The Scalp Microbiome and Hair Loss: What the Bacterial Research Suggests

The scalp has its own microbiome distinct from the skin elsewhere. AGA scalp shows measurable differences in microbial composition from non-balding scalp. The clinical significance is still being established.

The scalp microbiome is a rapidly evolving research area that intersects with AGA through the perifollicular inflammation pathway. While the evidence is not yet at the level of actionable clinical recommendations, the microbiome-AGA connection has mechanistic coherence and is driving research into adjunctive approaches.

Scalp microbial communities

The scalp is a sebaceous-rich environment that selectively enriches lipophilic organisms. The dominant scalp microbiota includes Malassezia species (the primary scalp fungi), Cutibacterium acnes (formerly Propionibacterium acnes), Staphylococcus epidermidis, and several other bacteria in lower abundance. This community is relatively stable within individuals but varies between individuals and is influenced by sebum production rate, washing frequency, and climate.

AGA microbiome differences

A small number of 16S rRNA sequencing studies comparing scalp microbiome between AGA and non-AGA men have found altered Malassezia species ratios and differences in bacterial diversity between balding and non-balding scalp regions. Specifically, some studies find relatively increased Malassezia globosa (a more lipolytic species associated with inflammatory scalp conditions) in AGA scalp. The increased sebum production of androgen-stimulated AGA scalp provides an enriched substrate for these organisms.

The inflammation link

The mechanism connecting scalp dysbiosis to AGA is primarily inflammatory. Malassezia lipases metabolise sebum triglycerides into free fatty acids — including oleic acid, which can penetrate the follicle wall and trigger keratinocyte NLRP3 inflammasome activation and IL-1 secretion. This inflammatory cascade contributes to the perifollicular lymphocytic infiltrates documented in AGA histology. Ketoconazole shampoo's benefit in AGA is partly attributed to its control of Malassezia overgrowth and the resulting reduction in proinflammatory fatty acid production.

Limitations of current evidence

Microbiome-AGA research is currently limited by small sample sizes, variable methodologies, lack of longitudinal data, and the challenge of distinguishing cause from effect. The scalp dysbiosis observed in AGA may be a consequence of the altered sebaceous and inflammatory environment of AGA rather than a primary cause of hair loss. Controlling for confounders (sebum production, washing frequency, climate) is methodologically challenging.