The scalp is part of the integumentary protecting system of the human head, a distinctive skin area with relatively high follicular density and sebum production and secretion. When covered with hair, the scalp is a protected environment that is a welcome setting to the inhabitation of microorganism population different from that present in other body areas. Biota imbalance on the scalp can give rise to infections associated with a variety of clinically manifested conditions. In recent years, efforts have been made to better identify scalp biota population and differentiate between a healthy and disease afflicted scalp; some are detailed in this article.


However, the connection between biota population, and its activity and effect on scalp skin innate immunity is largely unclear. To further understand the interplay between the scalp microbiome and its effect on skin cells, a combination of biome and omics research is to be employed in which biota’s activity is considered and drawn to map innate immunity components. For such projects, teamwork between microbiologists and omic biologists is essential. The most common scalp conditions are dandruff (DF) and seborrheic dermatitis (SD). They share cause and characteristics, with DF considered mild and SD being the more severe condition.


Knowledge that exemplify the importance of biota-innate immunity axis investigation as related to these conditions lies in the fact that at the cellular biochemical level SD is associated with attenuation of T cells activity and hence provides an indication of decreased immune responsiveness as well as activation of the alternative complement pathway. However it is not clear if the source of such reduced immunity is systemic and reflects overall health or is confined to the skin; or a combination thereof. If the disorder stems from overall health condition; topical treatment may provide limited results and a more comprehensive approach should be exercised.
When compared to skin in other body areas scalp skin is:

• Relatively thicker;
• Contains more blood vessels;
• Presents an environment rich with sebum and dense with sebaceous glands; and
• Inhabits unique combinations of biota domination.

DF is characterized by excessive shedding of dead skin cells from the scalp. These corneocyte clusters are bundles of cemented keratin filaments, which have retained a large degree of cohesion with one another and detach as such from the surface of the stratum corneum. In most cases, dandruff is not associated with apparent scalp redness; however, when detected, it involves the expression of inflammatory markers. Symptoms associated with dandruff are pruritus (66%); irritation (59%) and feeling of tight or dry scalp (25%). Distinguished from dandruff, SD is manifested in flakes that are greasy with yellow color accompanied with apparent clinical inflammation such as erythema. Unlike DF, SD may not be confined to the scalp area but can appear beyond the scalp particularly on the nasolabial folds, ears, eyebrows and chest. In SD, pruritus of the scalp is the most common symptom and lesions typically worsen during the winter, while sun exposure during the warmer months appears to improve the clinical appearance of the disorder.


The scalp is enriched with follicles. Therefore, compounds that can partition into sebum may be absorbed into deeper layers of the skin via the follicular route. As such, they may skip innate immunity components of the upper skin layer, imparting a direct effect to living cells of the epidermis. This makes scalp skin more vulnerable to such compounds and increases irritation and inflammation potential. Understanding the scalp permeability barrier is critical, as both DF and SD are more than just superficial disorders of the stratum corneum. In fact, the epidermis of such disorders is significantly altered, with hyper proliferation, excess intercellular and intracellular lipids and interdigitating of corneal envelope. In addition, as noted, DF and AD are associated with elevated sebum secretion. Access of sebum secretion, as well as hyper-keratinization, can be viewed as compensatory mechanisms for homeostasis imbalance; but they may generate the opposite reaction when creating ground for biota imbalance. It seems the evolution of innate immunity in highly complex biological environment such as the human body is not fast enough adapting to the evolution of the simple unicellular prokaryotes.


Analysis of human sebum drawn from the follicle reveals a mixture of triglycerides, fatty acids, wax esters, sterol esters, cholesterol, cholesterol esters and squalene. Upon secretion, triglycerides and esters get broken down by lipase enzymes and converted into triglycerides, monoglycerides and free fatty acids. It is thought that the free fatty acids play a key role in the initiation of the irritant response that involves hyper-proliferation of scalp skin cells. Moreover, it is postulated that an enzyme secreted by scalp yeast converts sebum components to unsaturated fatty acids such as oleic acid; this conversion may be correlated to DF formation. This was demonstrated when oleic acid applied to susceptible individuals’ scalps gave rise to DF formation even when specific biota was removed from the scalp. However, it does not mean that additional important biochemical paths for DF or SD disordered pathology are absent. Sebum may serve as nutrition to facilitate biota growth and metabolism, thereby enhancing proliferation of keratinocytes and their transformation to corneocytes as well as serve as a glue cementing corneocytes to form flakes.


People with scalp disorder typically carry it for many years and tend to use topical treatments such as shampoos with various antifungal and antimicrobial actives. Such practice is proven helpful only if the individual continues to use the products and often changing the product is necessary to maintain effectiveness. It is not known if such practice generates biota resistance over time and drives evolutionary changes to biota population that acquired tenacious strategies of self-protection. This symptomatic treatment requires continued product use, because the cycle of activation that led to the condition is not broken.



Scalp Biota Species


Prior to the availability of sophisticated genomic techniques, microbiology research was limited to procedures of isolation, maintenance, study and accuracy in identification of body commensal biota communities. Recent advances in techniques generated key information in the understanding of potential underlying cause and interplay between the biome and human cellular behavior.


Still, however, much is missing, even in the biota’s role per se. For example, while genomic sequencing can point toward a presence of biota; it cannot provide information on its livelihood or activity. If the biota sequenced is dead or not metabolically active, its contribution to the condition may be questionable. On the other hand, chemical components on the biota cell wall, even if it is not metabolically active, may stimulate an innate immunity mechanism to facilitate human cellular biological cascades.


Until recently, the consensus has been that Malassezia yeast (known as Pityrosporum), a lipid dependent organism, dominates scalp skin and therefore plays a pivotal role in scalp disorders. Members of this genus are numerous and some scientists classify them as lipid dependent and lipid independent. Identification of specific molecular markers is required to differentiate between species. Recent findings question this dogma. Before genetic sequencing, the study of this class was complicated because when isolated from the scalp it requires very particular culture conditions and its growth in culture may not replicate modalities that reflect the natural scalp environment. Genetic sequencing allowed further investigation of the Malassezia species as well as other biota communities on scalp. Market leaders such as Procter & Gamble and L’Oréal studied its sequencing and characteristics.


The fungus Malassezia globosa secretes lipase and other enzymes that metabolize triglycerides present in the sebum into unsaturated fatty acids such as oleic acid. During DF formation and presence, the levels of Malassezia increase 1.5 to 2 times its normal level. Oleic acid is a skin penetration enhancer that fluidizes the stratum corneum intercellular lipids, and can further partition into the sebum triggering an inflammatory response in susceptible individuals. This may disturb scalp environment homeostasis and result in irregular cleavage of connections between epidermal cells. Oleic acid can affect skin immunity by activating Langerhans cells, further fostering an inflammatory response. This is of key importance in the discussion of the follicular opening since the lower compartment of the follicle is heavily populated by protecting Langerhans cells. While DF is accompanied by itching, it does not exhibit visible signs of inflammation such as erythema that is manifested in AD. However, non-apparent inflammation is known to be involved in DF, giving rise to inflammatory mediators released from keratinocytes. Therefore, DF should be considered as an inflammation-involved disorder although it is not manifested clinically. In 2007, P&G published a study describing sequencing of the Malassezia globosa and Malassezia restricta genome. The Malassezia globosa genome was found to be 9Mb (mega bases), the smallest of any known, free-living fungi, but capable of expressing 4,289 proteins; some for glycolysis and variety of amino acids. Unlike all other species in this genomic fungi family, Malassezia globosa lacks the ability to produce fatty acid synthases. Although one may speculate that the inability to encode for the lipid synthase translates to reduced pathogenicity when compared to other species, this may not be the case and further investigation is required as Malassezia globosa has genes that can encode for proteases, members of the phospholipase C family, cell wall modifying enzymes and others that when released, may interact with scalp skin cells.


Last year, L’Oréal published a study characterizing key bacterial-fungal populations on DF scalps of Chinese volunteers. This study confirmed a previous hypothesis that, due to fungi domination on afflicted scalps, the overall biota population, bacteria included, presents abnormal representation. Species that are normal to skin areas where sebum is actively secreted such as Propionibacterium acnes and Staphylococcus epidermidis  were found on scalp skin as well. However, the DF afflicted population was typical to the absence of Propionibacterium granulosum and contained a more diverse staphylococcal biota. Malassezia globosa represented no more than 1% of the fungi population irrespective of the scalp condition. Using qPCR it has been demonstrated that DF scalp has significantly higher amounts of Malassezia restricta and Staphyloccoccus species and the ratios of Malassezia restricta to Propionibacterium and Staphylococcus to Propionibacterium were significantly higher in subjects with DF.


In a 2013 study, L’Oréal investigated scalp microbiome in French volunteers. The identification was conducted by cloning and sequencing the conserved ribosomal unit regions which are 16S and 28S-ITS for bacteria and fungi, respectively. Findings revealed that normal scalp inhabits mostly the bacterial commensals Staphylococcus epidermidis and Propionibacterium acnes and the fungi Malassezia restricta. DF afflicted scalp demonstrated lower levels of Propionibacterium acnes and higher levels of Staphylococcus epidermidis and Malassezia restricta. While these species were dominating scalp microbiome; other related species were identified in smaller quantities; for example, from the Propionibacterium family, Propionibacterium acnes inhabitation was 99.7% accompanied by 0.3% of Propionibacterium granulosum. In the Staphyloccoccus family, Staphylococcus epidermidis was present at 99.1% accompanied by Staphyloccoccus caprae at 0.5%. The role and importance of species in disorder formation does not necessarily correlate to their presence or percentage on the scalp but to their pathogenicity and for such, additional studies that combine microbiology with proteomics are required.


Novel Therapies


While new findings are available, they may be just the tip of the iceberg in terms of understanding the diversity of microbiome on the scalp and its role in health and disease. As additional information is revealed, more remains unknown and the importance and relevancy to treatment options is unclear. Although treating DF with antifungal agents is insufficient and new strategies are required, it has been proven effective. Resolution to persistence and repetition of DF and AD is needed.


Analysis of knowledge and its gaps raises numerous ideas for innovative treatment approaches; each should be weighted for its opportunities; limitations; feasibility; efficacy, safety and other product development related aspects. The strategies described below can be investigated alone or in combinations.

• Immune boosting. DF and AD are associated with immune deficiency, therefore enriching the scalp  with immunity boosting agents; i.e., antimicrobial agents such as specific lipids and peptides, may strengthen immunity and prevent growth of harmful biota.
• Sebum enrichment. The condition is associated with biota enzymatic breakdown of triglycerides to irritating fatty acids; changing sebum composition balance to reduce triglyceride feed may assist in reduction in formation of harmful fatty acids and enrich the scalp with protecting lipids.
• Addressing pH mantle. Biota population may require different acidity conditions. If such differences are understood and established as significant, changing the pH may foster growth of healthier biota population.
• Prebiotics. Enrichment of the scalp skin surface with compounds that nourish the biota of interest. For example, if specific nutrition for Propionibacterium acnes is provided, it may assist aiding the shift to a normal inhabited scalp.
• As noted, each one of these ideas requires further basic research for validation and an appropriate product development path. Its common denominator is the attempt to bring the scalp skin back to a healthy homeostasis. Treating the scalp with biota toxic compounds, while may be helpful temporarily, is not fostering the creation of healthy environment that will restore a healthy scalp.

References

• Wang L. et al. Characterization of major bacterial-fungal populations colonizing dandruff scalps in Shanghai, China shows microbial disequilibrium. Experimental Dermatology 24 (2015) 381-400.
• Dawson T.L. Malassezia globosa and restricta: breakthrough ujderstanding of the etiology and treatment of dandruff and seborrheic dermatitis through whole-genome analysis. J. Invest. Dermatol. 12 (2007) 15-19.
• Cecile C. et al. Dandruff is associated with disequilibrium in the proportion of the major bacterial and fungal populations colonizing the scalp PLOS ONE 8(3) (2013) e58203.
• Donnarumma G. et al. Analysis of the response of human keratinocytes to Malassezia globosa and restricta strains. Arch. Dermatol. Res. DOI 10.1007/s00403-014-1479-1