Dr Egert discusses the need more functional news from the scalp microbiome and asks when will the scales stop falling from our hair?
Reptiles such as snakes experience the shedding of ‘scales’ regularly, although it’s not only their body covered with scales, but their eyes too. This increasingly impairs their sight, as the scales get thicker and thicker. However, with every molting cycle, the animals get a clear view again.
One might say we need some more ‘eye-opening’ events – or studies – with relation to the scalp microbiome. Thinking beyond scale formation, or dandruff, we should think broader: what role could the human scalp microbiome play for scalp health and disease? Nevertheless, dandruff formation is still the ‘holy grail’ when it comes to the functional relevance of the scalp microbiota, at least from a cosmetic point of view.
The cosmetic opportunity
In view of the huge volume within the market for hair care products (€3 billion in Germany, €94 billion worldwide; both with a rising trend [1,2]), the scalp microbiome appears clearly under-investigated. Using “skin” and “microbiome” as keywords for a PubMed search on 06-07-21 delivered 3,325 hits, while “scalp” and “microbiome” delivered only 60 hits. In addition, studies including the functionality (genetic potential, gene expression, metabolite production) of the scalp microbiota, in addition to structural aspects (community composition), are still scarce. The same holds true for the in-vivo effects of cosmetic treatments / products targeting the scalp microbiota.
Nevertheless, two ‘eye-opening’ studies were recently published…
Developments in scalp microbiome research
In a recent landmark study, Saxena and colleagues  presented the first comprehensive analysis of the healthy and dandruff scalp microbiome. They performed structural molecular analyses of the bacterial (using 16S rRNA genes) and fungal (using ITS region sequences; the most widely sequenced DNA region for fungi) scalp community of a decently sized cohort of Indian women (140 subjects). They also performed metagenomic analyses to unravel the genetic potential of the scalp microbial communities. Moreover, they analyzed the association of microbiome data with scalp clinical and physiological data.
The researchers showed that the Cutibacterium acnes to Staphylococcus epidermidis ratio was higher in healthy scalp samples, while the same was true for a lower Malassezia restricta to Malassezia globosa ratio. Their findings largely corroborated results obtained from other geographic regions of the world (China, France, South America), which is significant, because microbiomes might behave differently in different regions.
Notably, on the human scalp and in the context of dandruff formation, S. epidermidis (the great skin immune stimulator and S. aureus fighter) and C. acnes (often associated with acne) seem to switch their ‘good guy’ – ‘bad guy’ images. This is a fine example that the microbial world can never be painted just black and white, even when seen through cosmetic eyes .
Saxena and colleagues could also prove higher relative abundances of several yet-uncultured Malassezia species in dandruff samples. Such a finding underlines the need for (molecular) in-vivo studies to unravel the functionality of the scalp microbiome, also with respect to efficacy testing, because yet-uncultured microbes that are not accessible in vitro might be functionally important.
Also, the metagenomic data were impressive. Metagenomics is the analysis of the complete genome of a microbial community. It provides an idea of the genomic potential of a microbial community. Here, the authors found more fungal genes with functional involvement in cell adhesion to host surfaces in the dandruff samples, which might facilitate fungal growth on affected scalp areas. In turn, significant enrichment of biosynthesis and metabolism pathways of vitamins, such as biotin, and other cofactors was observed in the bacterial microbiome of healthy scalp compared to the dandruff scalp. According to the authors, these results suggest a possible beneficial role of the bacterial scalp microbiome in supplying essential vitamins and amino acids to the host. They even draw a parallel to the human intestinal tract, where butyrate-producing bacteria play a pivotal role for the health and homeostasis of the gut epithelium.
As if that wasn’t enough, the same authors very recently published a study  in which they used the same cohort of probands and analytical methods to investigate the effect of coconut oil (in comparison to a neutral shampoo) on the microbiome of healthy and dandruff scalp. According to the authors, coconut oil is a well-established anti-dandruff treatment in countries such as India, containing antimicrobial actives such as lauric acid and improving skin barrier function.
Nicely in line with their previous observations, the 12-week long coconut oil treatment not only ameliorated the dandruff symptoms, but also shifted the scalp microbiota composition into a healthier direction. It increased the relative abundance of C. acnes and M. globosa in the dandruff samples, accompanied by a relative decrease in fungal pathogenesis pathways and an increase in healthy-scalp-related bacterial pathways, such as biotin metabolism.
The two studies by Saxena and colleagues provide new and existing hypotheses on the role the scalp microbiome might play in dandruff formation, and new targets for anti-dandruff strategies. The studies are fine examples of how the effect of cosmetic treatments on the scalp microbiota should be investigated in the future, not only structurally, but also functionally. I am not aware of other studies that have analyzed the effect of cosmetics on the scalp microbiota so skillfully.
The next frontier for the scalp microbiome
However, please let me add some critical comments and suggestions. Both dandruff studies discussed here are based only on relative changes of functional and structural microbial genes obtained with a next-generation sequencing technique. However, next-generation sequencing data have a compositional (quantitative, not relative) character, demanding special statistical methods . I would love to see the data re-analyzed with such methods. Will the main conclusions be the same? In addition, quantitative (qPCR, simple cultivation) data on the absolute abundance (cell numbers, gene numbers) of the main players would add much to the story. Are the cell numbers of C. acnes absolutely higher on healthy skin? Finally, transcriptomic and metabolomics data will be needed to demonstrate if, for instance, a scalp with a higher C. acnes to S. epidermidis ratio does really contain more biotin-producing enzymes and (even more important) biotin or not, and (most important) if this has any positive effect on scalp and hair health.
Nevertheless, the view on dandruff formation and treatment or prevention has definitely been made a bit clearer. Luckily, metagenomics was also discovered as a suitable tool to unravel the role of the microbiome for other scalp problems, such as hair loss (alopecia) .
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References https://de.statista.com/statistik/daten/studie/166763/umfrage/umsatz-mit-haarpflegemitteln-seit-2007/ https://de.statista.com/statistik/daten/studie/255850/umfrage/prognostiziertes-weltweites-marktvolumen-fuer-haarpflegeprodukte/ Saxena et al. Comparison of healthy and dandruff scalp microbiome reveals the role of commensals in scalp health. Front. Cell. Infect. Microbiol. 8, 346 (2018). https://thesecretlifeofskin.com/2019/03/19/the-staphylococci/ Saxena et al. Longitudinal study of the scalp microbiome suggests coconut oil to enrich healthy scalp commensals. Sci. Rep. 11, 7220 (2021). Gloor et al. Microbiome datasets are compositional: and this is not optional. Front. Microbiol. 15, 2224 (2017). Constantonou et al. The potential relevance of the microbiome to hair physiology and regeneration: the emerging role of metagenomics. Biomedicines 9, 236 (2021).