The impact of body lotions on the skin microbiome and ceramides

Moisturising products can help improve the look and feel of dry skin, but little is known about how these products may impact the bacterial ecosystem of the skin, the skin microbiome. Researchers Dr Barry Murphy and Dr Andrew Mayes from Unilever R&D explored the changes in skin health and the associated microbiome after using a body lotion for five weeks. They found that improvements in the skin’s barrier function, which included increases in essential skin lipids and ceramides, were supported by changes in the composition and function of the microbiome. Understanding how the skin microbiome contributes to skin health is important in developing more effective products to help address dry skin. Unilever has a large range of moisturising cosmetic products, including under the Dove and Vaseline brands.

Skin functions as the body’s first line of defence against invaders such as bacteria and viruses, as well as acting as a vital sensory organ. It helps regulate body temperature, protects us from external exposures such as sunlight and pollution, and helps maintain the balance of fluids in the body.

Skin is one of the body’s largest organs, covering nearly 25m², and makes up almost a sixth of our body weight. Due to its visibility and purpose, skin health and appearance can have significant impact on mental and physical health and wellness.

Dry skin affects around a third of the population globally. It is typically characterised by dry, itchy and rough skin, sometimes with flaking or scaling. It is caused by a lack of moisture, something that is usually regulated by the upper layers of the skin, the epidermis. There are a number of different factors that can contribute to dry skin, such as the environment (especially the weather), pollution, hot baths and showers, certain soaps, and genetics.

The stratum corneum is the outermost skin layer and is made up of specialised cells and lipids, which form a structure similar to a wall. In this so-called ‘bricks and mortar’ arrangement, the cells (corneocytes) are the ‘bricks’ and are held together by the lipid ‘mortar’ composed of ceramides, fatty acids, and cholesterol.

These essential lipids are produced by the skin to help prevent moisture loss due to evaporation and to block out irritants that may inflame the skin. Disruption or depletion of lipid levels often causes dryness and irritation.

Thus, ceramides, free fatty acids, and cholesterol play a vital role in ensuring that skin remains in good condition. These molecules also help maintain skin at a slightly acidic pH (between 5.4 and 5.9). This ‘acid mantle’ provides the ideal environment for the beneficial skin microbiome to thrive while protecting against invading pathogens.

Understanding dry skin and its microbiome

The human microbiome is a community of naturally occurring microorganisms, consisting mostly of bacteria but also fungi and viruses, and plays a vital role in human health. Each part of the body has its own microbiome, including the mouth, gut, urogenital system, and skin. Unilever scientist Michael Hoptroff suggests that a ‘balanced’ skin microbiome can help to support skin health, but when disturbed or ‘unbalanced’ it can contribute to skin problems. Alterations in the skin microbiome have been associated with a number of skin-related concerns, including acne, dandruff, and underarm odour.

Cosmetic products that support the function of a healthy skin microbiome can help maintain skin health. While a range of products exist that can effectively moisturise the skin, it is not known how these improvements in the skin are reflected in the skin microbiome. To progress this research area, Dr Barry Murphy, Dr Andrew Mayes, and their teams from Unilever R&D investigated how skin is impacted by product use and how this relates to the skin’s microbiome.

The researchers studied the impact of a body lotion on skin health, lipid composition and skin microbiome in a group of female subjects, between 18 and 55 years of age, with moderately dry skin on their lower legs. Participants applied the body lotion twice a day.

At the start of the study and after a five-week application period, skin health metrics were assessed using both visual and instrumental measures. A range of samples were also collected from the skin of the participants and analysed for lipid and microbiome changes using both qualitative and quantitative methods.

Changes to the skin

The research team showed that there were significant improvements in the visual dryness score, level of skin hydration, and stratum corneum cohesivity (all well-recognised skin health measures) after the five-week study period, demonstrating the effectiveness of the product. Although data were analysed after five weeks of use, positive changes to skin in terms of visual dryness were observed even sooner than this.

Stratum corneum cohesivity is a measure of how strong and resilient the upper layer of skin is. In order to test this, the researchers used sticky tape, similar to Sellotape™, to sample the surface of the skin. If the skin has lower levels of cohesivity, more protein from the skin is bound on the tape, providing evidence that the skin is less-well held together, i.e., it is less cohesive. Improvements in this measure demonstrated that the lotion application improved the strength of the skin and made it more moisturised.

Using state-of-the-art mass spectrometry techniques, the researchers were able to quantify the levels of over 500 lipid species. They discovered increases in a large number of lipids in all three classes, i.e., ceramides, free fatty acids, and cholesterol which are all essential in ensuring that skin cells are held tightly together – a feature of healthy and moisturised skin.

As well as the increase in the amount of lipids present, the researchers also investigated whether the composition of the lipids had changed. They discovered that the body lotion resulted in an increase in longer-chain lipids (both fatty acids and ceramides) – which are critically important for ensuring a more effective skin barrier.

Whilst the body lotion applied contained lipid ingredients with 16 or 18 carbon molecules in their structures, there was evidence that these molecules were elongated to longer chain lengths in the skin by the end of the study, with these longer molecules more beneficial for skin health.

The importance of ceramides to skin health is well established and accepted, with many hundreds of ceramide species identified in healthy skin and grouped into ‘classes’. The study’s finding that the lotion applied led to increases in the level of nine out of 12 ceramide classes is of particular significance. While lotions exist that contain synthetic ceramides, these are known to have limited penetration into the skin and cannot reproduce the complexity of the natural variety of ceramide species. Thus, the inclusion of the ceramide precursors in the product used in the current study has exciting implications for skin health. The data published suggests that the tested body lotion provides the skin with nutrients to boost the production of the range of essential skin lipids needed for well-nourished, well-moisturised and healthier skin.

Mapping the microbiome

The research team was also interested in how the product changed the skin microbiome after application.

A technique known as quantitative polymerase chain reaction (qPCR) was used to detect and quantify the amount of DNA from specific bacteria in the skin microbiome. The results showed an increase in the levels of a bacterium called Staphylococcus epidermidis, which has been shown to have beneficial effects on skin through modulation of inflammatory responses, the production of molecules that kill pathogens, and increasing the lipid content of the skin.

Interestingly, they also saw an increase in the levels of another genus of bacteria, Xanthomonas. It has been suggested by some that this is a potentially beneficial bacteria for skin despite it not normally being found on skin. However, the team were able to attribute this increase in Xanthomonas levels to residual DNA found in one of the ingredients, Xanthan gum (a common cosmetic ingredient produced by Xanthomonas), present in the body lotion, rather than changes in the skin microbiome.

Finally, the team used a newly patented and peer-reviewed approach called ‘Single Sample Network Analysis’ to compare co-occurrences of bacteria in the skin microbiome before and after five weeks of lotion application. Network analysis has previously been used to examine the differences between healthy and unhealthy skin with healthy skin having denser and more connected networks than unhealthy skin. This is the first time that this innovative method has been used in a study to investigate product effects.

Unilever and their collaborators at the Chinese Academy of Science found that Single Sample Network Analysis showed statistically significant improvements in network connectivity and density of members of the skin microbiome for every subject in the study. The networks were also less fragile after five weeks of using the lotion, suggesting a more robust and interconnected microbiome – something that is important for maintaining optimal skin health.

Conclusion

The lotion was able to improve signs of dry skin when applied for five weeks. This was achieved through improvements in skin barrier, lipids, and ceramides. The team’s study therefore provides further evidence that maintaining good skin barrier function and ensuring good hydration of the stratum corneum is important for skin health.

Together with these changes in skin, improvements were also seen in the microbiome, thus reinforcing the evidence of the importance of both skin composition and its microbiome in order to provide us with healthy skin. The skin microbiome is a complex and delicately balanced ecosystem. Understanding more about changes in the microbiome associated with cosmetic dry skin – and how its balance can be restored – is a vital consideration when developing products for cosmetic applications.