The living protective shield of your skin
The skin is our body’s largest organ and primary physical barrier against pathogens and harmful substances. Remarkably, the skin is not just a passive shield; it is a thriving ecosystem colonized by a diverse array of living microorganisms, including bacteria, fungi, viruses, and mites1, collectively known as the skin microbiota. This invisible ecosystem of microorganisms is occupying a wide range of skin niches and safeguarding our skin from external aggressors, while also contributing to the production of essential nutrients necessary for skin health. They even play a pivotal role in educating and regulating our immune system to ensure overall human health2,3. However, disruptions in this balance can lead to pathological skin conditions like eczema, atopic dermatitis, psoriasis, and acne4–6. The delicate equilibrium of the skin microbiota can be disrupted by a multitude of factors, ranging from changing pH levels, exposure to environmental toxins or skincare routines. Understanding this intricate relationship is key to unlocking the secrets of skin health.
Pre-, pro- and postbiotics
In recent years, the use of prebiotics, probiotics, and postbiotics in dermatology and cosmetics has surged in popularity, offering potential benefits like strengthening the skin barrier, reducing inflammation, and improving pathological skin conditions. Incorporating these biotics into skincare products represents a promising avenue for improving skin health, but their precise mode of action when topically applied remains largely unknown.
Prebiotics: a substrate that is selectively utilized by host microorganisms conferring a health benefit.
Probiotics: live microorganisms that, when consumed or applied to the body in adequate amounts, confer a health benefit on the host.
Postbiotics: metabolites secreted by live bacteria, or released after bacterial lysis providing physiological benefits to the host.
- Inulin: a well-known prebiotic that supports the growth of beneficial bacteria. It is a polysaccharide that can only be utilized by bacteria to generate short-chain fatty acids, including lactic acid. Lactic acid, in turn, is commonly used in various skincare products as it keeps the skin hydrated and influences skin pH.
- Butyloctanol: a unique biotic that has demonstrated its ability to target undesirable bacteria more effectively than beneficial ones.
- Lactic acid and pyruvic acid: postbioticsthat have been shown to inhibit the growth of undesirable bacteria while promoting the growth of beneficial ones. They also play a role in enhancing the skin barrier. The precise workings of these topically applied ingredients in vivo are still a subject of ongoing research.
Going beyond microbial composition
The researchers set up a multi-omics approach to gain a comprehensive understanding of how the triple-biotic complex impacts the skin microbiome. Participants with dry skin were applying either a shower gel and body lotion containing the triple-biotic complex or a standard shower gel and body lotion for a duration of 6 weeks. Interestingly, skin hydration significantly increased in both groups. The holistic approach encompasses sequence-based bacterial analysis, metagenomics, and untargeted mass spectrometry-based metabolomics. Recent advancements have introduced MS-based metabolomics as a powerful tool to characterize the chemical composition of the skin surface and correlate it with microbial communities8. Interesting small molecules to investigate are nutrients shaping the microbial community, byproducts of host-microbe interactions9 and agents of microbial “warfare,” including microbially-produced antibiotics10.
Protect your skin from opportunistic pathogens
The skin’s distinctive attributes create perfect growth conditions for both resident (symbiotic) microorganisms and transient (opportunist pathogenic) ones11. Analyzing the microbial composition, the researchers found that the overall microbial composition was not significantly changed. However, there was a significant reduction of opportunistic pathogens like Pseudomonas stutzeri, which is known to be associated with skin infections12, and Sphingomonas anadarae when using the triple-biotic complex. In addition, the commensal bacteria Staphylococcus equorum, Streptococcus mitis, and Halomonas desiderata significantly increased, indicating a potentially positive impact on skin health.
Moving beyond microbial composition, they also analysed the effects on metabolic pathways. Using the triple-biotic complex, several pathways were significantly enriched, including amine and polyamine degradation, sugar and sugar acids degradation, and aromatic compound biosynthesis. In the control group, several pathways were significantly depleted, including fatty acid and lipid biosynthesis, which are critical for maintaining the skin’s barrier function13. This reduction could potentially have a negative impact on skin health.
From minor alterations in microbial composition to substantial shifts in the skin metabolome
The researchers used untargeted mass-spectrometry and feature detection with MZmine214. Features were annotated either utilizing spectral matching (GNPS and NIST) or employing SIRIUS+CSI:FingerID15,16. They detected 3,508 metabolic features, revealing significantly separated metabolic profiles between standard and triple-biotic treated skin. Interestingly, the changes in the skin metabolome appeared more pronounced compared to the microbiome data. The triple-biotic treated skin exhibited a more profound modulation of several clinically relevant metabolites known to benefit skin health. These included long-chain/medium-chain fatty acids, fatty acid esters, and fatty acyls, all of which are essential components of skin lipids contributing to skin barrier functions13,17. Additionally, dicarboxylic acids, known for their antimicrobial and anti-inflammatory properties18, were more prominently affected.
The discriminant metabolites were subjected to correlation analysis with microbial composition and functional pathways. While the separation between groups based on microbiome data appeared relatively weak, interesting correlations emerged. Several microbes, including the opportunistic pathogens mentioned above and multiple species from the same genera, exhibited significant correlation with clinically relevant metabolite features. For example, both bacteria were negatively correlated with fatty acids and dicarboxylic acids, which are positively associated with skin hydration improvement. On the other hand, the commensal bacteria showed a positive correlation with skin hydration.
“biotics” promote bacteria that produce hydration metabolites
Although the researchers observed only minor alterations in bacterial taxonomic composition, they found substantial shifts in the skin metabolome. These findings suggest that even subtle shifts in microbial abundance can lead to significant effects on the skin. They demonstrate the importance of using an holistic approach, including mass spectrometry based metabolomics, to gain insight into the underlying mechanisms of topical pre/postbiotics for skin health. Products containing pre/postbiotics appear to benefit skin health by influencing the skin’s microbial composition toward bacteria that produce metabolites linked to improved skin hydration. It’s worth noting that the increase in skin hydration observed in the control group might be attributed to other ingredients, such as humectants commonly found in skincare products. The identification of specific bacteria and metabolites associated with skin hydration opens the door to the potential use of these factors as targets in the development of novel topical treatments aimed at optimizing and maintaining skin health.
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