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

Abstract

BackgroundBlue light therapy (BLT) is an Food & 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). MethodsA 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 one 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, two serial treatments of 50 J/cm2 separated by 24 hours, or two 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 two-sided t-tests. Comparisons in the primary outcome were made via mixed effects ANOVA with replicate as a random effect. ResultsAll BLT treatment regimens resulted in significantly fewer CFUs than their aggregate control plate CFUs (p<0.05 for all). Further, 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=0.017) and 50 J/cm2 (p=0.017).There were no improved antimicrobial effects with serial treatments when comparing two 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. ConclusionsBlue light therapy 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.