What microbes live in the human hair follicle and what is their role?

The hair follicle is less acidic, moister and more protected from ultraviolet radiation when compared with the rest of the skin. As a consequence, the microorganisms that live within hair follicles (the hair follicle microbiome) differ from those living on the surface of the skin. However, these have not been as comprehensively studied. In this review, the authors, from Germany, the UK and the USA summarize our understanding regarding the influence of the hair follicle microbiome in health and disease. An average person has roughly two million hair follicles. With that in mind, it is remarkable that folliculitis (infection of the hair follicle) is so infrequent. This is primarily achieved by the production of specific antimicrobial peptides by keratinocytes and immune cells. The microbiome, immune system and environment are in constant communication, and mutually regulate each other. Like a well‐tended garden, the hair follicle maintains a carefully regulated mix of bacteria, fungi and viruses. However, changes in the microbial flora are occasionally seen in disease. For example, the main colonising bacteria, Propionibacterium acnes , are implicated in acne. Although it is uncertain how much these contribute to development of the disease. Likewise, bacterial changes are seen in hidradenitis suppurativa, a related condition that causes boil‐like lesions, especially in the armpits and groin. Similarly, changes in mites and fungi have also been observed. Most of us carry small mites ( Demodex species) in hair follicles, mainly in greasier areas, such as facial skin. The number of these mites increases as we get older and they may be linked with conditions such as rosacea. Likewise, species of the fungi Malassezia, which frequently colonise the hair follicle, are linked with conditions such as seborrheic dermatitis and dandruff. Apart from direct infections such as bacterial and fungal folliculitis, the microbiome is altered in several hair loss disorders, such as androgenetic alopecia and alopecia areata. However, these can also be beneficial. Staphylococcus epidermidis , a normal skin and hair follicle inhabitant may protect against hair loss in alopecia areata. Nevertheless, how and how much changes in the microbiome contribute to the development of diseases remain a topic of great debate. For that reason, the authors conclude that our understanding of the hair follicle microbiome in health and disease is still in its infancy. Part of the problem is that the methods for surveying the microbiome, such as skin swabs, only obtain samples from the surface of the skin. Future studies, sampling deeper regions of the hair follicle, using techniques such as biopsies and organ cultures, will be essential to advance current notions and concepts. Linked Article:   Lousada et al. Br J Dermatol 2021; 184 :802–815.