Different photodynamic effects of blue light with and without riboflavin on methicillin-resistant Staphylococcus aureus (MRSA) and human keratinocytes in vitro

Methicillin-resistant Staphylococcus aureus (MRSA) is an important cause of infections in humans. Photodynamic therapy using blue light (450 nm) could possibly be used to reduce MRSA on different human tissue surfaces without killing the human cells. It could be less harmful than 300–400 nm light or common disinfectants. We applied blue light ± riboflavin (RF) to MRSA and keratinocytes, in an in vitro liquid layer model, and compared the effect to elimination using common disinfection fluids. MRSA dilutions (8 × 10⁵/mL) in wells were exposed to blue light (450 nm) ± RF at four separate doses (15, 30, 56, and 84 J/cm²). Treated samples were cultivated on blood agar plates and the colony forming units (CFU) determined. Adherent human cells were cultivated (1 × 10⁴/mL) and treated in the same way. The cell activity was then measured by Cell Titer Blue assay after 24- and 48-h growth. The tested disinfectants were chlorhexidine and hydrogen peroxide. Blue light alone (84 J/cm²) eliminated 70% of MRSA. This dose and riboflavin eradicated 99–100% of MRSA. Keratinocytes were not affected by blue light alone at any dose. A dose of 30 J/cm² in riboflavin solution inactivated keratinocytes completely. Disinfectants inactivated all cells. Blue light alone at 450 nm can eliminate MRSA without inactivation of human keratinocytes. Hence, a high dose of blue light could perhaps be used to treat bacterial infections without loss of human skin cells. Photodynamic therapy using riboflavin and blue light should be explored further as it may perhaps be possible to exploit in treatment of skin diseases associated with keratinocyte hyperproliferation.