HARALD VAN DER HOEVEN

CLR - Chemisches Laboratorium Dr. Kurt Richter GmbH

With consumers and a large part of the cosmetic industry, this is a common notion. Most cosmetic formulations on the market indeed contain ingredients which are either antimicrobial, bacteriostatic, or in another way somehow detrimental for microbial life. It is not for nothing that a large number of cosmetic products on the market make claims such as “microbiome-friendly” in order to set themselves apart from other products. The consumer is inclined to use products which claim to be mild to their skin, and by now the consumer has at some level understood that there is such a “thing” as the skin microbiome and that we should treat it with respect.

While it may seem intuitive to assume that cosmetic formulations inherently harm the skin microbiome, a question that may be unpopular from a marketing perspective must be raised: Are cosmetic formulations truly detrimental to the microbiome?

An examination of the scientific, peer-reviewed literature is called for. A scientific paper accessible on PubMed (1) is generally regarded as reliable and deserving of serious consideration. As anticipated, research papers discussing the adverse impact of cosmetics on the skin microbiome can be located through PubMed and various other scientific platforms.

Different peer-reviewed and serious-looking papers by Wallen-Russel (2) can be easily found on the internet. The particular paper just referenced mentions that “the use of synthetic chemical ingredients in modern cosmetics is postulated to be a cause of damage to the skin microbiome.” This is put in the context of the “skin allergy epidemic” and “an environmental factor in the Western world which is causing a catastrophic ecosystem collapse on the skin.” Concluding from the experiments described in this paper, it states: “The more synthetic ingredients contained in a product, the less of a positive effect on average biodiversity it appeared to have.”

This paper appears to be serious, even though the cosmetic products which belong to Wallen-Russel’s brand clearly showed to perform best in the study they performed.

Intuitively, cosmetic preservatives should have a negative impact on the skin microbiome. This was looked into and described in the scientific work of Pinto et al. (3). The effect of eleven preservatives commonly found in cosmetic products on Cutibacterium acnes, Staphylococcus epidermidis, and Staphylococcus aureus was assessed in vitro using 3D skin models and culture-dependent methods. In this paper, these three species are considered to be “the main three skin‐resident bacteria.” This is doubtful for S. aureus, but S. epidermidis and C. acnes indeed belong to the most abundant bacteria on (or in? We will discuss this later) the skin. The different preservatives all had, at some level, antimicrobial effects against the bacterial species mentioned above. The authors’ conclusion: “Taking together these data highlight the role of preservatives of skin resident microflora dynamics and could provide a reference for correctly choice preservatives and dosage in cosmetic formulations to preserve or restore homeostasis of skin microorganisms.”

Another example of a paper describing the negative effects of ingredients commonly used in cosmetic formulations was published in 2019 by Dobler et al. (4). The authors used an interesting approach: they “tested the ability of the Malassezia species and some bacterial strains to assimilate substances frequently used in dermal formulations” and “the hydrolysis of esters by Malassezia lipases was detected using High Performance Thin Layer Chromatography (HPTLC).” The Malassezia spp. (M. Furfur, M. Globosa and M. Sympodialis) and the bacteria (Corynebacterium minutissimum, Staphylococcus epidermidis, and Staphylococcus galinarum) were grown in culture, the study was performed in in vitro conditions. Their experiments elegantly showed that common cosmetic ingredients, not preservatives, but rather fatty alcohols, fatty acids, triglycerides and esters, in some way or form, had an influence on the microbial species mentioned. This included the promotion or inhibition of growth and metabolism.

From this paper, too, a logical and intuitive conclusion would be: Cosmetic products which we as consumers use frequently have an impact on the skin microbiome. The “need” for “microbiome-friendly” cosmetics which are formulated in such a way that they do not negatively impact the skin microbiome is merited.

Is this true, though? Two of the above-described scientific papers (and many more which can be found in the scientific literature) do not describe an in vivo situation, but rather that in a “petri dish.” The third paper was written by someone who can arguably be described as an “activist.”

In a global context, the most people live in environments and use (cosmetic) products that could adversely affect the skin microbiome. The extensive range of products and environments we encounter leads to constant changes in our skin microbiome. This supports Wallen-Russel’s notion of a "skin allergy epidemic" and an environmental factor causing a potential ecosystem collapse on the skin in the Western world.

Considering the scientific literature (5) highlighting that isolated indigenous populations devoid of external contact exhibit distinct skin microbiome compositions and lack common skin diseases, a clear inference emerges: our conventional practices may be flawed.

Arguably, the industrialized world doesn't necessarily suffer from a "skin allergy epidemic." Even if it does, various lifestyle factors likely contribute to it. With a global population of around eight billion, relocating everyone to a remote rainforest is obviously impossible. Most of us engage with cosmetic products which exert a potentially negative impact on our skin microbiome, as evidenced by the scientific papers mentioned above.

The paper of Oh et al. (6) is a must-read for anyone interested in the skin microbiome. It is a reference work in skin microbiome science that was written by people who work at the forefront of skin microbiome research. The article draws important and possibly unintuitive conclusions: “Despite the continuous perturbation that human skin undergoes in daily life, healthy adults stably maintain their skin communities for up to 2 years, similar to the stability observed in the gut.” Another important point made is: “We surmise that in the absence of major perturbations, dominant characteristics of skin microbial communities would remain stable indefinitely, a conclusion previously extrapolated for gut communities.” The context within which these results were obtained is of tremendous importance: intuitively, in our daily life we expose ourselves to many exogenous influences which continuously change (the use of different cosmetics such as skincare, cleansing or makeup products only one of them) which we would expect to have a significant impact on the long-term composition of our skin microbiome. However, this paper shows that there is no significant impact.

Numerous peer-reviewed scientific papers explore the impact of cosmetics or cosmetic raw materials on the skin microbiome composition. While certain papers highlight adverse effects, as mentioned earlier, others assert that the routine use of common cosmetic products does not disrupt the composition of the skin microbiome.

Like the paper by Oh et al. (6) on the temporal stability of the skin microbiome, the article by Bouslimani et al. (7) is also a must-read for anybody who works in the cosmetic industry, especially when interested in the impact of cosmetics on the skin microbiome. Researchers such as Rob Knight and Pieter Dorrestein were among the authors of this article, both of whom work at the highest level on the development of in-depth methods of analysis of the skin microbiome and chemistry, respectively.

In the study mentioned, microbial samples from skin of 12 healthy volunteers were collected. After a three-week wash-out phase when no cosmetic products were applied, the volunteers went through identical personal care routines for three weeks. A moisturizer was applied on the arm, a sunscreen on the face, an antiperspirant on the armpits, and a soothing powder on the foot. Additionally, a body wash was used daily. The results of this study showed that the antiperspirant and the foot powder, both of which were formulated to have an influence on the skin microbiome with Aluminum Zirconium Tetrachlorohydrex GLY (18.5%) and a high concentration of salicylic acid, respectively, indeed had an impact on the composition of the skin microbiome. The body wash, moisturizer and sunscreen, all of which included ingredients that, intuitively, should have an impact on the skin microbiome, such as parabens, phenoxyethanol and sodium benzoate, did not lead to important variations in the composition of the skin microbiome. For the analysis of the skin microbiome, 16S rRNA sequencing was performed following the Earth Microbiome Project protocols.

In a separate study conducted by Two et al. (8), the impact of common skin washing on the microbiome was examined. Six readily available skin cleansers were assessed, two of which featured heightened levels of potent antimicrobial agents, benzalkonium chloride or triclocarban. The remaining four cleansers lacked bactericidal properties and were preserved with standard cosmetic preservatives. Controlled washing and rinsing procedures were followed, skin surface samples were collected through swabbing, and microbial composition analysis was conducted using 16S rRNA sequencing. Notably, cleansers containing benzalkonium chloride or triclocarban demonstrated an effect on viable bacteria post-washing and drying, while the other four cleansers exhibited no distinct outcome compared to washing with water alone.

In yet another study by Jaksch et al. (9), two cohorts of 25 volunteers each were analyzed before and after daily use of two different commercially available shampoo formulations for two weeks. Both shampoos contained regular cosmetic ingredients, some of them intuitively considered to have some impact on the scalp microbiome, such as phenoxyethanol, potassium sorbate, lactic acid, and benzyl alcohol. Samples were taken from the surface of the scalp of the volunteers, and sequencing of PCR-amplified 16S rRNA and 18S rRNA genes was performed to analyze the bacterial and fungal communities of the scalp. As for the composition of these communities, the article reports that “both alpha diversity and beta diversity did not indicate any statistically significant differences between the scalp microbiota compositions before and after application of the two tested shampoo formulations.”

Rinse-off formulations, such as cleansers and shampoos, have a relatively short contact time with the skin and can therefore intuitively be less of a risk for the skin microbiome than regular leave-on cosmetic formulations such as creams and lotions. The paper by Callejon et al. (10) describes their experiments with three types of leave-on face skincare products: a hydrophilic formulation, a micellar formulation, and an oil-in-water emulsion. The study took place on the backs of 20 volunteers, and samples were collected 24 hours and four days after daily application of these formulations, and were compared to an untreated area. During 14 days before the start of the study, the volunteers were allowed to use a generic neutral shampoo and soft cleanser and to otherwise avoid any other product application. Here too, analysis of the skin microbiome took place by 16S rRNA gene sequencing. The results obtained in this study are in line with the articles mentioned above, i.e., “no dissimilarity was observed between the products and their controls, nor between each product, despite their different overall compositions linked to their functions.”

As discussed earlier, the outcomes observed in both rinse-off and leave-on cosmetic formulations warrant an examination of typical cosmetic preservatives to determine their potential impact on the skin microbiome. Most cosmetic formulations include preservatives or ingredients with inherent antimicrobial properties. Murphy et al.'s study (11) indicates that cosmetic preservatives indeed exhibit antimicrobial effects on certain common skin microbiome members in in vitro experiments. However, the study emphasizes a crucial point: current in vitro antimicrobial tests overlook a key aspect of the human microbiome: its ability to respond to external challenges and restore its composition from protected skin invaginations and glands. In vivo experiments, reflective of real-life consumer use, demonstrate that “despite the antimicrobial efficacy observed in vitro, preservative-containing products do not impact the skin microbiome in vivo”.

Murphy et al.’s (11) notion that the microbiota on the skin and to which cosmetic ingredients might be detrimental in vitro, are able to “re-seed its composition from skin invaginations,” is both extremely interesting and extremely important for the question of whether regular cosmetics are at all skin “microbiome-unfriendly” in real life. Might it be that microbes on the skin surface are, in essence, irrelevant for this discussion? Might it be that the microbes which are really of interest for this discussion, live inside the skin? And might they be protected against the antimicrobial effects of many common cosmetic ingredients or any other exogenous stresses?

A paper published by Liang et al. (12) postulates that skin surface microbes may be considered transient and that the bacteria in deeper epidermal and dermal regions should be regarded as the host indigenous microbiome. Important proof that microbes do indeed live under the surface of skin was found by 16S amplicon sequencing which showed that bacteria populate the upper stratum corneum (SC) layers, but become scarcer farther down the SC.

A paper by Acosta et al. (13) underlines this notion. It may be considered one of the most groundbreaking scientific articles on the skin microbiome of the last few years. According to the authors, a high abundance of bacteria can be found in the hair follicle with relatively few bacteria on the skin surface. Additionally, the abundance of microbial DNA on the skin surface is not associated with viable microbial cells. Bacterial DNA at the skin surface is readily removed from the skin and is continuously replenished by viable bacterial populations below the skin surface.

One of the experiments described in this paper elegantly showed that after sterilization of the skin surface and removal of microbial DNA, the skin surface was quickly repopulated with microbial DNA that was identical to that of the endogenous skin microbiome, which can only be explained by the replenishment by endogenous bacterial populations below the skin surface, as just mentioned. In this experiment, the surface bacterial DNA could be easily washed away, while the viable subsurface bacteria remained unperturbed. The authors conclude that the bacterial DNA on the skin surface can act as a “fingerprint” of the bacterial communities below, but nothing more than that.

Regarding assertions related to the skin microbiome, the most prevalent claim in the cosmetics market is undoubtedly the "microbiome-friendly" label. Based on the earlier discussion, one could deduce that typical cosmetic formulations that are not intentionally antagonistic to microbial life are inherently "microbiome-friendly" or, at the very least, not "microbiome-unfriendly."

The skin microbiome has become a prominent focus for the cosmetic industry, and consumers increasingly demonstrate understanding of and interest in this aspect. This leads to a pertinent question for cosmetic products: "What compelling claims can be made when 'microbiome-friendly' is no longer a distinctive feature?"

Drawing conclusions from the aforementioned in vivo experiments, the resident skin microbiome, often referred to as the "eigenbiome" or "neo-microbiome" (14), thrives beneath the skin surface, within hair follicles, and within the outer layers of the stratum corneum—referred to as the “epidermal skin microbiome.” In these regions, it enjoys considerable protection against potential adverse effects of cosmetic ingredients. Exploring the epidermal skin microbiome is particularly intriguing. While primarily residing within the stratum corneum, it is also found on the skin's surface, albeit in smaller quantities. Additionally, we continually shed material from our skin microbiome.

The continual renewal of microbial DNA on the skin's surface can be analogized to the ongoing replenishment of dead skin cells at the epidermal surface and their subsequent shedding through the desquamation processes. The scientific literature reports a substantial shedding rate of dead skin cells, ranging from 600,000 to 1,000,000 per hour (15). In a paper by Feuilloley (16), an intriguing observation is made: "each squama is carrying a mean of four germs." While these "germs" on dead skin cells ("squama") may no longer be viable according to Acosta et al.'s findings, it is reasonable to infer that, in alignment with the earlier discussion, the shedding of dead skin cells results in a significant release of microbial DNA into our immediate environment.

In line with the conclusions drawn from the combined papers of Feuilloley and Skowron et al., it is of interest to mention that a vast amount of microbial DNA is constantly shed from the skin. A paper by Meadow et al. (17) describes this “microbial cloud” that we are constantly releasing from our skin to be of valuable interest for forensic science.

All things considered, much like the constant renewal of the epidermis, the epidermal microbiome undergoes continuous renewal as well. The intriguing aspect for the cosmetic industry is that the resident epidermal microbiome resides within the stratum corneum. The general principle that the composition of a microbial community is influenced by its living environment (18) is highly relevant to the resident skin microbiome. The complex interactions between the resident skin microbiome and the skin itself highlight the skin as a true ecosystem, as eloquently discussed in a meeting report by Lebeer et al. (19).

These interactions can be positive, negative, or a combination thereof, depending on the composition of the resident skin microbiome and the environment in which it exists. The fact that the resident skin microbiome is situated inside the skin, shielded from the external environment, suggests that the skin itself shapes the environment in which it thrives. Consequently, it can be inferred that in the symbiotic relationship between our skin and its microbiome, the skin takes the lead.

This conclusion aligns with well-known aspects of certain skin diseases. For instance, Baviera et al. (20) observed that in atopic dermatitis, after barrier disruption the pH values range between 7 and 8, as compared to the pH of normal skin in the same setting, which is between 4.6 and 5.2. This elevated pH favors Staphylococcus aureus, a common pathogen implicated in the pathophysiology of atopic dermatitis. The elevated pH in atopic dermatitis stems, in part, from defects in the filaggrin gene, which disrupts the production of molecules such as trans-urocanic acid (tUCA) and pyrrolidone-5-carboxylic acid (PCA), which is crucial for maintaining a low pH in the stratum corneum (21).

In contrast to the diseased skin scenario, where altered skin microbiome composition results from a defect within the skin itself, it can be asserted that “healthy skin harbors a healthy skin microbiome” or at least not an unhealthy one. Healthy skin exhibits sound epidermal differentiation, contributing to an environment conducive to microbes considered healthy or, at the very least, not unhealthy. The pH of healthy skin is a reflection of a well-functioning skin.

For those members of the cosmetic industry who are interested in making skin microbiome claims, this inherently implies that, by “doing good” for the skin, you are automatically “doing something good” for the skin microbiome. This not only opens many doors from a scientific and marketing point of view, but also from a regulatory perspective.

Claims that extend beyond merely being "microbiome-friendly" to assert that "we are positively impacting the skin microbiome, and thus benefiting the skin," present significant regulatory risks. In 2021, Unilever encountered such challenges with certain Baby Dove products, a matter brought to the attention of the Advertising Standards Authority (ASA) in the UK (22). It appears more secure and straightforward to substantiate the reverse statement: "We are benefiting the skin, and therefore we are positively influencing the skin microbiome."