The skin is the largest organ of the human body and is home to a diverse array of microorganisms which can influence health. These are understood to play a significant role in the hosts immune reactions, wound healing, colonisation of pathogens and skin disorders. A great deal is still to be discovered on how these resident microbes influence health both at the site of where they reside but also the pathways to systemic health which are influenced by these skin microorganisms.1 The information we know so far is what we will look to unpack throughout this blog.
Microbiology and dermatology investigators are using pyrosequencing technology (DNA sequencing) to identify and characterize these microorganisms on the skin to understand these resident microbes and how their presence contributes to skin health and how they can modulate skin disorders.2 The skin serves as a barrier to a symbiotic relationship between host tissue and the communities of microbes which inhabit the epidermal and deeper layers of our skin, these include bacteria, viruses, and fungi. Grice et al. characterized four main phyla: Actinobacteria, Firmicutes, Proteobacteria and Bacteroides the three most common genera were as follows: Corynebacteria, Propionibacteria and Staphylococci.3
There is also a diversity to these colonies of microbes depending on the environment in which they reside. For example, certain areas of the skin are drier whereas some produce more sebum and others very little.4 Different bacteria tend to have a propensity for specific environments. Sweat glands, sebaceous glands and hair follicles are home to their own unique bacterial ecosystem.5
This relationship is modulated through signals from both the adaptive and innate immune systems and is under a constant barrage from endogenous and exogenous factors, such as environmental toxins, cosmetics, medications, allergens, pathogens, and injuries.6 These can cause a dysbiosis of the skin microbiome which is evidenced by Toll-like receptors (TLR) being released by our immune cells, these are an initial response mechanism of our immune system, to foreign invaders. This response also involves the release of antimicrobial products by keratinocytes.7.
The skin has 6 main functions. These are barrier protection, thermoregulation, sensation, excretion, absorption, and Vitamin D production. The protection is provided as a physical barrier but also from the properties of the microbiome. Maintaining the health of the microbiome ensures it serves the following functions:
- Prevents the colonisation of opportunistic pathogens.
- Maintains the optimal pH, this being relatively acidic to inhibit the growth of pathogenic bacteria
- Works with the immune system to release antimicrobial agents when needed, helping to manage inflammation.
- Reduces the exposure to harmful amount of UV radiation and endogenous substances.
- Assists in the wound healing process.
- Helps to keep the skin barrier intact and hydrated.8, 9
The skin’s thermoregulatory properties protect the body from cold or heat to maintain a core body temperature. This is achieved by our blood vessels responding to our environment through either vasodilation during cold periods or vasoconstriction during warm periods. Vasodilation is when our blood vessels dilate causing a reddening of the skin from increased blood flow and reducing the body’s temperature. Whereas vasoconstriction the vessels constrict reducing blood to retain the body’s heat.
The secretion and evaporation of sweat from the surface of the skin also helps to cool the body.
The skin is our first line to stimulation from our surroundings due to its sense of touch. This triggers a response when we touch or feel something, including things that may cause pain. This is important for patients with a skin condition, as pain and itching can be extreme in these situations and cause a great deal of distress.
Our skin is where we produce our Vitamin D. Once sunlight hits the skin, we synthesise cholecalciferol through our skin from a derivative of the steroid hormone cholesterol. This is then converted into its active form through the liver and kidneys. The skin also contains receptors for other steroid hormones (oestrogens, progestogens, and glucocorticoids) and for vitamin A.10
Skin microbiome through the ages
Skin microbes accompany us from infancy to old age. An infant’s skin is colonized by microorganisms when they are born, and this colonisation of the skin continues as the baby explores their environment.11 The ‘Hygiene Hypothesis’ proposed by Prof. David P. Strachan said that a lack of childhood exposure to infectious agents, symbiotic microorganisms and parasites increase the susceptibility of developing allergies and intolerances as it suppresses the natural development of the immune system. This being the most opportune time to educate the immune system.12
When children hit puberty hormone levels increase as does the production of sebum by sebaceous glands, these are the conditions that favour ‘lipid-loving’ microorganisms such a Propionibacterium acne. This natural development and change to microbial composition may cause or aggravate skin diseases such as acne vulgaris which affects almost 90% of teenagers and 11% of adults. Antibiotic treatment is often used to target P.acnes however researchers have suggested that not all strains of this microorganism have a negative effect. In fact, inhibiting P. acnes can do more harm than good. This idea is supported by research that suggests that P.acnes, fermented with glycerol could serve as a treatment to the skin by inhibiting the growth of MRSA and helping to manage the inflammatory response associated with skin issues such as eczema.13, 38
When teenagers become adults their skin microbiome reaches a certain equilibrium. However, this is the time when people must cope with mounting stress in life. Perceived levels of stress by the brain have a remarkable impact on skin conditions This is largely due to an increase in production of steroid hormones when adults’ resilience and ability to adapt to stress is exceeded.14, 15
The skin is the largest neuroendocrine organ with its own peripheral HPA to produce cortisol in response to psychological stressors. Cortisol is known to impair the skin barrier function.16
As we get older the skin microbiome diversity continues to change, with a study suggesting that older skin gives rise to the Corynebacterium and a decrease in Propionibacterium, the p. acne strain being the widely known dominant bacteria in acne sufferers explaining the predominance of this bacterium on the skin of younger individuals17.
People of same chronological age may have different biological skin ages due to different lifestyles, diets, genetic factors, and cosmetic use.
Dysbiotic skin microbiome
A healthy skin barrier is necessary for the commensal bacteria S.epidermis to inhibit the growth of pathogenic Staphylococcus aureus and maintain homeostasis of the microbial inhabitants of the skin. The commensal bacteria secrete antimicrobial agents which work in tandem with our immune system to kill pathogens and prevent biofilm formation and colonization of pathogenic bacteria.18 When the integrity of the skin barrier is disrupted, S epidermis is unable to suppress the colonisation of S. aureus, causing a loss of diversity.
- aureus happily resides on the skin as a commensal bacterium and would only cause an issue if the skin integrity becomes compromised.
Further factors driving this skin condition in addition to the loss of bacterial diversity is a reduction in filaggrin an important protein necessary to maintain the integrity of the skin barrier, that if compromised will cause an immune system response and drive further inflammation.19, 20
Acne sufferers are found to have the colonisation of the bacteria P. acnes in pilosebaceous glands. What starts as a commensal bacterium important for maintaining healthy skin and preventing the colonisation of other pathogens, can become an opportunistic pathogen in acne vulgaris.21
However further research has questioned the belief that P.acnes is the causal factor in the development of acne. Instead highlighting the delicate equilibrium between the skin microflora and P.acnes phylotypes becoming imbalanced thereby causing a loss of microbial diversity. It is this loss of diversity that is considered a significant causal factor of chronic inflammatory skin diseases.22
A more appropriate explanation for P acnes’ part in this skin disease is its role in the inflammatory response coupled with the cutaneous microbiota and innate immune cells excessive response from keratocytes of the follicle glands, and the increase in sebum production.
Furthermore, other contributing factors also play a part in the severity, incidence and persistent of this condition, these include environmental factors, hormones, stress levels, family history and stress as previously touched on.23
Psoriasis as with all the previous skin conditions mentioned present with an altered skin microbiome to that of healthy subjects. There is evidence to suggest that psoriatic flares are strongly associated with an altered skin microbiome and diversity of the microbiome is a central factor in the aetiology of this condition.24
Again, it is an inflammatory condition and psoriatic flares, and the management of such episodes are dependent on the integrity of the skin barrier and immune system response. These are tightly controlled by the resident microflora so when an overabundance of microorganisms such as fungal flora (candida albicans) and the bacterial flora(Corynebacterium species) as seen in psoriasis the this can be compromised leading to an inflammatory response.25,26
It isn’t just an alteration to skin microflora contributing to the pathogenesis of psoriasis but also a dysbiotic flora in the gut, this together with a severely compromised intestinal lining triggers the local and systemic immune response driving the systemic inflammation27.
Nutrients to support the health of the skin and help encourage a balanced skin microbiota
Evidence has suggested that probiotics have shown to improve presentation and ameliorate symptoms of chronic skin conditions including AD and acne. Probiotic and/or prebiotic formulas have been taken orally with success for some time, but emerging evidence suggests that it is also beneficial to skin barrier integrity to apply topically. Their benefits to these skin pathologies are due to their potential modulatory properties to gut and skin microbiota through restoring balance to gut bacteria composition and improving gut lining integrity. As an imbalance to gut bacteria and a compromised gut lining can drive chronic inflammation exacerbating skin conditions.28, 29
Probiotics given by topical application, could alter the skin microbiota producing a positive change in some pathological conditions of immune skin response. Among the mechanisms involved would be improving the skin barrier and an increase in secondary production of antimicrobial peptides.30
There are increasing studies that demonstrate the relationship between some pathological conditions of the skin, such as the atopic dermatitis, psoriasis, acne, or rosacea and microbial ecological dysbiosis
The impact environmental factors such as climate, including temperature and UV exposure, but also lifestyle factors, including alcoholism or nutrition on microbial communities remains to be established. However certain evidence has highlighted that excessive alcohol consumption has shown to diminish host resistance and nutrient and vitamin deficiency has been shown to impact on the skin microbiota balance, resulting in infection and skin barrier disturbance.2, 31
Frequent washing has been reported to disturb the skin barrier resulting in skin irritation and in changes in the microbiome on hand skin. Cosmetics, hygiene products, makeup and moisturizers have also been implicated in modifying the skin microbiome 32
To help encourage a healthy and robust skin microbiome it is important to address the following where appropriate:
- Improve the diet focusing on wholefoods, healthy fats/omega 3 sources, protein, plenty of vegetables, fibre.
- Avoid overuse of hand sanitisers or antibacterial agents
- Choose natural fibres over synthetic ones.
- Gently dry skin and avoid over brushing so as not to disturb commensal skin microbes
- Keep hydrated.33, 34, 35.
Gut microbiota in skin condition pathologies
The health of the gut microbiome can be a key player in the pathogenesis of skin conditions both positively and negatively. If the intestinal lining becomes hyperpermeable due to factors such as poor diet, stress, or because of an existing health condition then this gives the opportunity for undigested foods and toxins to gain access to the bloodstream causing immune system activation and systemic inflammation. This can be the driver to skin conditions.36 Furthermore, new evidence suggests that a more direct link to dysregulation of skin homeostasis is caused by the metastasis of gut flora and its metabolites to the skin.37
Gut issues caused or driven by the above factors affect absorption of key nutrients including the malabsorption of carbohydrates, protein and fats, vitamin and minerals and essential fats necessary to maintain skin homeostasis. This malabsorption can impact on one’s psychological state and on oxidative stress.38
The gut microbiome appears to influence the skin microbiome as in a beneficial way. SCFAs (short chain fatty-acids) resulting from fibre fermentation in the gut – propionate, acetate, and butyrate – are believed to play a pivotal role in determining the predominance of certain skin microbiomic profiles which subsequently influence cutaneous immune defence mechanisms.
Propionibacterium, for example, is a genus capable of producing SCFAs, predominantly acetate and propionic acid. Propionic acid can exhibit a profound antimicrobial effect against USA300 which is a strain of the antibiotic resistant form of methicillin-resistant Staphylococcus aureus (MRSA) associated with the rapid progression of chronic diseases including conditions of the skin.39, 40.
- The skin microbiome works with the immune system to produce antimicrobials when necessary to help manage inflammation. A healthy skin microflora is needed to help maintain the integrity of the skin’s barrier.
- A dysregulated skin microbiota is seen in all chronic skin conditions.
- Excessive washing with detergents, particularly in infants at risk of eczema, impairs skin barrier function and alters skin microbial composition thereby increasing the risk of developing eczema and sensitisation. However, further studies are needed to establish how the microbial ecology of the skin becomes established and stabilizes over the first years of life, and how variations influence immune development and disease risk.
- The composition of the skin microbiome changes throughout life due to changes in hormonal levels and psychological stressors that increase as we age.
- Lifestyle and environmental factors are also a contributing factor to skin health. These exogenous and endogenous factors interact with the physical and functional aspects of the skin barrier through effects on both host cells and the skin microbiome to alter both the integrity and the activity of the skin whether it be hormonal, metabolic, or an immune response.
- The health of the gut microbiota can affect our skin microbiome through several potential pathways both positively and negatively. These include the migration of microbes and their metabolites to the skin disrupting the balance of the microflora, increasing the susceptibility of systemic inflammation ensuing due to a leaky gut lining allowing undigested food particles or toxins to gain access to the blood stream activating the immune system.
- SCFAs, produced from fibre fermentation can affect the composition of skin microflora to positively influence the skin’s immune response. Research suggests that by modulating gut flora with probiotics you can modulate systemic immune responses and improve skin pathologies.
- Initial research has also looked at the use of probiotic formulas as topical applications to relieve and help manage skin diseases.
If you have questions regarding the topics that have been raised, or any other health matters, please do contact our team of Nutritional Therapists.
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- Yousef, H., Alhajj, M., & Sharma, S. (2020). Anatomy, Skin (Integument), Epidermis. StatPearls Publishing.
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