The scalp, despite being an often-neglected area of beauty and personal care, is essential for our health and wellbeing. The condition of the scalp determines the state of the skin in this area as well as our hair, with a healthy environment preventing irritation and disease and promoting optimal hair growth. However, as well as the scalp itself, it is also important to consider the scalp microbiome.
The role of the microbiome in scalp health, and the importance of keeping it balanced, was first recognized by a study investigating the hair follicles of the scalp area in the 1960s. The research identified that, “In the soil the densest populations of microorganisms are in the rhizosphere, the region that surrounds plant roots. The comparable region in the skin is the hair follicle” .
Over years of research, it has come to light that the scalp microbiome has a unique composition of bacteria and yeast which are integral to its health and function. In this blog, we take a look at scalp microbiome composition and how its dynamics influence the skin and hair, taking its role in dandruff as a key example.
Scalp microbiome composition
The scalp microbiome is generally less diverse relative to other areas of the body, which tend to have a bigger mix of microbial residents. It is dominated by two key types of bacterial species – Cutibacterium acnes and Staphylococcus epidermidis – both of which are normal residents of healthy skin but can have negative consequences under certain conditions .
Malassezia, a type of yeast, is another key resident of our scalp microbiome. Malassezia are attracted to our scalp as its sebaceous glands, which are present in high concentrations around our hair follicles, produce high levels of lipids that act as a source of food for the yeast. There are many different kinds of Malassezia yeast, but two key species that reside on our scalp are known as Malassezia restricta and Malassezia globosa – each of which are found in varying abundances in populations from different countries .
The role of the scalp microbiome in scalp and hair health
Research investigating scalp microbiome composition matters, as it can inform on important aspects of our health. As well as providing the first line of defense against harmful pathogens, these key microbial residents also play a defining role in the maintenance of healthy skin as well as hair growth. This is put at risk when an imbalance – also known as microbial dysbiosis – occurs due to the dense environment of the scalp microbiome and competition between its microbial residents [4,5,6].
Signs of an unbalanced scalp microbiome include flaking skin, itching and irritation. When severe, microbial imbalances can also lead to scalp skin disorders, such as dandruff, seborrheic dermatitis and tinea capitis – all of which have been shown to be associated with altered bacterial and fungal scalp composition [4,7].
Another under-researched field in the microbiome world is the impact of skin microbes on hair growth. The physical proximity of certain microbes to the hair root and associated glands suggests that these microorganisms may potentially influence hair growth, balding and sebaceous lipid production, potentially leading to inhibition of growth and dry, brittle strands, with microbial dysbiosis also shown to contribute to conditions such as androgenetic alopecia .
As for the direct cause of the scalp and hair conditions that can arise, Malassezia yeast have been widely implicated . However, a causative link between the yeast and development of such conditions has not yet been established, and more recent work has suggested that it may be an overall microbial imbalance rather than the direct effect of Malassezia or any other individual microbial resident. Here we will explore this in more detail, using dandruff to illustrate.
The role of the scalp microbiome in dandruff
Dandruff is a skin condition affecting the scalp of up to half of the world’s population . The condition is characterized by an itchy and flaky scalp and can cause significant psychological distress. Dandruff is generally associated with colonization of the skin by Malassezia and it often develops during puberty when a hormonal-driven increase in sebaceous lipid production occurs, providing additional fuel for the yeast .
Now, for the science behind the link. It has been a running theory that Malassezia yeast secrete several fat-digesting enzymes onto the skin of its host, which break up the lipids secreted by our sebaceous glands to release ‘free’ fatty acids . Only some of these fatty acids will be consumed by Malassezia. The unconsumed fatty acids are left behind and are thought to increase proliferation of keratinocytes – a type of skin cell located in the upper layers of our skin – and consequently cause dandruff-like flaking of human scalp skin  (7).
Malassezia yeast also produce additional inflammatory compounds while processing the sebum-derived lipids they use as food [13,14], further worsening the skin’s condition. Certain species have also demonstrated toxicity to skin cells, again suggesting an active role in the acceleration of dandruff .
However, more recently, research has suggested that Malassezia yeast alone are not the direct cause of dandruff. Instead, it has been proposed that the association between the yeast and dandruff could instead be an indication of microbial dysbiosis and that it is this overall imbalance that should be defined as the cause.
In support of this view, the yeasts M. restricta and M. globosa, as well as the bacteria C. acnes and S. epidermidis, have all been shown to be present in high abundance in normal, healthy scalps . Also suggesting that the cause of dandruff is not Malassezia or any other microbial resident alone, an increased M. restricta:M. globosa ratio and a reduction in the Cutibacterium:Staphylococcus ratio has been found in the setting of dandruff , as well as increased abundance of S. epidermidis bacteria on scalps suffering from dandruff  – highlighting again that it is rather a complex interplay between different members of the skin microbiome, rather than single species, that causes scalp conditions.
So, although Malassezia has been shown previously in several publications to be associated with dandruff, these controversial research findings highlight that the causative link between Malassezia and dandruff is missing and that other pathways are also involved.
Gaining a better understanding of these scalp microbiome dynamics is key, as this will have knock-on effects for management and treatment. In the case of dandruff, management is typically via antifungal therapies . However, as the precise roles of microbes in the aggravation of the condition are incompletely characterized, this may not be optimal. Although a reduction of the symptoms has been seen in alignment with a decrease in the proportion of Malassezia species on the scalp when using antifungal treatments, the improvement is usually observed when treatment is stopped, with the relapse restoring the initial symptoms – which may correlate with restoration of scalp microbiome balance.
Managing the scalp microbiome to prevent and treat scalp conditions
Although antifungal treatments may not be optimal for scalp conditions as the dynamics at play are still not fully understood, day-to-day management can help support the scalp microbiome.
One interesting area of beauty and personal care is the repurposing of common skin ingredients that can also help support the scalp and hair. Alpha-hydroxy acids (AHAs) and beta-hydroxy acids (BHAs), chemical exfoliants used often in skincare, can help clarify the scalp from product build-up, excess oil, dead skin and environmental irritants (such as pollen, dirt, pollutants), creating a healthier and more stable environment for the scalp microbiome [18,19]. Hyaluronic acid, which has been rising in popularity due to its benefits in facial care, has also been shown to preserve moisture levels and hydration on the scalp, combatting dry and irritated skin and helping inflammatory conditions associated with an imbalanced scalp .
Natural and gentle ingredients with antifungal properties, such as coconut oil, neem oil and tea tree oil, can also help support the scalp and maintain a healthy balance of bacteria and yeast when included in shampoos and other products, with these products shown to decrease dandruff and other symptoms of scalp infection .
Coconut oil and its formulations, for example, are commonly applied on the scalp in several parts of the world, including Asia and Africa, to support scalp health. In a recent study investigating the effect of coconut oil on the scalp microbiome, treatment with the oil increased the abundance of C. acnes and M. globosa. This is not surprising, as both species are lipophilic, which means that the fatty oil can be used by the microbes as a source of fuel. However, unexpectedly, the proportional increase of C. acnes and M. globosa was negatively correlated to dandruff parameters .
This work provides novel insights on the effect of coconut oil in maintaining a healthy scalp and in modulating the scalp microbiome. Such research also again suggests that it is the relative proportions and balance of microbes that influence scalp health and disease, rather than individual species, laying the way for future research into these dynamics and similar treatment approaches.
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