Folliculitis decalvans is characterized by a persistent, abnormal subepidermal microbiota.

Folliculitis decalvans (FD) is a chronic inflammatory disease of unknown aetiology. Although Staphylococcus aureus, frequently found on lesional skin, is thought to play a causal role, the importance of its involvement remains controversial. To examine the role of S aureus, we compared superficial and subepidermal microbiota in 20 FD patients who had S aureus on lesional skin and in 20 healthy controls using culture techniques and genomic identification, before and after an anti-staphylococcal treatment; we also screened for S aureus virulence factors. When present on lesional skin, S aureus colonized non-lesional and subepidermal skin in 80% of cases. These data imply a break in the epidermal barrier integrity and that an abnormal non-lesional skin microbiota persists in FD. S aureus had no superantigenic toxin in 31% of cases and no toxin specificity. Clinical improvement obtained in most cases upon treatment was associated with the disappearance of S aureus in all studied areas, with an incomplete restoration of normal microbiota and a significant increase in negative bacterial samples. This persistent unbalanced, subepidermal microbiota may act as a reservoir of abnormal flora and explain the chronicity of FD, suggesting new avenues of research to restore normal microbiota.