Skin Microbiome and Eczema: Microbial Mechanisms

What happens to the skin microbiome in eczema?

The microbiome in atopic dermatitis (eczema) undergoes a dramatic shift toward lower diversity and heavy colonization by a single pathogen. Healthy skin hosts hundreds of bacterial species in balanced communities, but eczema-affected skin often becomes dominated by Staphylococcus aureus, which can represent over 90% of bacteria during active flares. This collapse in microbial diversity correlates directly with disease severity.

Studies using DNA sequencing have revealed that even non-lesional skin in eczema patients shows altered microbial composition compared to healthy controls. The changes worsen as inflammation develops, suggesting a feedback loop between dysbiosis and disease activity. During flares, protective commensal species from genera like Cutibacterium, Corynebacterium, and beneficial Staphylococcus species decline sharply.

How does S. aureus drive eczema inflammation?

S. aureus doesn't just colonize damaged skin—it actively worsens the condition through multiple pathogenic mechanisms. This bacterium secretes toxins, including alpha-toxin and delta-toxin, that punch holes in keratinocyte membranes and trigger cell death. These same toxins also act as superantigens, activating massive immune responses that amplify the itching and inflammation characteristic of eczema.

The bacterium's proteases directly degrade proteins crucial to barrier integrity, including filaggrin breakdown products that normally help maintain skin hydration and pH. S. aureus also forms biofilms on eczema skin, which protect it from antimicrobial peptides and make colonization more persistent. Research has shown that the density of S. aureus on skin correlates with eczema severity scores and predicts flare risk.

What role does filaggrin play in microbial balance?

Filaggrin is a structural protein that bundles keratin filaments together, and its breakdown products acidify the skin and create natural moisturizing factor. Loss-of-function mutations in the filaggrin gene (FLG) are the strongest known genetic risk factor for atopic dermatitis, present in roughly 30% of European eczema patients. These mutations create a compromised barrier that allows increased water loss and penetration of allergens and microbes.

The filaggrin connection extends directly to microbial control. Filaggrin breakdown products lower skin pH, and S. aureus thrives in the higher pH environment created by filaggrin deficiency. The same breakdown products also support the production of antimicrobial peptides that normally suppress pathogen growth.

Without adequate filaggrin, skin becomes both physically leakier and immunologically less equipped to defend against colonization. This explains why FLG mutation carriers show earlier disease onset and more severe, persistent eczema.

Why do beneficial skin bacteria disappear in eczema?

Commensal bacteria that normally occupy healthy skin provide colonization resistance—they compete for nutrients and space while producing antimicrobial compounds. Staphylococcus epidermidis, the dominant species on healthy skin, secretes peptides and other molecules that selectively inhibit S. aureus growth. In eczema, the relative abundance of S. epidermidis drops significantly, removing this protective effect.

Studies suggest the inflammatory environment of eczema skin may favor S. aureus while creating inhospitable conditions for commensals. S. aureus is more tolerant of the immune mediators and altered lipid composition found in eczema lesions. The disrupted barrier also allows S. aureus access to deeper nutrient sources unavailable to surface commensals.

This competitive displacement becomes self-reinforcing: S. aureus worsens inflammation and barrier damage, which further selects for its own dominance. Recent work has shown that specific strains of protective S. epidermidis and S. hominis isolated from healthy skin can inhibit S. aureus and reduce inflammation in mouse models, highlighting the importance of these lost commensals.

Does microbial dysbiosis cause eczema or result from it?

The relationship between dysbiosis and eczema appears bidirectional rather than strictly causal in one direction. Barrier defects and immune dysregulation associated with eczema create conditions favorable to S. aureus colonization, suggesting the disease enables the dysbiosis. However, S. aureus toxins and proteases actively damage the barrier and drive inflammation, meaning the dysbiosis also perpetuates disease.

Longitudinal studies tracking infants before eczema onset have found increased S. aureus colonization precedes visible disease in some cases, supporting a contributory role. Twin studies and experiments transferring bacteria between mice demonstrate that microbiome composition can influence disease expression independent of host genetics. The most accurate model views eczema as a complex interaction where genetic susceptibility, immune activation, barrier dysfunction, and microbial imbalance all reinforce each other.

Environmental factors including cleansing practices, antibiotic exposure, and moisture levels modulate this interaction. The timing and severity of dysbiosis can influence disease trajectory, even if not the sole initiating cause.

The bottom line

Eczema involves a profound disruption of skin microbial ecology characterized by S. aureus dominance and loss of protective diversity. This dysbiosis actively worsens disease through toxin production, barrier damage, and immune activation, creating a cycle that perpetuates inflammation. Understanding these microbial mechanisms reveals why restoring balanced skin communities represents a rational therapeutic target.