Staphylococcus aureus photodynamic inactivation mechanisms by rose bengal: use of antioxidants and spectroscopic study

This study aims to follow the photodynamic and spectroscopic properties of dianionic rose Bengal disodium salt (RB) on Staphylococus aureus (S. aureus) in phosphate buffer solution (PBS) at pH 7.3. It focused on: (1) the effect of several reactive oxygen species (ROS) antioxidants used [such as sodium azide (NaN3), l-tryptophan (l-Trp) and d-mannitol] on the RB photodynamic efficiency as a mean to identify the main ROS attributed, and (2) the possible interactions of the RB with the important singlet oxygen quencher used namely tryptophan and/or between the dye and the bacteria S. aureus thanks to a spectroscopic study. The results showed that 20 µM of RB and 10 min of visible light (50 mW/cm2) with a light fluence dose of 30 J/cm2 are crucial for a good photodynamic action, achieving a reduction of 79.4% in the viability. Rose Bengal photodynamic action was in part inhibited by D-mannitol and l-Trp, indicating the mediation by.OH and 1O2, respectively. The high inhibition of the RB activity against S. aureus by l-Trp is not due only to its singlet oxygen quencher ability but it is mainly due to the interaction between RB and l-Trp as shown spectrophotometrically.