The skin microbiome is now recognized as a full stakeholder in skin health, on par with the lipid barrier or hydration. Yet as soon as a brand wants to build a “rebalancing” or “probiotic-like” claim, it runs into a recurring difficulty: how do you objectify a microbial imbalance, and more importantly, how do you demonstrate that a product actually corrects it? This guide reviews the scientific definition of dysbiosis, the biomarkers available, and the protocols that allow it to be linked to a measurable cosmetic benefit.
What exactly is skin dysbiosis?
Dysbiosis is not simply “fewer good bacteria, more bad ones.” Scientifically, it refers to a disruption of the functional balance of the microbiome, reflected in three types of change, which often combine:
- a shift in taxonomic composition (loss of diversity, dominance of an opportunistic species such as Cutibacterium acnes in acne or Staphylococcus aureus in atopic dermatitis);
- an alteration of the metabolic activity of microbial communities (reduced production of short-chain fatty acids, antimicrobial peptides, and pH-regulating metabolites);
- a spatial redistribution of bacterial populations across the skin’s different micro-environments (sebaceous, moist, and dry zones).
It is this functional dimension, not just the taxonomic one, that distinguishes true dysbiosis from ordinary individual variability — and inter-individual variability is itself high, which complicates the interpretation of raw sequencing data.
Why dysbiosis has become a claim topic
The scientific literature now links dysbiosis to several skin dysfunctions: impaired barrier function with increased transepidermal water loss (TEWL), heightened sensitivity, and a favorable ground for inflammatory conditions such as acne, atopic dermatitis, psoriasis, or dandruff. This mechanistic link opens the way to three main claim families, which should be clearly distinguished at the study design stage:
- prebiotics, which supply substrates that selectively nourish beneficial commensal bacteria;
- postbiotics, which cover inactivated microbial cells, metabolites, or fermentation filtrates with documented functional benefits;
- probiotic-like ingredients, which support the activity or colonization of commensal populations without introducing live microorganisms (EU cosmetic regulation excluding the use of live bacteria in finished products).
Each family calls for a different study design, with its own endpoints and significance thresholds.
How to objectify and monitor dysbiosis in practice
Assessing skin dysbiosis generally combines several levels of analysis, from sampling to functional biomarker:
- Sampling. The most common techniques are swabbing, tape stripping (adhesive strips such as D-Squame), and, less frequently, punch biopsy for more advanced clinical studies. The choice depends on the area studied (face, scalp, intimate zones) and the depth of skin layer targeted.
- Taxonomic analysis. 16S rRNA gene sequencing remains the reference method for mapping bacterial composition, with costs having dropped significantly in recent years, making larger cohort studies more accessible. For finer functional resolution, multi-omics approaches (shotgun metagenomics, metatranscriptomics, metabolomics) go beyond a simple species inventory to characterize the actual metabolic activity of communities.
- Associated functional biomarkers. Dysbiosis is most often accompanied by measurable changes captured with standard biophysical devices: Tewameter for TEWL, Corneometer for stratum corneum hydration, Sebumeter for sebum output, Cutometer for viscoelasticity. Cross-referencing these measurements with sequencing data builds a complete mechanistic case rather than a mere statistical correlation.
- Upstream in vitro and ex vivo models. Before the clinical study, laboratories use monoculture models (a single isolated strain, e.g. Staphylococcus epidermidis or Cutibacterium acnes) for fast mechanistic screening, or multi-species co-culture models that better reproduce the inter-bacterial interactions actually observed on skin. Ex vivo skin explants, in turn, allow the microbiome’s behavior to be studied within a full tissue environment — epidermis and dermis — while removing the inter-individual variability inherent to in vivo studies.
The table below summarizes the main approaches by level of evidence sought.
| Study level | Method | What it provides | Main limitation |
|---|---|---|---|
| Preliminary screening | In vitro monoculture model | Mechanism of action on an isolated strain | Does not reproduce inter-species interactions |
| Microbial interactions | In vitro multi-species co-culture | Partial reproduction of the skin ecosystem | Artificial culture medium, no tissue context |
| Tissue context | Ex vivo skin explant | Microbiome behavior on real tissue (epidermis + dermis) | Limited tissue viability window |
| Microbial composition | 16S rRNA sequencing | Quantitative taxonomic mapping | Does not capture functional activity |
| Microbial function | Multi-omics approaches (metatranscriptomics, metabolomics) | Actual metabolic activity of communities | Higher cost and analytical complexity |
| Clinical correlation | Biophysical biomarkers (TEWL, hydration, sebum) | Measurable link to a perceived skin benefit | Does not alone prove microbial causality |
Building a solid dysbiosis-friendly claim
The main pitfall for R&D and regulatory teams is the statistical shortcut: a change in microbial composition observed through sequencing does not, on its own, constitute proof of cosmetic efficacy. The most robust protocols systematically combine a microbiological criterion (diversity, relative abundance of a target species) with a functional or clinical criterion (TEWL, hydration, clinical severity score, consumer self-assessment), using a double-blind, placebo-controlled design whenever the claim targets a pathology or skin sensitivity.
For brands and laboratories looking to identify the providers and methodologies available for this type of study, the Skinobs platform references testing solutions dedicated to the skin microbiome, from in vitro screening through to clinical study.
Discover skin microbiome testing solutions on Skinobs → [click here]
