Heat-killed Propionibacterium acnes is capable of inducing inflammatory responses in skin

: The etiology of acne is a complex process, and acne is one of the most common skin disorders affecting millions of people. The pathogenesis of acne is closely associated with the bacterium, Propionibacterium acnes which was previously known as Corynebacterium parvum. Both viable and non‐viable P. acnes/C. parvum have been shown to induce an immunostimulatory effect in vivo, suggesting that even dead bacteria continue to activate an inflammatory response. Acne treatments with lasers or devices, induce a bactericidal effect through heat generation which may not address the immunogenic activity of P. acnes and the resulting acne inflammation. Therefore, we sought to determine whether killed P. acnes is capable of inducing an inflammatory response and therefore could be a contributing factor in acne. Direct heat treatment of P. acnes cultures with temperatures ranging from 50°C to 80°C reduced P. acnes viability. Both viable and heat‐killed P. acnes activated the p38 MAP kinase and its downstream substrate Hsp27. Stimulating keratinocytes with normal and heat‐inactivated P. acnes resulted in an induction of proinflammatory nitric oxide and IL‐8 production. Thus killed P. acnes is capable of inducing inflammation in skin suggesting that therapies that have both bactericidal and anti‐inflammatory effects may result in a more effective treatment of patients with acne than treatments that are bactericidal alone.