light. Several hundred species of skin bacteria form a complex and diverse community commonly termed skin microbiota. The balance of bacteria population in the community is important to keep skin health. Some published studies showed that nanoscale metallic particles can inhibit bacterial growth. So, it is important to know whether these particles can affect skin health by changing the balance of skin bacteria population when using sunscreen topically. Purpose: This study was aimed to evaluate whether nanoscale TiO2 and ZnO change the viability of skin bacteria and assess the effects of UV lights on the antimicrobial activities of nanoparticles.
Methodology:
Common skin bacterial strains were grown under anaerobic or aerobic conditions. Serial dilutions of bacterial cultures were dropped on top of nanoparticle-agar plates with or without further exposure to UVA light (320-400 nm) or UVB light (311 nm). Antimicrobial activities of ten nanoparticles were evaluated by counting bacteria colony numbers grown on top of the plates. Three of these particles are coated nanoparticles, which surfaces are covered with other materials.
Results:
Without UV exposure, coated TiO2 didn’t significantly affect the growth of skin bacteria. Uncoated ZnO particles showed dose-dependent antimicrobial activities. One hour-exposure to UVA enhanced bactericidal activities of uncoated ZnO (3.1 mg/mL) against Cutibacterium acnes, Staphylococcus haemolyticus, S. warneri, and S. epidermidis by 102 - 105 folds. Ten minutes-exposure to UVB enhanced about 100 folds bactericidal activities of uncoated ZnO (3.1 mg/mL) against C. acnes. Coated nanoparticles compromised the UVB bactericidal effects by increasing 10-100 folds of bacteria cell numbers.
Conclusion:
Nanoparticles in sunscreen products combining the influences of UV radiation may change skin microbiota in a complex manner. The information from this study could advance the knowledge about the safety of nanoparticles in sunscreens on skin health. The in vitro approach established from this study can help to evaluate the safety of cosmetics or ingredients on skin health.
