Optimization of the Antimicrobial Effect of Blue Light on Methicillin-Resistant Staphylococcus aureus (MRSA) In Vitro

Background and ObjectiveIn previous studies, we showed that irradiation with 405nm or 470nm 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 MethodsWe cultured 3x10(6) and 5x10(6) CFU/ml USA300 strain of MRSA and then irradiated each plate with varying fluences of 1-60J/cm(2) of 405nm or 470nm light, either once or twice at 6hours intervals. Next, we plated 7x10(6) CFU/ml and irradiated it with 45, 50, 55, or 60J/cm(2) fluence, once, twice, or thrice at the same 6hours intervals. In a third experiment, the same culture density was irradiated with 0, 165, 180, 220, or 240J/cm(2), either once, twice, or thrice. ResultsIrradiation with either wavelength significantly reduced the bacterial colonies regardless of bacterial density (P<0.05). At 3x10(6) CFU/ml density, nearly 40% and 50% growth of MRSA were suppressed with as little as 3J/cm(2) of 405nm and 470nm wavelengths, respectively. Moreover, 100% of the colonies were suppressed with a single exposure to 55 or 60J/cm(2) of 470nm light or double treatment with 50, 55, or 60J/cm(2) of 405nm wavelength. At 5x10(6) CFU/ml density, irradiating twice with 50, 55, or 60J/cm(2) of either wavelength suppressed bacterial growth completely, lower fluences did not. The denser 7x10(6) CFU/ml culture required higher doses to achieve 100% suppression, either one shot with 220J/cm(2) of 470nm light or two shots of the same dose using 405nm. ConclusionThe 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 7x10(6) CFU/ml. Lasers Surg. Med. 47:266-272, 2015. (c) 2015 Wiley Periodicals, Inc.