Blue 470-nm light kills methicillin-resistant Staphylococcus aureus (MRSA) in vitro

In a previous study, we showed that 405-nm light photo-destroys methicillin-resistant Staphylococcus aureus (MRSA). The 390-420 nm spectral width of the 405-nm superluminous diode (SLD) source may raise safety concerns in clinical practice, because of the trace of ultraviolet (UV) light within the spectrum.

Here we report the effect of a different wavelength of blue light, one that has no trace of UV, on two strains of MRSA--the US-300 strain of CA-MRSA and the IS-853 strain of HA-MRSA--in vitro.

We cultured and plated each strain, and then irradiated each plate with 0, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 25, 30, 35, 40, 45, 50, 55, or 60 J/cm2 of energy a single time, using a 470-nm SLD phototherapy device. The irradiated specimens were then incubated at 35 degrees C for 24 h. Subsequently, digital images were made and quantified to obtain colony counts and the aggregate area occupied by bacteria.

Photo-irradiation produced a statistically significant dose-dependent reduction in both the number and the aggregate area of colonies formed by each strain (p < 0.001). The higher the dose the more bacteria were killed, but the effect was not linear, and was more impressive at lower doses than at higher doses. Nearly 30% of both strains was killed with as little as 3 J/cm2 of energy. As much as 90.4% of the US-300 and the IS-853 colonies, respectively, were killed with an energy density of 55 J/cm2. This same dose eradicated 91.7% and 94.8% of the aggregate area of the US-300 and the IS-853 strains, respectively.

At practical dose ranges, 470-nm blue light kills HA-MRSA and CA-MRSA in vitro, suggesting that a similar bactericidal effect may be attained in human cases of cutaneous and subcutaneous MRSA infections.