Skin flora is another important player in human health and disease prevention. Being exposed to the outside environment and in close relation with the inside environment, skin microflora interact with other microbes, and with skin human cells including immune cell system. The skin is the largest human organ and even larger if you consider that total number of bacteria on an average human skin has been estimated at 1012, and that up to 1 billion bacteria inhabit a typical square centimeter of skin (1). The healthy human skin microbiome corresponds to approximately 1000 bacterial species that represent 2 million genes (2). A common inter-individual microbiome has been defined (2). Four major phylum of bacteria have been identified as part of this microbiome: Bacteroidetes, Firmicutes, Proteobacteria and Actinobacteria. Based on 16S sequencing analysis from the inner elbow of 5 healthy human subjects, 113 phylotypes belonging to these four bacterial phyla have been identified, and in this analysis, proteobacteria was the dominate species. Bacterial communities and density greatly depend on the body site where they reside and condition of the skin (1, 2, 3), it is therefore important to consider these variability to investigate skin microbiota researches and any skin health disorders. Skin flora (including scalp flora) contribute to the equilibrium of the skin, and desequilibrium can result in disease (e.g., dermatitis, eczema, psoriasis, acne and dandruff) (1-4). Skin flora disequilibrium can be therefore a clinical sign for skin diseases (the NIH HMP working group et al, 2009, Genome research). The skin can be exposed to abiotic stressors (pollution, smoke, UV…) and exogenous pathogens. The flora defend against these insults through competitive exclusion and biological reactions (co-aggregation, biosurfactant expression, bacteriocin production, defensins…) that can trigger activation of the immune system (release of immune mediators) (5, 6). Several studies have reported a link between the gut microflora and skin microbiota, related to the release of immune mediators in systemic circulation (2, 7). A recent interesting functional study of skin microflora (high throughput sequencing) has provided access to the major functions carried out by dominant skin colonized taxa including Corynebacterium, Staphylococcus and Propionibacterium (8). Furthermore, parallels found between the complexity and identity of mouse skin microbiota and human skin flora reinforces the interest of preclinic studies related to skin flora.
Microbiota as a potential targets for promoting skin health
Probiotics & skin flora. We have chosen to expand into this interesting field, which has opened up innovative research avenues in cosmetic & neutraceutic ingredients. Beyond their potential benefit in the intestinal microbiota realm, it is thought that both oral probiotic’s ingestion and topical application of ‘bacterial extract could i) improve barrier function ii) help to balance beneficial vs. harmful microflora iii) upregulate immune response to opportunistic pathogens iv) downregulate inflammatory reactions in response to allergens v) modulate host gene expression to promote function and delivery of functional proteins to prevent skin diseases (see below the interview with A. Gueniche). In particular, skin microbes have been shown to promote photoprotection (9 and references therein), modulate cutaneous inflammation related to atopic dermatitis (10-13), improve skin barrier and reactive skin (14), and activate antioxidant and antimicrobial defenses (15). Specifically, Vitreoscilla filiformis extract (a hydrothermal species) and Bifidobacterium longum extract topically applied were shown to be beneficial on atopic dermatitis (AD) and reactive skin, possibly by stimulating antimicrobial peptide (e.g., organic acids or bacteriocins) and immunomodulatory pathways (e.g., IL10, TGF) (10-15). The probiotic strain Lactobacillus salivarius LS01 (DSM 22775), taken orally, has been shown to modulate cytokine secretion from type 1 and 2 helper cells and could be considered as an important adjunctive therapy in the treatment of adult AD (16). Lactobacillus paracasei ST11 has been reported to improve skin resistance to physical and chemical aggression by reducing inflammation. L. johnsonii was also shown to modulate cutaneous immune function altered by UV exposure (9). Moreover, a recent study has demonstrated some probiotic effect of Lactobacillus paracasei ST11 in reducing scalp disorders (e.g., dandruff) (SFA Journey on Microbiota, ‘Le microbiote la medicine de demain’). [...]
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