The effect of probiotics and prebiotics on human disease and health is a current issue that has attracted significant interest in the industrial and scientific community.
The progression of Next Generation Sequencing (NGS) which, among other interesting developments, enables the analysis of the complete microbial community (the microbiome) of any given setting, has provided an additional technique for analyzing how probiotics and prebiotics function.
Scientists are just beginning to discover the impact of pre/probiotics on both gut health and general wellbeing. Novel and exciting research has confirmed the gut-brain-skin axis hypothesis: a concept that suggests the importance of the gut microbiome plays a crucial role in the regulation of skin inflammation induced by stress and the skin homeostasis.
Comparatively less research has focused on the analysis of topical prebiotics. This article aims to shed some light on the issue in relation to Labskin’s experience and the available literature.
Figure 1. Probiotic Lactobacilli Bacteria.
The article will begin by explaining what probiotics and prebiotics are. Probiotics are microorganisms that function as part of the commensal microbiota of the human body and are known to be advantageous to health. Notable examples of these are gut bacteria, for example are Bifidobacteria and Lactobacilli.
These microorganisms live within the intestines and ferment nutrients that the human body is largely unable to digest. Further nutrients are produced during this fermentation process which are beneficial to health, regulate natural defenses such as the immune system, and shield the body from pathogenic microbes.
There are also prebiotics present, which are inherently connected to probiotics as they are nutrients, cannot be digested by humans, and facilitate the helpful bacteria that allow the human organism to function while inhibiting the development of pathogenic microbes. In a simple analogy, probiotics are the machine whereas prebiotics are the ghost in the machine.
Natural compounds found in the human diet, like Inulin type fructans, β-Galacto-oligosaccharides, and α-Oligoglucans have been shown to exhibit distinct prebiotic effects on beneficial bacterial groups in the gut, for example Bacteroides, Lactobacilli, and Bifidobacteria. The specificity of these compounds is due to the ability of these groups of bacteria to digest sugars and obtain energy from them.
Enzymes codified in their genomes disintegrate the oligosaccharides which enables the microbes to feed on this source of energy, which confer an advantage against other gut dwellers that do not include these enzymes.
Microorganisms that can feed on Inulin and additional fructans contain enzymes known as β-fructofuranosidases (or fructanases). To digest β-Galacto-oligosaccharides they will contain β-Galactosidases and to feed on α-Oligoglucans, they will have α-glucosidases (α-amylases) that can disintegrate the α1→6 bond between glucans.
This process occurs in the very specific environment of the bowel. While this environment is diverse and variable, it has some well-defined features, for example it is nutrient-rich, has a high water content, is predominantly neutral in pH, and is largely an anaerobic niche.
Skin Could be Considered a Desert When Compared to the Gut
Compared to the bowel, the skin can be likened to a desert. The environment on human skin is less rich, much drier, and is generally more acidic (with a pH ranging between 4-4.5) compared to the gut.
The skin’s environment is also considered to be predominantly aerobic. More environmental complexity can be identified on the skin as it is the largest organ of the human body which means that highly distinct niches can be found.
Broadly speaking, three main types of environmental conditions can be discovered in human skin. Hands and arms represent a dry environment, feet, umbilicus, inguinal crease, and axilla represent wet environments, and oily environments can be found on the back and face, among others.
All three can be classed as significantly aerobic, but pockets of anaerobiosis will be found in all of them, normally within hair follicles and sebaceous glands.
When compared to the gut environment, the amount of water and nutrients required for the growth of microbial communities is very limited and the skin does not possess a source of oligosaccharides similar to the one provided in the diet.
As such, it is logical that the most widespread groups of bacteria located on human skin, Cutibacteria, Corynebacteria, and Staphylococci, do not possess genes that codify for the enzymes required to digest those prebiotics.
This shows that natural selection and evolution have been effective. Searches in multiple protein and gene databases return no results for available β-Galactosidase or fructanases in those three microbial groups.
All of them appear to contain α-amylases (α 1→4), but only Cutibacterium acnes contains a gene which codifies for an α-amylase (α1→6). In Labskin’s experience, C. acnes does not have the ability to grow with α-Oligoglucans as they provide its sole carbon source.
Could Oligosaccharides be Used as Topical Prebiotics?
The question which then arises is whether oligosaccharides can be employed as topical prebiotics. They do not appear to have a prebiotic effect in Staphylococcus epidermidis, which is the most well-known beneficial commensal on human skin. Oligosaccharides do not support the development of other primary dwellers of the skin.
It is beyond the scope of this article to discuss whether Cutibacteria and Corynebacteria should be considered harmful or beneficial, but it is important to note that there is a significant body of evidence that confirms the presence of C. acnes pathogenicity at the strain level.
Recent studies have demonstrated that the skin of people with Psoriasis and Atopic Dermatitis contains a smaller number of Lactobacilli in general, and Lactobacillus iners more specifically. It could be argued that oligosaccharides can be effective for alleviating these skin conditions, but this is a hypothesis that requires further investigation.
What about essential oils like Cannabidiol, Camphor, Thymol and Carvacrol?
This leads researchers to ask if there are any alternatives. Viewing this challenge from an evolutionary and ecological perspective, it may be useful to identify which nutrients available in the skin can function as prebiotics, and which natural compounds from alternative sources can have an equivalent prebiotic effect.
Certain products on the market provide prebiotics that contain lipids such as squalene, which is a lipid that is naturally created by sebaceous glands, but some studies suggest that squalene peroxidation may play a role in triggering inflammation in Acne vulgaris.
Oat proteins have also been included in formulations that claim to have a prebiotic effect, but there is negligible evidence to support this. Considering essential oils, a number of these are included in cosmetics and have been proven to have an antimicrobial effect in various microorganisms from distant groups.
Camphor, cannabidiol, carvacrol, and thymol are all chemical compounds originating from plant-based essential oils. They exhibit antibacterial effects against both Gram- and Gram+ bacteria. As such, certain essential oils will not function as prebiotics.
In summary, there are several possibilities that are being investigated in the field of topical prebiotics, but knowledge continues to be very limited in this area. Before prebiotic claims can be supported, further research will be required to gain evidence. Reproducible models to analyze the microbiome/host interactions at the skin site, such as Labskin, will be fundamental to achieve this task.
Produced from materials originally authored by Dr. David Caballero-Lima from Labskin.
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