The Scalp and Hair Follicle Microbiome in Alopecia: Current Evidence and Future Directions

The scalp and hair follicle harbor a specialized microbial ecosystem composed of bacteria, fungi, viruses, and mites that interact closely with the follicular epithelium, sebaceous glands, and local immune system. Microbial alterations have been reported in androgenetic alopecia, alopecia areata, and lymphocytic and neutrophilic cicatricial alopecias. However, carefully characterizing the scalp microbiome remains challenging because most studies have relied on superficial scalp swabs that incompletely represent deeper follicular compartments where hair cycling, immune privilege, and inflammatory signaling occur. Recent advances in laser-capture microdissection, shotgun metagenomic sequencing, fluorescence in situ hybridization, and other spatial techniques have demonstrated region-specific differences in microbial communities across follicular compartments, reshaping investigation of follicular dysbiosis. This review summarizes the healthy scalp microbiome, examines disease-specific microbial alterations across major alopecia subtypes, reviews methodological advances and limitations, and discusses emerging microbiome-directed diagnostic and therapeutic strategies. Although dysbiosis is increasingly recognized across multiple alopecia phenotypes, current evidence remains insufficient to determine whether microbial alterations initiate disease, amplify inflammation, arise secondary to follicular injury, or reflect disease activity. Future longitudinal and compartment-specific studies integrating microbial, immunologic, and transcriptomic data will be essential to define the role of the hair follicle microbiome in alopecia pathogenesis and to guide targeted therapeutic interventions.