The Scalp Microbiome — What a Decade of Research Has Actually Settled

The two most studied groups are:

**Cutibacterium acnes** — a bacterium that breaks down sebum into free fatty acids, helping maintain the scalp's slightly acidic pH (around 4.5–5.5).

**Malassezia species** — a genus of fungi that feeds on sebum triglycerides. At least 14 species have been identified, but *M. globosa*, *M. restricta*, and *M. furfur* are the most common on the human scalp (Byrd et al., 2018).

In a balanced state, these organisms coexist without causing symptoms. When the balance shifts — a state researchers call **dysbiosis** — the downstream effects are familiar to anyone who has dealt with an itchy, oily, or flaky scalp.

**Oil regulation.** Your sebaceous glands produce sebum at a rate determined by genetics, hormones, and environment. But how that sebum *behaves* on your scalp — whether it spreads evenly, oxidises into irritants, or gets broken down by bacteria into soothing free fatty acids — is largely a microbiome question. A 2020 review in *Microorganisms* found that individuals with seborrheic dermatitis consistently showed reduced microbial diversity and an overrepresentation of *Malassezia restricta* relative to *Cutibacterium* species (Park et al., 2020). It's not how much oil you produce, but which microbes are processing it.

**Inflammation and itch.** When Malassezia overgrows, it releases oleic acid by breaking down sebum triglycerides. In roughly 50% of the population, oleic acid penetrates the stratum corneum and triggers an inflammatory response — redness, itching, and accelerated skin-cell turnover that shows up as visible flakes (Zisova, 2009). The other 50% don't react the same way, which is why two people with identical Malassezia counts can have completely different symptoms. This is a host-sensitivity issue, not purely a microbial one.

**Follicle environment.** The hair follicle is an immunologically active structure surrounded by sebaceous glands, nerve endings, and a local microbiome. Research published in *Nature Communications* (2019) demonstrated that the follicular microbiome differs measurably from the surface microbiome, and that follicular dysbiosis correlates with conditions like folliculitis. This does not mean the microbiome *causes* hair loss. It means the follicle's local environment is one modifiable variable — and it is the variable that topical scalp care can most directly influence.

**Over-washing with sulfate-based shampoos.** Sulfate surfactants (SLS, SLES) are effective at stripping the lipid film that the microbiome depends on. A 2017 study in *Annals of Dermatology* showed that subjects who switched to a mild, amino-acid-based cleanser showed measurable improvements in scalp barrier function within two weeks, with no change in sebum production rate. You don't need to produce less oil — you need to stop stripping the environment that keeps oil processing in balance.

**Antibacterial shampoos used long-term.** Zinc pyrithione, selenium sulfide, and ketoconazole are effective anti-fungal agents for acute flare-ups. But long-term daily use reduces overall microbial diversity, which the literature consistently associates with poorer scalp health. Use targeted treatments during flare-ups, then transition to a maintenance routine that supports microbial diversity.

**Product residue and occlusion.** Heavy styling products and silicones create an occlusive layer that alters the local anaerobic/aerobic balance, favouring one microbial group over another.

**Stress, diet, and hormones.** A 2021 study in the *Journal of Investigative Dermatology* found that psychological stress significantly altered the cutaneous microbiome within 48 hours. Cortisol elevation, high-glycaemic diets, and hormonal fluctuations all alter sebum composition — upstream factors that topical care alone cannot fully address.

**1. Cleanse gently, not aggressively.** Use a mild, sulfate-free cleanser. Frequency should be determined by your own sebum production rate — there is no universal "wash every X days" rule.

**2. Support the acid mantle.** The scalp's optimal pH is 4.5–5.5. Products within this range support *Cutibacterium* activity and suppress *Malassezia* overgrowth. Products above pH 6.0 do the reverse.

**3. Add actives that address the local environment.** Adenosine (0.75%) has been shown to support dermal papilla cell signalling. Platycladus orientalis leaf extract has demonstrated microcirculatory and anti-inflammatory effects. These are scalp-environment ingredients — not "hair growth" ingredients, and the distinction matters.

**Probiotic scalp treatments** — no topical probiotic has yet demonstrated consistent, reproducible colonisation of the scalp. The concept is promising; the products on the market are premature.

**Microbiome testing** — scalp swab sequencing is commercially available but has not been validated as a tool for guiding individual treatment. The inter-individual variability is so high that a single swab tells you less than it appears to.

**Dysbiosis and hair loss** — the link remains correlational, not causal. Dysbiosis is present in most hair-loss conditions, but whether it is a cause, a consequence, or a parallel phenomenon has not been determined.

We will update this article when new data changes these conclusions.