The team searched four databases, PubMed, Cochrane, Disbiome and Scopus, from their inception to May 2024, screening 608 records against strict inclusion criteria. Only randomized controlled trials, case-control studies and cohort studies that directly measured microbial diversity or abundance in atopic dermatitis patients relative to healthy controls made the cut. The final synthesis included 23 studies: eight randomized controlled trials, ten case-control studies and five cohort studies. Methodological quality was assessed with validated instruments, including the Cochrane risk-of-bias tool for randomized trials and the Newcastle-Ottawa Scale for observational work, and the meta-analyses followed PRISMA 2020 reporting standards with random-effects modeling to accommodate the expected heterogeneity between studies.

The headline numbers are striking. Across the pooled studies, the bacterial alpha diversity of atopic dermatitis skin was significantly reduced compared with healthy skin, with a standardized mean difference of -1.36 and a p-value below 0.00001. In practical terms, affected skin hosts a far less varied bacterial community, a signature of dysbiosis that has been repeatedly observed but never before quantified with this degree of statistical confidence across study designs. Low diversity is more than a descriptive curiosity: ecosystems dominated by one or few opportunistic species lose the buffering capacity that a rich commensal community provides, leaving the barrier vulnerable to colonization and inflammation.

The second major finding concerns Staphylococcus aureus, the notorious opportunistic pathogen that colonizes the lesions of most moderate-to-severe patients. The meta-analysis found a dramatically increased abundance of S. aureus in atopic dermatitis skin, with a standardized mean difference of 3.49 and a p-value below 0.00001, and the effect was most pronounced in lesional, actively inflamed skin. This aligns with mechanistic work showing that S. aureus exploits epidermal barrier defects to penetrate the skin, where its virulence factors trigger cytokine expression and amplify type 2 inflammation. Earlier longitudinal studies have also shown that S. aureus expansion tracks disease flares and recedes when flares resolve, and that skin pH-dependent S. aureus abundance can predict worsening disease severity, reinforcing the idea that the bacterium is not merely a passenger but an active participant in pathogenesis.

The story of Staphylococcus epidermidis, the skin’s most abundant commensal, proved more nuanced. Across the pooled data, S. epidermidis abundance was elevated overall in atopic dermatitis skin, but the analysis revealed strain-dependent variability that complicates any simple good-bug-versus-bad-bug narrative. Certain S. epidermidis strains are genuinely protective: they ferment glycerin into short-chain fatty acids, inhibit S. aureus biofilm formation, and boost innate immunity through activation of gamma delta T cells and induction of antimicrobial effectors such as perforin-2. Other strains, however, carry deleterious traits. The protease EcpA, produced by some S. epidermidis isolates, can damage the epidermal barrier, and extracellular serine proteases from the species have been shown to elicit type 2-biased immune responses in patients. In other words, the same species can heal or harm depending on which strain dominates, a finding with direct consequences for how live biotherapeutics should be designed and screened.

These microbial shifts sit at the intersection of the two classical pillars of atopic dermatitis. A defective barrier, driven in part by filaggrin loss-of-function and reduced epidermal lipids, creates a dry, alkaline microenvironment that favors S. aureus over commensal staphylococci. Meanwhile, the type 2 immune skew of the disease suppresses the production of antimicrobial peptides, giving the pathogen an additional survival advantage. The result is a self-reinforcing loop: barrier breakdown invites colonization, colonization drives inflammation, and inflammation further degrades the barrier. Interrupting that loop at the microbial node is therefore an attractive strategy, and the review’s authors argue that microbial dysbiosis contributes meaningfully to disease progression rather than simply accompanying it.

Supporting this view, several of the intervention studies included in the synthesis showed that effective treatments also repair the microbiome. Biologic therapies such as dupilumab have been shown to rapidly reduce S. aureus burden and improve stratum corneum hydration, while tralokinumab increased microbial diversity in adults with moderate-to-severe disease in the ECZTRA 1 randomized trial. Emollient-based regimens shifted microflora balance and improved barrier function in children with mild disease, and the skin microbiome of treated patients has been observed to normalize gradually over the course of therapy. Coal tar, one of the oldest eczema remedies, acts partly through aryl hydrocarbon receptor-dependent induction of antimicrobial peptides that reshape the cutaneous bacterial community. Even conventional approaches such as topical corticosteroids and dilute bleach baths have measurable microbiome effects, though not every popular remedy holds up: apple cider vinegar soaks, for instance, failed to alter the bacterial microbiome in a controlled study.

These observations are fueling a growing pipeline of microbiome-targeted therapies. Live biotherapeutics based on selected commensal Staphylococcus strains, topical probiotics and postbiotics, prebiotic formulations that feed protective commensals, and barrier-restoring emollients that indirectly discourage pathogen expansion are all under active investigation. The meta-analysis supports the rationale for such approaches, particularly those that aim to restore microbial balance while simultaneously strengthening the skin barrier. The strain-level complexity of S. epidermidis, however, is a caution: any therapeutic microbe must be selected not just by species but by functional genotype, and manufacturing consistency, colonization stability and long-term safety remain open questions that regulators and developers will need to address.

The study is not without limitations, as the authors acknowledge implicitly through their rigorous quality assessments. Heterogeneity across studies was substantial, reflecting differences in sampling sites, sequencing methods, patient age groups and disease severity, and observational designs cannot fully disentangle cause from consequence in the dysbiosis-inflammation relationship. Subgroup and meta-regression analyses in meta-analyses of this size also carry limited statistical power. Nonetheless, the consistency and magnitude of the two central effects, reduced diversity and S. aureus expansion, across randomized, case-control and cohort designs make microbial dysbiosis one of the most robustly quantified features of atopic dermatitis biology.

For the millions of patients who live with the itch, sleep disruption and infection risk of eczema, the message from this synthesis is ultimately hopeful. The skin microbiome is not a fixed fingerprint but a dynamic, modifiable ecosystem that responds to treatment, and its restoration tracks clinical improvement. As microbiome-targeted interventions mature from concept to clinic, measuring and managing the skin’s bacterial balance may become as routine in dermatology as measuring blood pressure is in cardiology, turning an invisible ecosystem into a concrete therapeutic target for one of the world’s most burdensome chronic diseases.

Subject of Research: Skin microbiome dysbiosis in the pathogenesis of atopic dermatitis

Article Title: The role of skin microbial shifts in the pathogenesis of atopic dermatitis: a systematic review and meta-analysis

Article References: Mohd Noor, N. A., Sulaiman, N. N. Y., Lim, S. M., Ramasamy, K., & Ismail, M. F. (2026). The role of skin microbial shifts in the pathogenesis of atopic dermatitis: a systematic review and meta-analysis. Archives of Dermatological Research, 318(1), Article 513. https://doi.org/10.1007/s00403-026-04990-0

Image Credits: AI Generated

DOI: 10.1007/s00403-026-04990-0

Keywords: atopic dermatitis, skin microbiome, Staphylococcus aureus, Staphylococcus epidermidis, dysbiosis, meta-analysis, eczema, alpha diversity, skin barrier, microbiome-targeted therapy, biotherapeutics, dermatology