Is Cutibacterium (previously Propionibacterium) acnes a potential pathogenic factor in the aetiology of the skin disease progressive macular hypomelanosis?

Progressive macular hypomelanosis (PMH) is a skin condition that normally causes symmetrically distributed hypopigmented macules on the front and back of the trunk, but rarely the face. To date, the pathophysiology of the condition is not well understood, but a role for the anaerobic skin bacterium Cutibacterium (previously Propionibacterium) acnes in the development of the disease has been proposed due to its sole presence within lesional, but not normal peri-lesional, skin. The success of antimicrobials in the treatment of PMH also provides circumstantial evidence that this association may be causal, although this is still to be proven. More recent culture and metagenomic typing studies indicate that strains of C. acnes subsp. elongatum (type III) may be important in the aetiology of the condition, which would help to explain why PMH does not normally affect the face since such strains are rarely present there, and why no association between this condition and acne vulgaris is found; acne appears to primarily involve type IA strains from C. acnes subsp. acnes (type I). In this review, we summarize current knowledge on the relationship between C. acnes and PMH, and re-examine previous challenges to the view that the bacterium plays a role in the condition against the backdrop of newly emerged data.