The complex causes of dandruff

Dandruff is a prevalent skin condition affecting the scalp, characterized primarily by heavy flaking and sometimes mild itchiness. While often perceived as a simple issue, its underlying causes are complex, involving a delicate interplay of genetic, environmental, and microbial factors. It's important to understand that dandruff is not a result of poor hygiene, but rather an excessive growth of skin cells and a disturbed scalp microbiome. This condition can

sometimes lead to social or self-esteem problems for those affected, highlighting the need for a clearer understanding of its mechanisms.

The Enigma of Its Origins: Genetic and Environmental Factors

The precise cause of dandruff remains unclear, but current understanding points to a combination of genetic predispositions and environmental influences. The condition may worsen during the winter months, suggesting that environmental factors play a role in its manifestation or severity. Unlike common misconceptions, dandruff is not linked to poor hygiene, which means that simply washing hair more frequently does not address the root cause of the problem. Instead, the core issue lies in the accelerated turnover of skin cells on the scalp.

Normally, skin layers continuously replace themselves, with old cells being pushed outward, dying, and flaking off. For most individuals, these flakes are microscopic and invisible. However, in people with dandruff, this cell turnover process is unusually rapid. It is hypothesized that skin cells on a dandruff-affected scalp may mature and shed within two to seven days, a stark contrast to the approximately one month it takes for individuals without the condition. This rapid shedding results in dead skin cells accumulating into larger, often oily clumps, which are visible as white or grayish flakes on the scalp, skin, and clothing.

The Microbial Landscape: Fungi and Bacteria at Play

For a long time, the primary suspect in dandruff causation was the fungus *Malassezia furfur*, also known as *Pityrosporum ovale*. While this species naturally resides on human skin, both in those with and without dandruff, research in 2007 identified a more specific culprit: *Malassezia globosa*. This scalp-specific fungus metabolizes triglycerides found in sebum, a natural oil produced by the skin, through the action of lipase enzymes. This process generates oleic acid as a byproduct. In individuals experiencing dandruff, the levels of *Malassezia* are typically 1.5 to 2 times higher than normal.

The oleic acid produced by *Malassezia globosa* can penetrate the stratum corneum, the outermost layer of the epidermis. In susceptible individuals, this penetration triggers an inflammatory response, disrupting the skin's natural balance and leading to the erratic shedding of stratum corneum cells. This explains the characteristic flaking associated with dandruff. Beyond fungi, bacteria are also implicated in the condition. Studies have shown that *Staphylococcus capitis* can be up to 100 times more abundant on dandruff-affected scalps, suggesting a significant bacterial contribution to the condition.

The Bacterial Balance: Propionibacterium vs. Staphylococcus

Recent studies utilizing advanced sequencing techniques have shed more light on the bacterial communities inhabiting the scalp microbiome. Eleven bacterial phyla were detected on dandruff-affected scalps, with the majority of sequences assigned to Actinobacteria (64.9%) and Firmicutes (32.5%). Within these phyla, *Propionibacterium* (63.3%, Actinobacteria) and *Staphylococcus* (32.4%, Firmicutes) comprised over 95% of the total bacterial sequences. Specifically, 99.7% of the *Propionibacterium* identified belonged to *P. acnes*, and 94.9% of the *Staphylococcus* were various *Staphylococcus* species, including *S. epidermidis*, *S. capitis*, and *S. caprae*.

Interestingly, research revealed a shift in the bacterial balance in individuals with dandruff. *Propionibacterium* decreased from 70.8% to 50.2% in the dandruff group, while *Staphylococcus* increased from 26.0% to 43.5%. Furthermore, the proportion of other less abundant bacteria also saw an increase in the dandruff group, from 3.2% to 6.4%. Redundancy analysis indicated that 33 genera were related to dandruff severity, with *Staphylococcus* showing a significant positive correlation. Conversely, only two genera, *Propionibacterium* and *Labrys*, exhibited a significant negative correlation. These findings suggest that dandruff is strongly associated with an imbalance between the two dominant and reciprocally inhibited bacterial genera on the scalp: *Staphylococcus* and *Propionibacterium*. While fungi, particularly *Malassezia* species, were long considered the primary cause, the relationship between bacteria and dandruff has been observed to be stronger than that between fungi and dandruff in some studies, highlighting the intricate microbial interactions at play.