Visible 405 nm SLD light photo-destroys methicillin-resistant Staphylococcus aureus (MRSA) in vitro

Infections with MRSA remain a growing public health concern, prompting the need to explore alternative treatments instead of the on-going effort to develop stronger drug-based therapies. We studied the effect of 405 nm blue light on two strains of MRSA-US-300 strain of CA-MRSA and the IS853 strain of HA-MRSA-in vitro.

We cultured and plated each strain, following which bacteria colonies were irradiated with 0, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 25, 30, 35, 40, 45, 50, 55, or 60 J cm(-2) energy densities-just once-using a Solaris superluminous diode (SLD) device. Specimens were incubated at 35 degrees C for 24 hours. Then, digital images obtained were quantified to obtain colony counts and the aggregate area occupied by bacteria colonies.

Blue light irradiation produced a statistically significant dose-dependent reduction in both the number and the aggregate area of colonies formed by each bacteria strain (P<0.001). Maximum eradication of the US-300 (92.1%) and the IS-853 colonies (93.5%) was achieved within 9.2 and 8.4 minutes of exposure, respectively. The longer the irradiation the more bacteria were eradicated. However, the effect was non-linear as increases of energy densities between 1.0 and 15 J cm(-2) resulted in more bacteria death than similar increases between 15 and 60 J cm(-2).

At low doses, blue light photo-destroys HA-MRSA and CA-MRSA in vitro; raising the prospect that phototherapy may be an effective clinical tool in the on-going effort to stem MRSA infections.