When it comes to good health, high microbial diversity in the gut is well-established as beneficial. But does the same principle apply to skin health or is there a more nuanced answer? With an increasing number of cosmetics products claiming to be microbiome-friendly, this is an important question with serious health implications.
There is a broadly held view that a reduction in microbial diversity precedes a microbial imbalance (dysbiosis). This phenomenon can be found throughout nature, where high biodiversity leads to healthy ecosystems. More specific to the skin microbiome, populations that are less exposed to Western practices and pollution are understood to have higher skin microbial diversity and be less prone to develop skin allergies and disorders (1).
However, in practice, measurements of skin health often reveal that it is not diversity per se that is the marker of wellbeing. Rather, a sudden change in skin or scalp microbiome diversity (either a decrease or an increase) is often associated with skin and scalp disorders.
That – perhaps counterintuitive – outcome raises a question: does the measure of microbial diversity actually matter when it comes to skin and scalp health? Or is it merely a statistical artifact, with little practical relevance?
Defining microbial diversity
Before we dive into this complex topic, we must understand how microbial diversity is defined. The skin microbiome is a diverse ecosystem that is composed of many different microbial species, including viruses, fungi, mites, and bacteria. The composition of this ecosystem is affected by many factors, including anatomical location, pH, moisture and sebum levels, the presence or absence of hair follicles, as well as age and genetic factors.
Importantly, diversity is not just a question of the amount of microbial species present, but species diversity. Species diversity is defined by two main components – species richness (the total number of individual microbes within that community) and species composition (the total number of different species present, i.e. types of viruses, fungi, mites and bacteria). In other words, an ecosystem rich in species diversity has both a large number of individuals and a large species composition.
With these definitions in mind, let’s consider some real-world examples of the impact of microbial diversity on skins and scalp health.
When high microbial diversity indicates good skin health
There is a substantial body of literature supporting a correlation between high skin microbial diversity and good skin and scalp health. We can illustrate this point with a couple of representative examples.
Example 1: Atopic dermatitis & Straphylococcus aureus
A large majority of atopic dermatitis (AD) patients have their skin colonized by the pathogenic bacteria Staphylococcus aureus. A reduction in microbial diversity has been found to correlate with increasing disease severity (2). For example, in AD, the proportion of S. aureus and commensal S. epidermidis are significantly increased during flare-ups. Hence, microbial communities at disease sites are significantly different in AD patients compared to healthy control sites. It seems that an increased abundance of these species leads to a growth inhibition (suppression) of other community members in the same niche, and as a consequence the diversity is reduced.
Similarly, another research finding suggests that a decrease in microbial diversity and an increase in the population of staphylococcus is not only a hallmark of AD, but that this shift precedes the flare of AD (2). Only once diversity is increased (i.e. re-balanced), and the staphylococcus abundance returns to normal levels, does the AD flare resolve and skin return to its healthy baseline conditions.
Example 2: Chronic disease, allergies & bacterial resistance to antibiotics
A published theory goes so far as to suggest that chronic diseases are caused by microbiome shifts towards reduced diversity (3). For example, a significant loss of microbial biodiversity has been observed in polluted and urban areas, and is associated with increases in allergies, asthma and other chronic diseases. This loss of diversity is also correlated with an increase in bacterial resistance to antibiotics – making it all the more important to implement strategies to restore bacterial diversity.
When high microbial diversity indicates bad skin health
The situation becomes more complicated when we consider the evidence supporting the claim that low microbial diversity is linked to good scalp health.
Example 1: The dandruff or psoriatic scalp
A metagenomic analysis of a healthy scalp showed that the diversity of the fungal population was significantly lower in the healthy scalp compared to the flaking/dandruff scalp (4). Similarly, another research group observed a significantly higher abundance of Malassezia globosa in the healthy scalp as compared to the dandruff scalp. The abundance of Malassezia restricta did not show a significant difference between the healthy and flaking/dandruff scalp. However, the ratio of M. restricta to M. globosa was observed to be significantly higher in the dandruff scalp (5).
Another publication reported that increased diversity of the scalp microbiome and the relative abundance of Pseudomonas were associated with the severity of scalp psoriasis. (6)
It seems therefore that the microbial diversity level has reversed implications for skin and scalp health. One plausible explanation may be that the hair on the scalp provides a better physical protection against external environmental impacts compared to exposed facial skin. Assuming that the scalp is exposed to less harsh and more stable extrinsic impacts, its microbiome may have less pressure to develop the high diversity needed to cope with hostile conditions (a similar hypothesis is discussed below). Indeed, studies have revealed that the scalp microbiome is characterized by a rather low bacterial diversity, as compared to the other body sites (7). This would imply that the health status of the scalp can be worsened by unbalancing the skin microbiome through shifting the low baseline diversity to a higher diversity. Here’s food for thought – does the balding scalp have a differing, higher microbial diversity, similar to skin, compared to a hairy scalp?
Example 2: pubescent and senior skin Did you know that before puberty and in old age, our skin microbiome diversity is high, and in middle life its lower and stable? Note its often the pubescent and senior population who experience more severe skin disorders, such as acne, allergic contact dermatitis, skin dryness, itchiness and infections. (8, 9)
No ‘one size fits all’ answers?
The broadly held view that high microbial diversity is always associated with health benefits is based on the long known and scientifically well-founded fact that our gut profits from high bacterial diversity (10). However, we need to be cautious about extrapolating too broadly from that fact. What may be valid for the gut does not necessarily count for skin, and what holds true for skin, does not automatically apply to scalp.
Certain parts of our body are better suited for more diversity, other parts for less diversity. While high diversity in our gut microbiome is beneficial, the same diversity level in other body parts, such as eyes or ear canal, would indicate disease.
This is partly because microbes in the gut have to deal with a constant flood of challenging and varying tasks, such as acidity from the stomach, diverse substances to digest, and exposure to pathogenic intruders. By contrast, microbes in the eye, ear canal or the skin, can afford to be less diverse, and to have less specific and dedicated skill sets, as they have to cope with less challenging tasks.
An overrun of these niches with too many species would lead to competition within the community members and ultimately lead to dysbiosis. Hence, implications of diversity in the microbiome for health depends on the context.
Even then, is diversity the right metric?
Is skin and scalp microbial diversity actually an informative measurement? Perhaps ‘diversity’ is too reductive as a metric. The more important factor may be the nature of the affected species. That is, whether commensal or pathogenic microbes are replaced by the previous commensal or pathogenic inhabitants of the same niche on skin, and whether the absolute number of affected species is affected. In other words, diversity by itself should not conclude anything about skin health. Rather, it is the characteristics and the abundance of the microorganisms that is important.
To critically discuss the significance of diversity measures for health or disease, a review was dedicated to this very topic with the title “Microbiome: Should we diversify from diversity?” (11). Studies on microbiome diversity are flooding the current literature, yet lessons from ecology clearly demonstrate that diversity is just one factor to consider when analyzing an ecosystem, along with its stability, structure and function. Measures of diversity may be a useful tool for interpreting metagenomic data, but the question remains as to how informative they are, what insight they may provide into the state of the microbiome and how microbe-host interactions are affected. There is a need to broaden our approach to the analysis of microbiome data if we are to better understand this complex ecological community and its role in human health and disease.
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References
1. Wallen-Russell C, Wallen-Russell S. Meta Analysis of Skin Microbiome: New Link between Skin Microbiota Diversity and Skin Health with Proposal to Use This as a Future Mechanism to Determine Whether Cosmetic Products Damage the Skin. Cosmetics. 2017;4(2):14.
2. Baldwin H, Aguh C, Andriessen A, Benjamin L, Ferberg AS, Hooper D, et al. Atopic Dermatitis and the Role of the Skin Microbiome in Choosing Prevention, Treatment, and Maintenance Options. Journal of drugs in dermatology : JDD. 2020;19(10):935-40.
3. Blaser M. Antibiotic overuse: Stop the killing of beneficial bacteria. Nature. 2011;476(7361):393-4.
4. Saxena R, Mittal P, Clavaud C, Dhakan DB, Roy N, Breton L, et al. Longitudinal study of the scalp microbiome suggests coconut oil to enrich healthy scalp commensals. Scientific Reports. 2021;11(1):7220.
5. Saxena R, Mittal P, Clavaud C, Dhakan DB, Hegde P, Veeranagaiah MM, et al. Comparison of Healthy and Dandruff Scalp Microbiome Reveals the Role of Commensals in Scalp Health. Frontiers in cellular and infection microbiology. 2018;8:346.
6. Choi JY, Kim H, Koo HY, You J, Yu DS, Lee YB, et al. Severe Scalp Psoriasis Microbiome Has Increased Biodiversity and Relative Abundance of Pseudomonas Compared to Mild Scalp Psoriasis. Journal of clinical medicine. 2022;11(23).
7. Shibagaki N, Suda W, Clavaud C, Bastien P, Takayasu L, Iioka E, et al. Aging-related changes in the diversity of women’s skin microbiomes associated with oral bacteria. Sci Rep. 2017;7(1):10567.
8. Grice EA, Kong HH, Conlan S, Deming CB, Davis J, Young AC, et al. Topographical and temporal diversity of the human skin microbiome. Science (New York, NY). 2009;324(5931):1190-2.
9. Luna PC. Skin Microbiome as Years Go By. American journal of clinical dermatology. 2020;21(Suppl 1):12-7.
10. Lozupone CA, Stombaugh JI, Gordon JI, Jansson JK, Knight R. Diversity, stability and resilience of the human gut microbiota. Nature. 2012;489(7415):220-30.
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