A thrice-daily application of pulsed blue light at 450 nm at 3-hour intervals over the course of 3 days can suppress Propionibacterium acnes (P acnes), a bacterium responsible for acne vulgaris, study research published in the Journal of Photochemistry & Photobiology, B: Biology suggests.

In this 3-series study, researchers first irradiated P acnes with 3 mW/cm2 irradiance and 33% pulsed blue light at 450 nm. Radiant exposure of bacteria reached 5 J/cm2, 10 J/cm2, or 20 J/cm2, and bacteria were irradiated 3 times at 0 hours, 3 hours, 6 hours, or 4 times at 0 hours, 4 hours, 24 hours, 48 hours, or 0 hours, 3 hours, 6 hours, or 24 hours. In the series 2 experiments, researchers irradiated bacteria with 2 mW/cm2 irradiance, as well as radiant exposure of 3.6 J/cm2 or 5 J/cm2. Irradiation was applied 3 times per day for 3 days or 3 times a day for 4 days. The pulse blue light rate was maintained at 33% into the series 3 experiments, and series 3 also used 2 mW/cm2 irradiance.

Irradiation led to dose-dependent reductions in bacterial survival compared with control patients. Bacterial survival was lowest with 20 J/cm2 radiant exposure (0.5%) vs 5 J/cm2 (89%). When bacteria were irradiated 4 times at 0 hours, 3 hours, 6 hours, and 24 hours, the percent bacterial survival was 0% with 20 J/cm2 radiant exposure. Similar percent bacteria survival was found when bacteria were irradiated at 0 hours, 3 hours, and 6 hours (61%, 12%, and 0.2%, respectively).

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In the series 2 experiments, repeated irradiation 3 times per day at 5 J/cm2 for either 3 or 4 days led to 100% bacterial suppression. The use of 33% pulsed 450 nm light at 2 mW/cm2 irradiance and 5 J/cm2 radiant exposure thrice daily over the course of 3 days was associated with 100% suppression in bacteria and 0% survival.

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A potential limitation of the study was its in vitro nature, which may limit the ability to identify the full extent of pulsed blue light on acne bacteria in patients with acne.

The researchers wrote that the results of the study “clearly show that longer duration of exposure to blue light potentiates its antibacterial effect, perhaps more so than a shorter exposure at higher irradiances and radiant exposures.”

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Bumah VV, Masson-Meyers DS, Tong W, Castel C, Enwemeka CS. Optimizing the bactericidal effect of pulsed blue light on Propionibacterium acnes – A correlative fluorescence spectroscopy study. J Photochem Photobiol B. 2020;202:111701.