Enzyme-mediated photoinactivation of Enterococcus faecalis using Rose Bengal-acetate

Rose Bengal-acetate (RB-Ac) is a pro-photosensitizer claimed to diffuse into target cells, where the acetate groups are hydrolyzed and the photosensitizing properties of Rose Bengal (RB) are restored. Despite promising results on tumor cells, the interaction of RB-Ac with bacteria has never been investigated. This study aimed to assess the interaction of RB-Ac with Enterococcus faecalis and to evaluate its potential use in antimicrobial photodynamic therapy (aPDT). Spectrofluorometry was used to assess the ability of E. faecalis to hydrolyze the RB-Ac compound. Fluorescence microscopy was employed to observe the distribution and to evaluate the cellular uptake of the RB produced. The antibacterial efficiency of RB-Ac-mediated aPDT was assessed by flow cytometry in combination with the LIVE/DEAD® staining. Results showed that RB-Ac was successfully hydrolyzed in the presence of E. faecalis cells. The RB produced appeared to incorporate the membrane of bacteria. Higher concentrations of RB-Ac resulted in higher incorporation of RB. The blue-light irradiation of RB-Ac-treated samples significantly reduced bacterial viability. Less than 0.01% of E. faecalis survived after incubation with 200 μM RB-Ac during 900 min and blue-light activation. The current report indicates that E. faecalis cells can hydrolyze the RB-Ac compound to produce active RB. The use of RB-Ac did not appear to allow cytoplasmic internalization of the RB produced, which rather incorporated the membrane bilayers of E. faecalis. The use of RB-Ac did not provide additional advantages over RB in terms of PS localization. Nonetheless, sufficient RB was produced and incorporated into the membranes of bacteria to elicit effective aPDT.