Anti-dandruff, microbiome-based

Dandruff is not an infection in the usual sense. Malassezia is present on essentially every adult scalp, including scalps that never flake. The question a science page has to answer is therefore what changes.

A fungus that cannot make its own lipids#

Malassezia species are lipid-dependent: their genomes lack a functional fatty acid synthase gene, so they cannot synthesise the long-chain fatty acids they need for their own membranes and must take them from the environment. On a human scalp the environment is sebum.

To get at them, the fungus secretes lipases and phospholipases that hydrolyse sebum triglycerides into free fatty acids and glycerol. It then consumes the saturated fatty acids selectively and leaves the unsaturated ones — oleic acid principally — on the skin surface.

This is the pivot of the whole subject. Free oleic acid applied to the scalp of dandruff-susceptible individuals is reported to reproduce flaking, while the same application on non-susceptible individuals does not. The proposed sequence is that unsaturated free fatty acids penetrate a compromised stratum corneum, disrupt barrier lipid organisation, and trigger the accelerated, disordered desquamation that appears as flakes.

Three factors are therefore required together: the fungus, enough sebum to feed it, and individual susceptibility of the barrier. That explains what a fungus-only model cannot — why dandruff appears at puberty when sebaceous output rises, why it concentrates on sebaceous sites, and why two people with similar Malassezia loads differ.

What the conventional actives do#

Zinc pyrithione and selenium disulfide are broadly fungistatic, and zinc pyrithione has been shown to disrupt fungal metal ion homeostasis. Ketoconazole is an azole: it inhibits lanosterol 14α-demethylase, blocking ergosterol synthesis and destabilising the fungal membrane. Piroctone olamine is a hydroxamic acid whose activity is attributed to iron chelation and consequent disruption of fungal energy metabolism.

All of them reduce the fungal population, which reduces lipase activity, which reduces free fatty acid load. None of them addresses the susceptibility term, which is why relapse on discontinuation is the norm rather than a formulation failure.

What the microbiome approach adds#

Prebiotic formulations use oligosaccharides — alpha-glucan oligosaccharides made enzymatically from sucrose and maltose by glucosyltransferase are the common example — chosen because commensal scalp bacteria can metabolise them and Malassezia, a lipid feeder, cannot. The intent is to shift the balance rather than to suppress a population.

Probiotic and postbiotic formulations apply organisms or their metabolites, most often lactic-acid bacteria, aiming at surface pH and at the bacterial side of the community. Note that the scalp balance in question is genuinely three-way — Malassezia, Cutibacterium and Staphylococcus — so a claim about “rebalancing” is only meaningful when it says which of the three it moves.

Plant oils such as tea tree and neem carry real in-vitro antifungal activity; the honest caveat is that in-vitro minimum inhibitory concentrations do not translate directly to a rinse-off product with seconds of contact time, and tea tree oil is itself a recognised contact sensitiser.