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 (NaN 3 ), 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/cm 2 ) with a light fluence dose of 30 J/cm 2 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 1 O 2 , 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.