Pulsed 450 nm blue light significantly inactivates Propionibacterium acnes more than continuous wave blue light

Infection with Propionibacterium acnes is ubiquitous, and drug resistant strains have been on the rise as the use of pharmaceutical antimicrobials continues to engender the emergence of further resistant strains. In previous studies, we showed that treatment with blue light serves as an alternative to pharmaceutical intervention. As a part of our ongoing effort to improve the antimicrobial efficacy of blue light, we studied the effect of pulsed 450 nm light on P. acnes in vitro and compared two pulsed rates with continuous wave irradiation. We irradiated cultures of P. acnes at various irradiances and radiant energies either singly or repeatedly at various time intervals, using printed micro-LEDs, with the goal of finding the lowest combination of irradiance and radiant energy that would yield 100% bacterial suppression. Our results show that treatment with 33% pulsed light gave the best result compared to 20% pulsed wave or continuous wave. Timing irradiation to coincide with the replication cycle of P. acnes produced a significantly better antimicrobial effect. Furthermore, repeated irradiation at 3-h or 4-h interval enabled significant bacterial suppression even at lower irradiances; thus, making single irradiation at high irradiances unnecessary. Moreover, combining repeated irradiation with appropriate duration of treatment and 33% irradiation pulse rate gave optimal 100% [7 log₁₀] bacterial suppression.