How to rebalance the skin microbiota after sun exposure?

The skin is inhabited by billions of diverse tiny microorganisms, including bacteria, fungi, and viruses. These microorganisms are crucial for maintaining skin health and form the skin microbiota. However, there are times when its balance is disrupted, and thus unbalanced: we call this situation a dysbiosis.

This disruption could potentially be caused by the sun, according to recent studies. Indeed, the UV rays from the sun are believed to be capable of both reducing and increasing the proportion of bacteria in the skin microbiota. These changes can be harmful to the skin, leading to conditions and inflammations.

If your skin microbiota becomes unbalanced due to prolonged exposure to the sun, here are the solutions available to you to restore the normal balance.

Method No.1: Bacteriotherapy.

Cutaneous bacteriotherapy involves placing one or more pure cultures with beneficial health properties onto a person's skin area. The applied microbiota can be:

• Probiotics.

  The probiotics have the ability to optimise, maintain and restore the skin's microbiota. Topical applications of probiotics have been reported to induce natural defence mechanisms. This boosting of the skin's natural defences is due to competition with pathogens for nutrients, modulation of mucosal immune functions, and the production of anti-microbial metabolites, such as bacteriocins. All these mechanisms can contribute to the rebalancing of the skin microbiota impacted by the sun.

• Postbiotics.

  Postbiotics are compounds that can be heat-killed or physically killed bacteria, or even cellular lysates. Bacterial cellular structures, enzymes, and excreted bacterial factors are added, but the bacteria no longer replicate. However, the enzymes can still be active, which could aid in rebalancing the microbiota.

• Fermentation products or supernatants.

  Bacteria are not added, but the supernatants containing their antioxidants, amino acids, lipids or vitamins are added.

It is possible to apply highly concentrated live bacteria (probiotics), which allows for significant efficacy. Probiotics or postbiotics can be applied in a skin emollient, cream, or a medium suitable for the skin.

However, the use of pro- and postbiotics also presents a series of drawbacks. Bacteria are cultivated in sugar-rich environments; therefore, it may be more challenging for the bacteria to adapt to a sebum-rich environment. Moreover, the application of large quantities of bacteria could trigger an immune response from the skin, resulting in irritations and side effects.

Thus, many cosmetic products claiming to contain probiotics actually only contain "dormant" probiotics, which are metabolically active but unable to grow. These molecules retain their activity and can be beneficial for the skin, posing no danger when applied topically.

Method No. 2: The transplantation of a skin microbiome.

During a skin microbiome transplantation, the skin microbiome of a healthy individual is transferred into the skin of another person with the aim of improving their condition. The benefit lies in the fact that the microbiome is transferred into its natural environment.

For instance, S. epidermidisproduces anti-microbial peptides that can benefit the skin's immune defence systems by selectively inhibiting S. aureus, a pathogenic bacteria, whilst maintaining a normal skin microbiota.

Despite its simplicity, this approach has several drawbacks. Only a few bacteria can be sampled from an individual's skin. It is essential to perform a culture step to obtain adequate quantities of bacteria. Lastly, it is also possible to transmit potential pathogenic taxa.

Method No. 3: Stimulation through prebiotics.

In this process, prebiotics are added to the skin to stimulate the growth of specific beneficial microbes. A prebiotic is an ingredient that promotes the growth of desirable organisms (probiotics). Examples include fructo-oligosaccharides, glucomannan oligosaccharides like those from konjac, andinulin.

In cosmetic formulations, prebiotics can be applied to the skin microbiota, where carbohydrates selectively stimulate the activity and growth of beneficial microorganisms. It has been observed that prebiotics boost the immune system in vitro and in vivo, and promote the growth of probiotics at the expense of pathogens, such as Salmonella typhimurium and Clostridium perfringens.

This method offers several advantages. There is no need to work with live bacteria, which reduces the risk of skin immune reactions. The mechanism of action of this method is indirect. Prebiotics are generally well-defined compounds whose side effects have been thoroughly studied.

This method also has its drawbacks. The approach is indirect, hence the results may be less straightforward than when using probiotics. Prebiotics can also stimulate non-targeted bacteria that are present in low abundance. The effect of prebiotics can be unpredictable due to the variability of the skin microbiome, physiology, and immune response among different individuals. Lastly, prebiotics may or may not be soluble in skincare products, which poses challenges.

However, it should be noted that no study has been conducted on the rebalancing of the skin microbiota specifically in the case of sun exposure.