The scalp is not a sterile surface.
Like the rest of our skin, it is home to a community of microorganisms. Bacteria such as Cutibacterium and Staphylococcus, together with yeasts of the genus Malassezia, are normal inhabitants of a healthy scalp.
Their presence alone is therefore not the problem.
A more useful question is: what kind of environment are they living in, and how well is that environment functioning?
01 / THE SCALP IS AN ECOSYSTEM
The scalp is a distinctive microenvironment because of its high density of hair follicles and sebaceous glands. Sebum, sweat, keratin, pH, the skin barrier and local immune responses all contribute to the conditions in which microorganisms live.
A healthy scalp is therefore not a scalp without microorganisms.
It is a scalp in which the skin barrier, sebum, microorganisms and the immune system coexist in functional balance.
When one part of this system changes, the others may change with it.
02 / THE MICROBIOME AND SKIN BARRIER WORK BOTH WAYS
The microbiome does not exist independently of the skin.
Sebum composition, hydration, pH and the integrity of the stratum corneum influence which microorganisms are able to thrive. At the same time, microbial metabolism can alter the local skin environment.
This means that barrier disruption, excess sebum, inflammation and changes in microbial communities may coexist and influence one another.
That is why I would not reduce the idea of balance to the quantity of a single bacterium or yeast.
Balance is a property of the ecosystem as a whole.
03 / THERE IS NO SINGLE “IDEAL” MICROBIOME
There is no universal microbial profile that defines a healthy scalp in every person.
The skin microbiome varies between individuals and is influenced by skin physiology, sebum production, age, environment and other factors.
Variation is normal.
It becomes more clinically relevant when changes in the microbial environment occur alongside changes in skin function: itching, inflammation, scaling, discomfort or disruption of the skin barrier.
Dandruff is a useful example of why the presence of one microorganism is not enough to explain a condition.
04 / DANDRUFF IS NOT SIMPLY “TOO MUCH MALASSEZIA”
Malassezia naturally inhabits sebum-rich areas of the skin, including the healthy scalp.
Its presence alone therefore does not explain dandruff or seborrhoeic dermatitis.
Malassezia is lipophilic. Its lipases use lipids from sebum and produce metabolites that, in susceptible individuals, may contribute to irritation and disruption of the epidermal barrier.
Research into dandruff and seborrhoeic dermatitis has also identified changes in the relationships between Malassezia, Cutibacterium and Staphylococcus communities.
But microorganisms are only part of the story.
Sebum composition, individual susceptibility, barrier integrity and the host inflammatory response also influence whether disease develops.
Two people can therefore harbour similar microorganisms while their scalps behave very differently.
05 / CLEANSING IS NOT THE ENEMY OF THE MICROBIOME
The fact that the scalp has a microbiome does not mean it should be washed as infrequently as possible.
Sebum, sweat, shed corneocytes, environmental particles and cosmetic residues accumulate on its surface.
Removing that build-up is part of normal skin hygiene.
The important distinction is between effective cleansing and unnecessarily aggressive cleansing.
A good formulation should remove accumulated material without creating persistent dryness, irritation or unnecessary disruption of the skin barrier.
Good cleansing therefore does not work against the microbiome.
It helps maintain an environment in which the skin and its microbial communities can function.
06 / WHAT DOES “MICROBIOME FRIENDLY” ACTUALLY MEAN?
The microbiome has become an attractive concept in cosmetics.
Microbiome friendly.
Rebalancing.
Microbiome reset.
Biology is less simple.
We do not have one ideal microbial composition that every product should “restore”, nor can the state of an ecosystem be reduced to the abundance of a single microorganism.
For that reason, I find it scientifically more meaningful to talk about supporting the conditions in which the scalp can function normally.
That means appropriate cleansing, preservation of the skin barrier, management of excessive sebum build-up and formulations that are well tolerated by the scalp.
A comfortable scalp without persistent inflammation, itching or pronounced scaling tells us clinically far more than a cosmetic claim that a product has “reset the microbiome”.
07 / WHEN SCALP CARE IS NO LONGER ENOUGH
The microbiome should not become an explanation for every scalp problem.
Persistent redness, pronounced scaling, intense itching, burning, pain or other changes may indicate a dermatological condition that requires assessment.
Seborrhoeic dermatitis, psoriasis, contact dermatitis and other conditions are not simply an “imbalanced microbiome”.
Cosmetic care can help maintain the scalp environment.
Disease requires diagnosis and, when indicated, treatment.
BALANCE, NOT STERILITY
One of the most important lessons of modern microbiome science is that clean skin is not sterile skin.
The scalp is an ecosystem.
The aim of good scalp care is therefore not to eliminate microorganisms or promise a cosmetic “reset”.
It is to support an environment in which the skin barrier, sebum, microorganisms and immune system can function together.
This is also one of the principles behind the EASTOK approach: not to sterilise the scalp, but to cleanse, support and care for it as a biological system.