Dandruff and seborrheic dermatitis are very common multifactorial conditions mainly related to unbalanced scalp microbiome, unbalanced sebum composition and an impaired skin barrier function. During the lasts decades our knowledge about human microbiome has deepened and in certain skin conditions its role has become staring. Dandruff and seborrheic dermatitis are one of the most studied diseases in which scalp dysbiosis leads to clinical signs and symptoms.  
Originally, the main interest focused on Malassezia genus since it was isolated as the principal agent and the fact that these conditions improved with antifungals reinforced its role. Later studies have broadened the microbiological scenario to include bacteria adding complexity to our understanding of these pathologies. Xu et al. reported that bacteria had a stronger relationship with dandruff severity than fungi (1). 
Many microbiome studies were conducted in different locations and although all showed that scalp dysbiosis was linked to dandruff and seborrheic dermatitis, the identified microorganisms showed controversial results probably due to ethnic and geographical variations and heterogeneity of microbiological identification methods (2- 3-4). A recently published systematic review (5-Table) of fungal and bacterial shifts in dandruff and seborrheic dermatitis found an increase Malassezia restricta/Malassezia globosa ratio and a reduction in Cutibacterium/Staphylococcus ratio. Researchers also suggested that unclassified microorganisms, as uncharacterized Malassezia, play an important role in health and dandruff and need further investigations (6-4).

The imbalance in microbiological communities and the altered physiological parameters were found not only in lesional but also in non-lesional skin in affected subjects, so dandruff could be thought of as a systemic process (4).
Scalp dysbiosis has been studied mainly at a taxonomic level but it is also important to understand the interactions between microbiological communities and between the host and the microorganisms. Cutibacterium and Staphylococcus showed a reciprocally inhibited relationship in some studies (1). S. epidermidis and Staphylococcus spp. were positively correlated with dandruff scores, TEWL and symptoms, but negatively correlated with hydration, suggesting their potential role in causing skin barrier damage. Also M. restricta and Aspergillus penicilloides were positively correlated with sebum level (1). An area even less studied is the functional microbiome, Saxena et al. highlighted its role. They found that N-glycan biosynthesis pathway (essential for maintaining the fungal cell wall integrity) was enriched in the dandruff scalp and also that biosynthesis and metabolism pathways of vitamins and cofactors was significant enriched in the bacterial microbiome of healthy scalp compared to the dandruff scalp (6).
Treatment clinical studies showed that rebalancing scalp microbiome is one of the fundamental aspects in dandruff and seborrheic dermatitis therapeutics. Research has demonstrated that clinical improvements are correlated to changes in microbiome profile and clinical relapses can be predicted by recolonization.  Recently, Massiot et al. (7) investigated clinical and microbiome scalp changes using a selenium disulfide-based shampoo after a ketoconazole shampoo treatment phase in patients with moderated to severe seborrheic dermatitis.  As expected, ketoconazole shampoo provided an improvement in clinical symptoms, an increased fungal diversity and significant decrease of Malassezia spp. Selenium disulfide-based shampoo showed an additional benefit in clinical scores associated with a low Malassezia spp. count and a decreased of Staphylococcus spp. level. Selenium disulfide-based shampoo appears also as a good option for the maintenance phase after ketoconazole treatment in subjects with mild to moderate seborrheic dermatitis.

Further taxonomic and functional microbiome studies will lead to exploring new lines of investigation and innovative treatments aimed at restoring scalp homeostasis.


Bibliography

1.Dandruff is associated with the conjoined interactions between host and microorganisms. Zhijue Xu, Zongxiu Wang,  Chao Yuan, Xiaoping Liu et al. Nature Scientific Reports | 6:24877 | DOI: 10.1038/srep24877.
2.Dandruff Is Associated with Disequilibrium in the Proportion of the Major Bacterial and Fungal Populations Colonizing the Scalp. Cécile Clavaud, Roland Jourdain, Avner Bar-Hen, Magali Tichit, Christiane Bouchier et al. PLoS ONE 8(3): e58203. doi:10.1371/journal.pone.0058203.
3.Characterization of the major bacterial–fungal populations colonizing dandruff scalps in Shanghai, China, shows microbial disequilibrium. Lili Wang, Cécile Clavaud2, Avner Bar-Hen, Meng Cui et al. Experimental Dermatology, 2015, 24, 381–400.
4.Dysbiotic Bacterial and Fungal Communities Not Restricted to Clinically Affected Skin Sites in Dandruff. Renan C.Soares, PedroH.Camargo-Penn1, Vanessa C.S.deMoraes et al. Front. Cell. Infect. Microbiol. 6:157. doi: 10.3389/fcimb.2016.00157
5.Skin microbiome alterations in seborrheic dermatitis and dandruff: A systematic review. Rong Tao, Ruoyu Li, Ruojun Wang. Experimental Dermatology. 2021;00:1–8.
6.Comparison of Healthy and Dandruff Scalp Microbiome Reveals the Role of Commensals in Scalp Health. Rituja Saxena, Parul Mittal, Cécile Clavaud, Darshan B. Dhakan et al. Front. Cell. Infect. Microbiol. 8:346.
doi:10.3389/fcimb.2018.00346.
7.Continuous clinical improvement of mild-to-moderate seborrheic dermatitis and rebalancing of the scalp microbiome using a selenium disulfide–based shampoo after an initial treatment with ketoconazole. Philippe Massiot PhD , Cécile Clavaud PhD, Marie Thomas MSc, Alban Ott MSc, Audrey Guéniche Pharm D, PhD, Ségolène Panhard MSc, Benoît Muller PhD, Céline Michelin MSc, Delphine Kerob MD, Anne Bouloc MD, Pascal Reygagne MD. J Cosmet Dermatol. 2021;00:1–11