Optimization of the antimicrobial effect of blue light on methicillin‐resistant Staphylococcus aureus (MRSA) in vitro

Background and Objective In previous studies, we showed that irradiation with 405 nm or 470 nm light suppresses up to 92% methicillin‐resistant Staphylococcus aureus (MRSA) growth in vitro and that the remaining bacteria re‐colonize. In this study, the aim was to develop a protocol that yields 100% MRSA growth suppression. Materials and Methods We cultured 3 × 10 6 and 5 × 10 6 CFU/ml USA300 strain of MRSA and then irradiated each plate with varying fluences of 1–60 J/cm 2 of 405 nm or 470 nm light, either once or twice at 6 hours intervals. Next, we plated 7 × 10 6 CFU/ml and irradiated it with 45, 50, 55, or 60 J/cm 2 fluence, once, twice, or thrice at the same 6 hours intervals. In a third experiment, the same culture density was irradiated with 0, 165, 180, 220, or 240 J/cm 2 , either once, twice, or thrice. Results Irradiation with either wavelength significantly reduced the bacterial colonies regardless of bacterial density ( P <  0.05). At 3 × 10 6 CFU/ml density, nearly 40% and 50% growth of MRSA were suppressed with as little as 3 J/cm 2 of 405 nm and 470 nm wavelengths, respectively. Moreover, 100% of the colonies were suppressed with a single exposure to 55 or 60 J/cm 2 of 470 nm light or double treatment with 50, 55, or 60 J/cm 2 of 405 nm wavelength. At 5 × 10 6 CFU/ml density, irradiating twice with 50, 55, or 60 J/cm 2 of either wavelength suppressed bacterial growth completely, lower fluences did not. The denser 7 × 10 6 CFU/ml culture required higher doses to achieve 100% suppression, either one shot with 220 J/cm 2 of 470 nm light or two shots of the same dose using 405 nm. Conclusion The bactericidal effect of blue light can be optimized to yield 100% bacterial growth suppression, but with relatively high fluences for dense bacterial cultures, such as 7 × 10 6 CFU/ml. Lasers Surg. Med. 47:266–272, 2015. © 2015 Wiley Periodicals, Inc.