Propionibacterium acnes, a major member of normal skin microbiota, is subdivided into 6 phylotypes: IA1, IA2, IB, IC, II and III. This study investigated P. acnes subgroups on the face and back in patients with severe acne and in healthy controls. In 71.4% of patients with severe acne, P. acnes phylotypes were identical on the face and back, whereas this was the case in only 45.5% of healthy controls. The healthy group carried phylotypes IA1 (39.1%) and II (43.4%), whereas the acne group carried a high predominance of IA1 (84.4%), especially on the back (95.6%). In addition, the single-locus sequence typing (SLST) method revealed A1 to be the predominant type on the back of patients with acne, compared with a wide diversity in the healthy group. We report here that severity of acne on the back is associated with loss of diversity of P. acnes phylotype, with a major predominance of phylotype IA1. The change in balance of cutaneous P. acnes subgroups might be an inducing factor in the activation of P. acnes, which could trigger inflammation.

Hyper keratinization was higher in healthy skin of adult women with acne compared with controls, confirming that microcomedo is crucial in the development of acne lesions. We also demonstrate that the repartition of comedones and microcomedones is inhomogeneous with a great number in the mandibular area where acne lesions are located.

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Background Acne has long been understood as a multifactorial chronic inflammatory disease of the pilosebaceous follicle, where Cutibacterium acnes (subdivided into six main phylotypes) is a crucial factor. In parallel, the loss of microbial diversity among the skin commensal communities has recently been shown as often accompanied by inflammatory skin disorders. Objective This study investigated the association of C. acnes phylotype diversity loss and the impact on Innate Immune System (IIS) activation. Methods The IIS response of skin after incubation with phylotypes IA1, II or III individually and with the combination of IA1 + II + III phylotypes, was studied in an in vitro skin explant system. The inflammatory response was monitored by immunohistochemistry and ELISA assays, targeting a selection of Innate Immune Markers (IIMs) (IL‐6, IL‐8, IL‐10, IL‐17, TGF‐β). Results IIMs were significantly upregulated in skin when being incubated with phylotype IA1 alone compared with the combination IA1 + II + III. In parallel, ELISA assays confirmed these results in supernatants for IL‐17, IL‐8 and IL‐10. Conclusion We identify the loss of C. acnes phylotype diversity as a trigger for IIS activation, leading to cutaneous inflammation. These innovative data underline the possibility to set up new approaches to treat acne. Indeed, maintaining the balance between the different phylotypes of C. acnes may be an interesting target for the development of drugs.