Abstract
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.
Highlights
Light induces excitation of the photosensitizer (PS) molecules to the singlet excited state which can be converted into the triplet excited compound
This section will include two components: (1) the evaluation of the rose Bengal disodium salt (RB) photodynamic effect against S. aureus bacterium and its mechanisms involved by quenching the main reactive oxygen species (ROS) produced and (2) the spectrophotometric study to reveal the possible interactions between the tryptophan and the dye and/or the dye and the bacterium
The RB sensitization procedure on S. aureus inactivation was performed in phosphate buffer instead of the culture medium because this latter is providing additional protection as the antioxidants themselves
Summary
Light induces excitation of the photosensitizer (PS) molecules to the singlet excited state which can be converted into the triplet excited compound. The RB, is a cyclic compound that contains three aromatic rings in a linear arrangement and an oxygen atom in the centre of the ring, which absorbs light in the visible spectrum (Costa et al 2010), exhibiting intense absorption bands in the green area of the visible spectrum (480–550 nm) (DeRosa and Crutchley 2002) with maximum absorption around 549 nm in diluted aqueous solutions (Xu and Neckers 1987) According to these properties, we investigated in this current study the mechanisms of photodynamic action of this PS, against a Gram-positive bacterium (S. aureus), used as a pathogenic microorganism model. Singlet oxygen reacts primarily with five amino acids, which are tryptophan, histidine, tyrosine, methionine and cysteine to form peroxides (Michaeli and Feitelson 1995 and Michaeli and Feitelson 1997) and in high yields especially for tryptophan, tyrosine and histidine residues, both free and on proteins (Gracanin et al 2009)
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