The skin – the largest organ of the body – acts as the body’s primary protective barrier and first line of defense against foreign invaders.
Without a strong protective outer skin barrier, your body becomes more susceptible to harmful microorganisms (known as microbial pathogens) and can also be triggered to call for an inflammatory response, which can lead to other unwanted effects in the body if not kept in check.
But how does the skin microbiome fit into this?
Well, in addition to our body’s own intrinsic defences that protect us against infection from microbial pathogens and disease, our resident skin bacteria also play a key role by helping to maintain a stable environment and regulate our immune system. Here we will explore this delicate relationship, as well as what happens in the event of skin microbiome imbalance.
Skin microbiome and immune protection
A healthy, balanced and fully functioning skin microbiome contributes to the barrier function of the skin and helps to maintain a stable skin surface environment, which protects against pathogens and helps mediate environmental stress . The skin microbiome also communicates with the body’s immune cells to influence their behaviour, regulating immune responses and helping to keep your body stable and healthy .
Through these mechanisms, our microbiome provides support for both our innate (the non-specific defence system that you were born with, providing the first line of defence in our immune response) and adaptive (learned or acquired immunity, specific to foreign pathogens) immune responses – for example, by exerting targeted antimicrobial and anti-inflammatory activity.
Let’s delve into this symbiosis in a bit more detail…
Antimicrobial activity and defence against pathogens
First off, the very presence of our friendly, commensal skin microbes can help prevent the invasion of harmful pathogens by directly outcompeting and killing the threatening microbes. The human skin is home to distinct bacterial and fungal communities, and bacterial populations inhabiting the same areas on our skin compete for nutrients and other resources [3,4]. To do so, they use strategies such as antimicrobial production and secretion of digestive enzymes and other substances to suppress other rising bacterial species .
As well as outcompeting invading pathogens, these processes help maintain a stable environment on our skin and can influence local immune responses [2,4]. For example, antimicrobial peptides (AMPs) and cytokines – produced by skin cells (keratinocytes) and immune cells – form an important part of the innate immune system, as they provide a highly potent defence against a broad range of microbes . Bacteria found on our skin, such as Staphylococcus epidermidis, can enhance AMP production in keratinocytes [2,5] and trigger increased production of certain defensive cytokines [5,6]. Other Staphylococcus types have also been shown to inhibit toxin production in pathogenic bacteria [7,8].
Our inflammatory response
So, our body’s immune responses in the skin are critical for defence against pathogens. However, when thrown out of balance, dysregulated immune reactions can also have the opposite effect, leading to inflammation.
To manage this, the cells and microbes of the skin can communicate to regulate our immune system’s inflammatory response. So much so, that the skin microbiome is essential to ensure sufficient defence while also maintaining a stable and balanced environment, known as ‘immune homeostasis’, and preventing inflammatory disease [2,9].
Essentially, this all means that the presence of certain bacteria – such as S. epidermidis and Acinetobacter bacteria [2,10] – can help keep inflammation at bay, but if imbalances in our resident bacterial populations occur, an inflammatory response can be triggered.
The role of the microbiome in inflammatory disease
In line with this concept, an increasing number of microbes are classified as ‘pathobionts’ – that is, microbes that can cause harm under certain circumstances, even if ‘harmless’ under others . When the skin microbiome is disrupted in this way, this can lead to inflammation, irritation, dryness, itchy skin and worsen our skin condition, challenging the immune response . This may also contribute to the progression of inflammatory skin conditions such as acne, atopic dermatitis and psoriasis as a result of an unregulated inflammatory response [9,12,13].
Acne vulgaris (better known just as ‘acne’) is a highly prevalent inflammatory skin condition. Although several factors contribute to its development, such as excess sebum, a key part of acne development is the microbiome and its interactions with the innate immune system.
The bacteria Cutibacterium acnes play a key role in the acne development. Although some C. acnes bacteria can actually benefit your skin, following the concept of the pathobiont, a loss of microbial diversity and balance between certain types can trigger acne. Imbalances can act as a trigger for immune system activation, leading to skin inflammation. However, once microbiome diversity is restored to its ‘normal’ state, inflammation is suppressed through improved regulation of innate immunity .
Atopic dermatitis is another inflammatory skin condition where pathogenesis involves key interactions between immune responses and microbial communities . Again, multiplefactors contribute to its pathogenesis, such as skin barrier dysfunction, but also microbial dysbiosis and immune dysregulation, with the skin microbiome of atopic dermatitis patients showing considerable variability and various organisms associated with exacerbating or alleviating the skin inflammation [4,15].
In the case of atopic dermatitis, it is believed that lack of microbiome diversity during the early stages of life can affect the maturation of innate and adaptive immunity leading to chronic inflammation. Research has shown infants with atopic dermatitis to have low quantities of bacteria known as ‘Bifidobacterium’ and ‘Bacteroides’ [16,17].
Supporting your skin microbiome to keep you healthy
To help keep your immune system running smoothly, maintaining a diverse but balanced skin microbiome is key.
Age, gender, ethnicity and immune status are thought to be the most important factors that that affect each individual’s skin microbiome  but there are some day-to-day considerations that help provide balance and stability.
Hygiene and personal care routines
Daily hygiene is a key factor that can modify the skin microbiome.
Handwashing and disinfectants, for example, have been shown to be effective in reducing pathogens on the hand (see our previous article here). However, over-washing and excessive use of beauty and personal care items may negatively impact your skin microbiome by reducing the variety of microbes in our skin microbiome, or benefitting new and potentially harmful species [12,18].
Certain beauty products and cosmetic ingredients have also been shown to persist on the skin for weeks after use, altering the chemical environment where skin microbes live and promoting or inhibiting the growth of certain bacteria . For instance, the lipid components of moisturizers can provide nutrients and promote the growth of microbes such as Staphylococcus and Cutibacterium bacteria .
Maintaining a healthy skin pH
Our skin has a naturally acidic skin pH at 4.1–5.8  – and this should be considered when thinking about hygiene and personal care practices.
Research has demonstrated a link between abnormal pH and skin diseases, such as atopic dermatitis, acne vulgaris and xerosis (dry skin). Although the relationship is complex, research into our skin’s physiological make-up has shown that the acidic nature of the ‘stratum corneum’ (the outermost layer of our skin) can impact our skin’s integrity, antimicrobial defence mechanisms and the protective function of our skin barrier [20,21]
Cosmetic ingredients such as lactic acid, citric acid and sorbic acid that help maintain an acidic environment may also help support a healthy skin microbiome , which can subsequently provide immune-system-boosting properties to the skin as well as helping to maintain skin structure and function .
Maintaining a healthy gut
The gut appears to have a direct influence on the skin microbiome, via an intimate relationship known as the gut–skin axis, with the gut and its own microbiome now linked to skin health and homeostasis in numerous studies (read more here).
So, nourishing your gut with healthy foods such as whole foods, fibre and probiotics can help strengthen your skin. Short-chain fatty acids resulting from fibre fermentation in the gut, for example, can promote the growth of skin microbes that influence immune defence and regulate skin inflammation .
Altering the gut microbiome to prevent and treat disease – via skin microbiome modulation – is an exciting area of research. Recent work has suggested that probiotic supplementation presents promising potential in the role of prevention and management of various skin disorders such as acne. For instance, probiotics can suppress acne-causing C. acnes bacteria by secreting an antibacterial protein, which inhibits the growth of the threatening strains .
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