Ting Li, Dongni Yao, Feng Cheng, Xiaozhou Mou, Qiong Bian
Hair loss is common and can markedly impair quality of life, yet the contribution of the scalp microbiome to alopecia remains fragmented across disease subtypes and mechanistic levels. This review aims to construct a clinically relevant and mechanistically coherent framework for the scalp microbiome-hair axis. We first summarize the composition and physiological functions of the scalp microbiome, including its roles in barrier integrity, immune regulation, and nutrient metabolism. We then compare dysbiosis patterns across androgenetic alopecia, alopecia areata, seborrheic dermatitis-associated hair loss, and folliculitis/scarring alopecia. This comparison identifies a shared pathogenic layer-barrier disruption, innate immune activation, altered lipid or metabolite processing, and impaired anagen support-together with subtype-selective mechanisms, including immune-privilege collapse in alopecia areata, androgen-lipid-microbiome coupling in androgenetic alopecia, Malassezia-centered inflammatory ecology in seborrheic dermatitis, and infection/biofilm-driven injury with fibrosis in scarring disease. We further organize the available evidence into a 3-tier signaling cascade linking upstream microbial triggers to midstream NF-κB, JAK-STAT, PI3K/AKT, and Wnt/β-catenin integration and downstream outcomes such as stem-cell quiescence, dermal papilla dysfunction, vascular regression, immune-privilege loss, and fibrosis. We also evaluate probiotics, postbiotics, and engineered microbial systems, whose potential benefits may arise from coordinated anti-inflammatory, ecological, regenerative, and pro-angiogenic effects. However, translation is limited by methodological heterogeneity, weak causal evidence, strain stability, targeted delivery, manufacturing quality control, and regulatory uncertainty. Overall, this framework clarifies shared and subtype-specific roles of scalp dysbiosis and defines priorities for precision, microbiome-guided alopecia therapy.