A low acriflavine dose strongly potentiates the antimicrobial effect of blue light on Staphylococcus aureus

Staphylococcus aureus infections are difficult to treat in chronic wounds due to biofilm formation and are frequently compounded by antibiotic resistance, necessitating the development of alternative therapeutic approaches. This study investigated the antimicrobial effect of 470 nm blue light, alone and in combination with the photosensitizer, acriflavine, against S. aureus. Planktonic cells and preformed biofilms of S. aureus SH1000 (methicillin-sensitive) and BH1CC (methicillin-resistant) strains were exposed to 470 nm blue light at varying intensities. A reduction of 4.9 and 5.3 log₁₀ was observed in the viability of BH1CC and SH1000 planktonic cells respectively when exposed to blue light at 28 mW cm^-2 for 4 h compared to unexposed cells. The effectiveness of blue light inactivation was reduced at 14 and 7 mW cm^-2, and no inactivation was observed at 3.5 mW cm^-2. Exposure to a combination of 5 μM acriflavine and blue light (3.5 mW cm^-2) significantly reduced BH1CC viability by 6 log₁₀ (p = 0.0079) when compared to blue light alone after 1 h. No SH1000 cells survived 1 h exposure to 3.5 and 1.75 mW cm^-2 combined with 5 μM acriflavine. Incubation of S. aureus strains with any of the tested concentrations of acriflavine in the dark produced no loss of viability, confirming the synergistic action of blue light combined with acriflavine. These results demonstrate that 470 nm blue light is lethal to S. aureus even at very low intensities and that this antimicrobial activity can be significantly enhanced by acriflavine at much lower concentrations than previously reported. These data also suggest that the antimicrobial mode of action for acriflavine is likely to be at least partly light mediated, a finding that has not previously been recognised.