The scalp, like other areas of the human body, has its own unique microbiome and plays a role in maintaining our scalp’s health but… what exactly lives there?
The main bacterial species that have been identified on the healthy scalp are Cutibacterium acnes and Staphylococcus epidermidis, though many others are present. Both C. acnes and S. epidermidis are members of the healthy microbiome and can be both commensal (healthy) and pathogenic (harmful) – as in acne vulgaris or “acne” (1, 2).
More well-known as being present on the scalp are Malassezia fungal (yeast) species which have received considerable attention for their potential role in the development of dandruff. The cause of social or self-esteem problems for many, dandruff is a skin condition affecting the scalp (primarily, but not exclusively) of approximately 50% of the adult population worldwide, the main symptoms of which are flaking, scaling, and mild itchiness (3). A more advanced and inflammatory condition which shares a common origin with dandruff is known as seborrheic dermatitis – these terms are sometimes used interchangeably (4).
The development of both dandruff and seborrheic dermatitis depend on sebum (an oily substance produced by our sebaceous glands), metabolism of Malassezia fungi, and individual susceptibility. The sequence of events leading to each condition is understood to occur in four phases (2, 4):
- Human scalp Malassezia fungi interact with the outermost layer of the skin – the epidermis
- Inflammation causes symptoms such as itching and redness of the skin (erythema)
- Proliferation (rapid cell multiplication) and differentiation in the epidermis are disrupted
- As a result, the skin’s barrier function is disrupted, negatively affecting hydration and the antimicrobial layer
While neither condition is exclusive to this area, the scalp is well suited to the growth of Malassezia fungi as sebum provides a nutrient source for the Malassezia, which require fatty acids for growth triggering the cascade of events outlined above. Patients with dandruff have been shown to secrete greater amounts of sebum than those with a healthy scalp.
The role of these fungi has been indicated beyond just this association, as lipase enzymes produced by Malassezia break down sebum into by products – including oleic acid. Around half of the world’s population are sensitive to the presence of oleic acid, which causes accelerated skin cell renewal leading to the ‘shedding’ dandruff sufferers experience (5).
The predominant Malassezia species present on the healthy scalp are M. restricta and M. globosa. While an imbalance of the microbiome (dysbiosis) is associated with the onset of dandruff, this is attributed in particular to an increased ratio of M. restricta to M. globosa. Indeed, a lower M. restricta to M. globosa ratio is characteristic of a healthy scalp (2, 5).
Let’s take a closer look at a few of our scalp’s top residents…
Malassezia restricta
The fungus Malassezia restricta is normally associated with scalp dandruff but provides a fascinating example of how genomics – the study of an organism’s complete set of DNA – helps to explain microbiome dysbiosis.
Studies on the gut microbiome have linked a defect in a human gene known to be important for immunity to fungi, to higher levels of M. restricta on the gut surface. Researchers speculate that this genetic weakness allows the growth of M. restricta beyond normal levels, causing dysbiosis in the gut microbiome. It is this dysbiosis that causes susceptibility to developing chronic stomach conditions, such as Crohn’s disease, in these individuals. However, the complexities and interconnections do not stop there (6).
Separate research shows how groups of dandruff sufferers have higher ratios of Malassezia to Cutibacterium (as in C. acnes), and Cutibacterium to Staphylococcus species on their scalps (2). As with the gut, it is this microbial imbalance of greater M. restricta that leads to the manifestation of dandruff. It is important to note here that unlike the more harmful strain of C. acnes isolated from acne lesions, M. restricta may not be a pathogenic microbe. While imbalance of this microbe can lead to skin conditions, many healthy people have M. restricta living harmlessly in both their gut and their scalp microbiome so it may play an essential role in the overall health of the local microbiome or simply be benign.
Malassezia globosa
The other most abundant fungus in both the healthy and dandruff scalp is Malassezia globosa which, with M. restricta, constitutes the fungal core of the scalp though other Malassezia species are present in differing proportions.
In contrast to M. restricta, the abundance of M. globosa is much higher in the healthy scalp and is reduced where dandruff occurs. Thus, it negatively correlates with M. restricta.
Sequencing of this fungus’ complete set of genetic material (its genome) revealed it has the smallest genome size of any free-living fungus – potentially an adaption to the limited niche in which this fungus lives i.e., our skin and that of other warm-blooded vertebrates (7)!
M. globosa lacks certain enzymes and secretes high levels of lipase enzymes that break down sebum. . Further, the particular combination of enzymes secreted may be such to enable these fungi to survive and thrive on human skin.
Cutibacterium bacteria
While Malassezia fungi claim the spotlight when it comes to dandruff, an interesting observation between Cutibacterium and scalp health has been made.
The metabolism pathway of one of our B vitamins – biotin – was found to be enriched in the healthy scalp compared to the scalp of dandruff sufferers. Biotin is an essential scalp nutrient for hair growth and scalp health, with its function being the production of the protein keratin which is essential for healthy hair growth.
How does this relate to Cutibacterium? Well, in addition to being the most abundant bacterial species in a healthy scalp microbiome, Cutibacterium species also carry the genes for biotin synthesis and transport – suggesting it plays a key role in providing the scalp with biotin (2). We ourselves cannot synthesise biotin so need to get it from other sources – such as microbes (including but not limited to Cutibacterium bacteria).
Treatment
With dysbiosis of the scalp microbiome driving dandruff symptoms, it makes sense that rebalancing the microbiome may alleviate symptoms and provide a target for treatments.
Indeed, findings echoed by multiple studies undertaken in different geographies, suggest that the antagonistic relationship between M. restricta and M. globosa fungal species could be used as a biomarker to help diagnose and prevent dandruff.
Dandruff is typically treated with antifungal therapies which aim to reduce barrier impairment caused by Malassezia fungi and, therefore, primary symptoms of dandruff – such as flaking. Currently available antidandruff therapies largely consist of incorporating active ingredients into shampoos to reduce fungal growth.
Many ingredients have been identified as having potential antifungal qualities but their efficacy within the shampoo format have been called into question. One recent study examined the effectiveness of various identified ingredients in shampoo treatments.
The results showed previous studies on some ingredients had not taken into consideration the shampoo format including dilution by water when used in the real-life setting. However, they validated the use of zinc pyrithione as an antidandruff treatment due to its ability to inhibit Malassezia growth on the scalp when used in a shampoo (8).
While there are many additional microbial species present in the local scalp microbiome, it is clear that the Malassezia fungal species are the most clinically relevant for their role in dandruff.
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Reference
- The diversity and abundance of fungi and bacteria on the healthy and dandruff affected human scalp, Grimshaw et al, 2019
- Comparison of Healthy and Dandruff Scalp Microbiome Reveals the Role of Commensals in Scalp Health, Saxena et al, 2018
- Seborrheic Dermatitis and Dandruff: A Comprehensive Review, Borda and Wikramanayake, 2015
- A Comprehensive Pathophysiology of Dandruff and Seborrheic Dermatitis – Towards a More Precise Definition of Scalp Health, Schwartz et al., 2013
- Genotype Analyses of Human Commensal Scalp Fungi, Malassezia globosa, and Malassezia restricta on the Scalps of Patients with Dandruff and Healthy Subjects, Hiruma et al, 2014
- The microbiome and immunodeficiencies: Lessons from rare diseases, Pellicciotta et al, 2019
- Malassezia Fungi Are Specialized to Live on Skin and Associated with Dandruff, Eczema, and Other Skin Diseases, Saunders et al, 2012
- Effect of zinc pyrithione shampoo treatment on skin commensal Malassezia, Leong et al, 2020
