The follicular microbiome in hair-follicle-associated inflammatory diseases: A systematic review

Hair follicles represent distinct cutaneous microenvironments that host microbial communities. Dysbiosis has been increasingly associated with inflammatory skin and hair disorders, but available studies were limited in scope and sample size, limiting confidence in the conclusions that can be drawn.

To assess the bacterial microbiome of hair follicles across inflammatory hair-follicle-associated skin and hair diseases and identify recurrent alterations.

We searched six databases, including MEDLINE and Embase, for studies published from 2007 to August 2024. Studies using culture-independent methods to characterize the bacterial microbiome in inflammatory hair-follicle-related conditions were included. Owing to expected heterogeneity, a qualitative synthesis was conducted based on the JBI Manual for Evidence Synthesis and reported in line with PRISMA.

Fifty-five studies involving 1,811 patients from 19 countries were included. Methods varied substantially across sampling, storage, DNA extraction, sequencing, and analysis. Cutibacterium and Staphylococcus most frequently showed altered relative abundance. Across diseases, Staphylococcus showed increased relative abundance, particularly in scarring alopecias, seborrheic dermatitis/dandruff, and acne. Reduced Cutibacterium abundance was reported in seborrheic dermatitis/dandruff, whereas Cutibacterium acnes appeared more prevalent in androgenetic alopecia. Findings regarding Cutibacterium were inconsistent in scarring alopecias and acne. Results in alopecia areata and rosacea were inconclusive, whereas hidradenitis suppurativa suggested enrichment of anaerobic opportunists such as Porphyromonas and Prevotella.

Despite substantial methodological heterogeneity, recurrent genus-level shifts involving Cutibacterium and Staphylococcus were observed across several diseases. Their causal significance remains unclear. Larger, standardized studies with functional analyses are needed to clarify biological relevance and therapeutic potential.