The Scientific Basis of "Scalp Microbiome": How Shampoos Balance "Cleansing" and "Microbial Stability"

The Scalp Is Not Just "Skin That Needs Washing"

For decades, the core logic of shampoo formulation had only one mandate: cleansing. Wash away sebum, dirt, and dandruff; leave it feeling fresh; task complete.

But recently, a research direction disrupting this paradigm has emerged: the scalp microbiome.

The scalp is the body's second-largest microecological system (after the gut), hosting over 100 microbial species on its surface, including bacteria, fungi, viruses, and arthropods. These microorganisms, together with sebaceous glands and hair follicles, form a highly precise ecosystem. As long as it remains balanced, the scalp stays healthy; once disrupted, dandruff, excess oil, sensitivity, and hair loss follow.

Between 2024–2025, the scalp microbiome care market surpassed 1 billion RMB, with a YoY growth of 91%—significantly outpacing the broader haircare market's 22% growth. A paradigm shift from "cleansing the scalp" to "nourishing the microbiome" is actively reshaping R&D logic across the haircare industry.

I. The Basic Architecture of the Scalp Microbiome: Who Lives on Your Head?

Healthy scalp microbial communities are not chaotic; they exhibit a clear, structured dominance hierarchy.

Core Resident Microbes:

• Cutibacterium acnes: The dominant beneficial bacterium in healthy scalps, observed as the primary species in both Chinese and French populations. It breaks down sebum to produce short-chain fatty acids, maintains a weakly acidic scalp environment (pH 4.5–5.5), inhibits harmful microbial overgrowth, and synthesizes protective skin compounds.

• Malassezia: The most prevalent fungal genus on the scalp. In healthy states, it maintains a commensal relationship with the host. Once sebum overproduction or immune dysregulation occurs, its pathogenicity activates. It secretes lipases to break down sebum, releases toxic fatty acids, triggers inflammation, and leads to dandruff and seborrheic dermatitis.

• Staphylococcus epidermidis: A common skin commensal that forms an antagonistic balance with C. acnes, jointly maintaining overall scalp microbial stability.

Signals of Imbalance: When the ratio of Malassezia restricta to Malassezia globosa increases, dandruff severity worsens. Furthermore, Malassezia abundance positively correlates with itching scores and clinical disease severity. Sensitive scalps are characterized by lower bacterial diversity, and diversity itself is a core indicator of microbial resilience.

II. What Happens to the Scalp Microbiome After 28 Days of Shampoo Use?

This question only received a systematic experimental answer in 2025.

A study published in BMC Microbiology (Impact Factor: 4.2) was the first to systematically analyze structural and functional changes in the scalp microbiome before and after 28 days of shampoo use, utilizing 16S rDNA and ITS high-throughput sequencing.

Key Results:

• Shampoo use significantly increased scalp moisture content (P=0.04)

• Relative abundance of beneficial Cutibacterium rose from 11.58% to 28.66%

• Relative abundance of conditionally pathogenic Malassezia decreased from 81.34% to 64.31%

• Bacterial metabolic pathways shifted: Glycolysis and phospholipid synthesis pathways were activated by Day 28; the pathogen-saprophyte nutritional mode (fungal side) decreased by 14.3%, shifting toward a healthier functional profile.

Conclusion: The synergistic effect of Chinese honeylocust (Gleditsia sinensis) ferment filtrate and mild surfactants in the formula maintains the ecological niche of beneficial bacteria like Cutibacterium while suppressing opportunistic pathogens like Pseudomonas. This mechanism provides a clear formulation design target for developing "microbiome-friendly" haircare products.

III. Cleansing Power vs. Microbial Stability: Where Lies the Core Conflict?

The primary function of shampoo is "cleansing," but the act of cleansing inherently disrupts the microbiome—an unavoidable tension between "cleansing" and "microbial stability."

The Cost of Over-Cleansing

Surfactants, while removing sebum and dirt, simultaneously alter three key parameters of the scalp microenvironment:

• Sebum Levels: Sebum is the primary carbon source for Malassezia, but also the nutritional foundation for beneficial bacteria like C. acnes to maintain their ecological niche. Excessive sebum removal can disrupt the competitive balance between harmful and beneficial microbes.

• pH Value: Sulfate-based primary surfactants (SLS/SLES) are highly cleansing but inherently alkaline. Post-wash scalp pH can temporarily rise to 6–7, which coincidentally matches the optimal growth pH for Malassezia—meaning alkaline formulations may temporarily create favorable conditions for fungal proliferation.

• Microbial Diversity: Long-term use of shampoos with high concentrations of broad-spectrum antimicrobials (e.g., excessive ketoconazole or triclosan) may eradicate commensal beneficial bacteria alongside pathogens, reducing diversity and lowering scalp ecological resilience. The 2025 China Scalp Health Industry White Paper notes that 25.9% of long-term anti-dandruff users experience worsening symptoms due to single-ingredient resistance development in Malassezia.

IV. How Do Formulators Design "Microbiome-Friendly" Shampoos?

Industry consensus on "microbiome-friendly shampoo" formulation revolves around four key dimensions:

① Surfactants: Choose mildness to minimize ecological disruption

Biosurfactants (rhamnolipids, sophorolipids) achieve 85%–95% sebum clearance in lab conditions, with a skin irritation index <1.0 (vs. ~6.5 for traditional SLS), and 7–14 day biodegradation rates of 60%–95%. Crucially, sophorolipids possess inherent antimicrobial and anti-inflammatory properties, assisting in scalp microenvironment regulation during cleansing—a functional dimension entirely absent in traditional synthetic surfactants.

However, cost remains the core barrier to scale: biosurfactant raw materials, making full replacement of traditional surfactants unrealistic short-term. Thus, the mainstream solution currently uses amino acid-based primary surfactants + betaine co-surfactants to minimize chemical stress on the microbiome, supplemented with microbiome-active ingredients.

② Ferment Filtrates & Prebiotics: Actively "Feed" Beneficial Bacteria

Chinese honeylocust ferment filtrate was proven in a 28-day human trial to effectively increase Cutibacterium abundance while decreasing Malassezia. Similarly, plant ferment extracts (e.g., Lactobacillus, Bacillus ferment filtrates) are increasingly common in scalp serums. Their mechanism mirrors gut prebiotics—providing nutritional substrates to strengthen the competitive advantage of beneficial microbes.

③ Precision Preservative Selection: Balancing Sterilization vs. Symbiosis

Preservatives in microbiome-focused formulas face dual pressure: insufficient dosage leads to microbial contamination/non-compliance, while excessive dosage may collateral-damage beneficial flora. The 28-day high-throughput sequencing study explicitly indicates that optimizing the ratio of sodium benzoate to humectants (e.g., Polyquaternium-10) is key to ensuring preservative efficacy while protecting core microbial functions.

④ pH Locked at 4.5–5.5: The Physical Moat for Microbial Stability

Healthy scalp pH is ~4.5–5.5. This weakly acidic range is the optimal survival zone for beneficial bacteria like C. acnes while effectively suppressing Malassezia overgrowth. Maintaining shampoo formulation pH within this range is the most fundamental and effective single-parameter control for microbiome-friendly formulations.

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Key Takeaways

The relationship between shampoo and the scalp microbiome is fundamentally a dynamic balance of intervention and equilibrium. Over-cleansing disrupts the flora; under-cleansing allows pathogens to thrive—both extremes compromise scalp health.

The surge of scalp microbiome studies since 2025 provides the first molecular sequencing evidence that: shampoo is not merely a cleansing tool, but an active regulator of the scalp microbiome. Consequently, formulation logic has fundamentally shifted: from "how to wash cleaner" to "how to ensure beneficial microbes thrive after washing."