The skin microbiome’s role in human health & wellbeing is becoming better understood within the personal care and beauty industry. In the personal care market, we observe a steady increase of newly launched Skin Care products with “Microbiome” claims. Over the past three years (2020 – 2022) the numbers have doubled [1]. By supporting the skin barrier and immune function, the microbiome layer provides a first line of defence against external aggressors such as ultraviolet (UV) radiation [2]. However, sun care and the use of UV filters is one area of personal care where more work is required to understand the relationship with the skin microbiome.
What is the skin microbiome and how is it impacted by UV radiation?
The skin microbiome is a diverse community of microorganisms, including bacteria, fungi, archae and viruses, that reside on human skin (Fig. 1) and has interactions with the environment [3]. These microorganisms and their whole spectrum of molecules produced play an important role in supporting our skin health and overall wellbeing – for example, by shielding our skin from potential pathogens and irritants by supporting our protective skin barrier and immune system [4].
As we have learnt more about this complex ecosystem on our skin, we have seen that protecting the microbiome supports healthier skin. To maintain a balanced and healthy microbiome, both the quantity and quality of microorganisms makes a difference. The skin microbiome adapts to internal and external factors, such as age, lifestyle, and environment. As the microbiome resides at the boundary between an individual and the environment, the skin microbiome is clearly influenced by environmental factors. For example, increasing urbanization might disturb the skin ecosystem with a decrease in diversity [5].
There are few evidenced studies exploring UV radiation and the skin microbiome. Obviously, high-energy UV irradiation kills microbes. This principle is common practice for disinfection with UVC lamps. However, even UVA and UVB at erythemal doses are sufficient to change bacterial compositions for more than 24 hours as shown by Burns et al. [6]. They observed clear increases in some bacterial groups such as Cyanobacteria and decreases in others such as Lactobacillaceae in UV–exposed subjects.
In another study Patra showed that cellular and immune response to UV was dependent on skin microbiome [7]. For example, immune suppression to UV was reduced in the presence of an intact microbiome on mice skin. The authors suggested that it might be beneficial to protect a healthy skin microbiome against UV-induced shifts. Given the evidence presented here, therein lies the question, might UV filters support the microbiome upon UV stress? Several UV filters have also been tested in the industry and proved to have no detrimental effect on the skin microbiome, but no studies have fully explored this relationship, or considered whether there could be any active benefit by UV filters for the skin microbiome.
Recognizing the need to investigate the impact of UV exposure on the skin microbiome and any potential advantages of UV filters, we designed and executed a pioneering clinical study to explore these relationships.
dsm-firmenich’s clinical trial on UV irradiated skin microbiome
The two key objectives in the clinical pilot study were:
1. To evaluate the impact of erythemal UV exposure on the skin microbiome
2. To determine the protective effect of an SPF 20 sunscreen versus placebo on the UV-exposed microbiome
Ten pre-menopausal subjects were recruited, each providing skin samples from four skin zones on their upper middle back, i.e., untreated/unexposed, untreated/exposed, placebo and SPF20 treated on exposed skin. The time points just before and 2 hours after erythemal UV exposure (2 Minimal Erythema Doses) were considered. Samples were collected by skin swabbing for the microbiome analysis by bacterial 16S rRNA gene sequencing.
(i) Changes in skin microbiome diversity
The analysis from sequencing of the skin samples revealed the bacterial percentage compositionin relative abundances. This refers to how common or rare a species is relative to all other species present in a community. Apart from significant inter-individual differences, both, UV irradiation and treatment, had a distinct impact on the relative abundances and diversity of the different bacteria found within the skin microbiome (Fig. 2).
(ii) Protecting Lactobacillus crispatus
Not only diversity but also quality matters in terms of what species are present when we talk about a healthy microbiome. The sequencing data were statistically analyzed for UV-sensitive bacteria which were positively associated with the sunscreen treatment. The bacterial species Lactobacillus crispatus has been identified that was readily reduced after UV exposure and on the same time protected by the sunscreen treatment (Fig. 3).
Lactobacillus crispatus is present in and on humans [8-11] and was found the most abundant Lactobacillus species our skin study. Yet little is known around its dynamics on skin. Abundance is thought to be highest after birth and decreases while we age [10]. It has a key function in the vaginal microflora and is commonly used in probiotic treatment. It is known to preserve the balance of a healthy microbiome and prevents pathogens. It produces lactic acid creating an acidic environment and releases antimicrobial compounds [12]. Consequently, L. crispatus is a component of the innate immune system. As seen before, UV filters can protect the abundance of L. crispatus during UV exposure. We suggest that UV filters preserve the functionality of such key players and ultimately, may help to preserve skin barrier function and resilience upon UV stress.
in vitro UV tests on individual bacterial species confirmed UV protection of microbiome by specific UV filters
Having observed that specific microorganisms such as L. crispatus can benefit from the additional UV protection, further in vitro experiments on individual bacteria species were conducted that supported our findings. Single UV filters and combinations thereof applied to the bacteria before UV irradiation showed selective protective behaviour reflected in the bacterial survival rate: Effective protection of L. crispatus populations upon UV radiation was observed with octocrylene, zinc oxide, avobenzone and ethylhexyltriazone. Also, the UV filter combination as used in the in vivo study and the combination avobenzone/bemotrizinol/ethylhexyltriazone performed well. A smart UV filter combination allows the Sun Protection Factor (SPF) to be maximized while protection of UV-sensitive bacterial key players can be maintained to strengthen UV-exposed skin resilience.
In parallel, Cutibacterium acnes has been tested. It is known that some strains occur as commensals and some as opportunistic pathogens associated with acne formation [13, 14]. Therefore, the scope was different here: C. acnes abundance rather needs to be controlled or reduced rather than to be stimulated. On the same time, you want to maintain S. epidermidis – a normal skin commensal – which was used as reference strain here. Some filters specifically reduced C. acnes population, while keeping S. epidermidis in good shape such as avobenzone, octisalate, and octocrylene. As a result, a smart selection of the UV filters in a sunscreen might reduce C. acnes population and mitigate inflammatory acne breakouts. These findings suggest the preferred use of those filters for mild sunscreen formulations that target for acne-prone skin.
What do the results mean for consumers?
(i) You can empower natural skin resilience
It has been proposed that UV protection can be expanded to the microbiome: Sunscreens not only protect skin (cells) but also its skin microbiome. It is easily understood by consumers as everybody knows why we apply sunscreens. The rationale is to preserve functional microbiome by inhibiting the environmental factor UV at the very beginning before any harm has occurred to skin (microbiome).
It has been shown that a selection of UV filters not only shielded against harmful UV radiation, but also enhanced the skin’s natural resilience by protecting the survival of beneficial bacteria within the skin microbiome. Using microbiome-friendly sun care products containing optimized filter combinations can therefore support skin health. These can help to maintain skin structure and function by preserving a natural skin microbiome. This increased resilience can help protect against the damaging consequences of environmental stressors such as UV radiation and counteract the effects of skin ageing [10].
For more information on the skin microbiome, please visit our Secret Life of Skin content hub: https://thesecretlifeofskin.com/
References
- Source: GNPD database (Mintel; www.mintel.com) – No. of launches in PC 2020 vs. 2022.
- Patra, V., I. Gallais Serezal, and P. Wolf, Potential of Skin Microbiome, Pro- and/or Pre-Biotics to Affect Local Cutaneous Responses to UV Exposure. Nutrients, 2020. 12(6).
- Byrd, A.L., Y. Belkaid, and J.A. Segre, The human skin microbiome. Nat Rev Microbiol, 2018. 16(3): p. 143-155.
- Flowers, L. and E.A. Grice, The Skin Microbiota: Balancing Risk and Reward. Cell Host Microbe, 2020. 28(2): p. 190-200.
- McCall, L.I., et al., Home chemical and microbial transitions across urbanization. Nat Microbiol, 2020. 5(1): p. 108-115.
- Burns, E.M., et al., Ultraviolet radiation, both UVA and UVB, influences the composition of the skin microbiome. Exp Dermatol, 2019. 28(2): p. 136-141.
- Patra, V., et al., Skin Microbiome Modulates the Effect of Ultraviolet Radiation on Cellular Response and Immune Function. iScience, 2019. 15: p. 211-222.
- Bouslimani, A., et al., Molecular cartography of the human skin surface in 3D. Proc Natl Acad Sci U S A, 2015. 112(17): p. E2120-9.
- Lebeer, S., et al., Selective targeting of skin pathobionts and inflammation with topically applied lactobacilli. Cell Rep Med, 2022. 3(2): p. 100521.
- Kim, H.J., et al., Aged related human skin microbiome and mycobiome in Korean women. Sci Rep, 2022. 12(1): p. 2351.
- Lepargneur, J.P., Lactobacillus crispatus as biomarker of the healthy vaginal tract. Ann Biol Clin (Paris), 2016. 74(4): p. 421-7.
- Wang, S., et al., Antimicrobial Compounds Produced by Vaginal Lactobacillus crispatus Are Able to Strongly Inhibit Candida albicans Growth, Hyphal Formation and Regulate Virulence-related Gene Expressions. Front Microbiol, 2017. 8: p. 564.
- Mayslich, C., P.A. Grange, and N. Dupin, Cutibacterium acnes as an Opportunistic Pathogen: An Update of Its Virulence-Associated Factors. Microorganisms, 2021. 9(2).
- Lee, Y.B., E.J. Byun, and H.S. Kim, Potential Role of the Microbiome in Acne: A Comprehensive Review. J Clin Med, 2019. 8(7).
