Antimicrobial effects of blue light therapy against cutibacterium acnes: optimal dosing and impact of serial treatments.

BACKGROUND: Blue light therapy (BLT) is a Food and Drug Administration cleared modality used in dermatology as an effective treatment of acne. The primary purpose of this study is to determine if there are dose-dependent antimicrobial effects of BLT against Cutibacterium acnes (C. acnes) .

METHODS: A known strain of C. acnes was grown on chocolate agar in a controlled laboratory environment under anaerobic conditions for 1 week. After 1 week, 2-3 colonies of C. acnes were isolated and transferred to broth medium to incubate for 2 or 7 days. Broth vials (treatment arm) then underwent 1 of 6 different blue light dosing treatment regimens and a duplicate broth vial served as a control left open to the same environment. The BLT regimens were a single treatment of 25 J/cm2 , 50 J/cm2 , 75 J/cm2 , 100 J/cm2 , 2 serial treatments of 50 J/cm2 separated by 24 hours, or 2 serial treatments of 75 J/cm2 separated by 24 hours. The Omnilux Blue device (415 nm wavelength) was used for all BLT treatments and delivered, on average, 1.68 ± 0.004 J/min. Following treatment, the control and treatment broth samples were plated on chocolate agar and allowed to grow for 7 days. After 7 days, plates were counted and colony forming units (CFUs) were calculated. Six trials were completed for each BLT dosing regimen based on an a priori power analysis of 6 individual 2-sided t -tests. Comparisons in the primary outcome were made via mixed-effects analysis of variance with replicate as a random effect.

RESULTS: All BLT treatment regimens resulted in significantly fewer CFUs than their aggregate control plate CFUs ( P  < .05 for all). Furthermore, in 2-way comparison of CFUs between BLT treatment groups, a single treatment of 75 J/cm2 did lead to significantly less growth than 25 J/cm2 ( P  = .017) and 50 J/cm2 ( P  = .017). There were no improved antimicrobial effects with serial treatments when comparing 2 doses of 50 J/cm2 with a single dose of 100J/cm2 , nor were 2 doses of 75 J/cm2 more efficacious than 100 J/cm2 . Using the Omnilux Blue device, it took 44.8 minutes to deliver a 75 J/cm2 dose.

CONCLUSION: BLT is an effective antimicrobial agent against this single virulent strain of C. acnes . Treatment dosing of 75 J/cm2 was identified to be the most effective dose per unit time. Serial treatments did not lead to superior antimicrobial effects over a single, high-dose treatment.