Background  Blue light in the 400–420 nm range has been shown to reduce the levels of Propionibacterium acnes (P. acnes) in the skin. P. acnes has been postulated to be a critical trigger for inflammatory acne. Thus, treatment with 420 nm-intense pulsed light should reduce inflammatory activity in acne.

Aim  To evaluate the clinical and histological effects of 420 nm-intense pulsed light treatment on acne in animal model.

Method  Inflammation acne animal model was constructed by intradermal injection of P. acnes of rat auricular. Levels of tumour necrosis factor alpha (TNF-α) and matrix metalloproteinase 2 (MMP-2), markers of inflammation implicated in acne, were assessed in treated and untreated animals by immunohistochemistry and quantitative polymerase chain reaction (PCR).

Result  Treatment with 420 nm intense pulsed light led to marked improvement after 6 biweekly treatments. Immunohistochemistry and PCR showed that TNF-α and MMP-2 levels correlated with the extent of acneiform activity and were reduced by treatment with 420 nm light.

Conclusion  A 420-nm intense pulsed light may exert its beneficial effects on inflammatory acne by reducing the levels of P. acnes and secondarily reducing inflammation induced by the bacteria.