Merocyanine 540-mediated photodynamic therapy against Staphylococcus aureus planktonic and biofilm cells

Photodynamic inactivation of Staphylococcus aureus planktonic and biofilm cells by a photosensitizer, merocyanine 540 (MC 540) was investigated. For the planktonic experiments, the antimicrobial activity was enhanced with the increasing light dose, but an increase in light dose could not further improve the antimicrobial activity if the maximum excitation level attainable lay below the necessary minimum threshold level. A complete inactivation was achieved when the excitation level of MC 540 lay somewhere above the threshold level. The relationship between antimicrobial activity and the excitation level of MC 540 revealed that the more MC 540 was excited, the more S. aureus cells were killed. For the biofilm experiments, the antimicrobial activity was enhanced with the increasing light dose. No viable cells were detected when exposed to 15 μg/ml MC 540 and a light dose of 600 J/cm² or 20 μg/ml MC 540 and a light dose of 450 J/cm². The results from both planktonic and biofilm experiments suggest that the antimicrobial activity of photodynamic inactivation against S. aureus is closely related to the excitation level of MC 540.