PHOTODYNAMIC INACTIVATION MEDIATED BY ROSE BENGAL AND ERYTHROSINE INDUCE CHANGES ON PHYSICOCHEMICAL PROPERTIES OF Escherichia coli SURFACE

Photodynamic inactivation is a technique that consists in the excitation of a nontoxic photosensitizer by means of light of an appropriate wavelength. As a result, reactive oxygen species are generated, which are responsible to induce bacterial inactivation. This method has become an interesting alternative for controlling pathogenic bacteria in food field, once it is cheap, eco-friendly and less likely to cause development of antimicrobial resistance. This study evaluated the antimicrobial effectiveness and the changes on physicochemical properties of Escherichia coli surface subjected to photodynamic inactivation mediated by rose bengal and erythrosine. A bacterial suspension (107 CFU/ mL) was transferred to 24-well microplates containing increasing concentration of rose bengal or erythrosine. Following, the plates were incubated in the dark (10 min) and irradiated with a green LED light source (530 nm) for more 10 min. The number of survival cells were determined after 10-fold serial dilution and plating onto Plate Count Agar. The physicochemical changes on bacterial surface induced by photodynamic treatment were assessed by the sessile drop contact angle method using a video-based optical contact angle measurement instrument. The solvents applied were water, formamide and -bromonaphthalene. Control cultures received green light alone or photosensitizer alone. No changes on bacterial survival and surface physicochemical properties was observed when cells were exposed only to the light source or only to the compounds alone. On the other hand, when 50 mol/ L of erythrosine or 20 mol/ L of rose bengal was combined with irradiation, no cultivatable cells were recovered. Regarding to the physicochemical characteristics, E. coli was predominantly hydrophilic (24.48 mJ/ m2), electron donor and less prone to accept electrons. The apolar component of surface tension was increased from 24.41 mJ/ m2 (control) to 30.98 and 30.28 mJ/ m2 when erythrosine and rose bengal was employed, respectively. At the same time, the polar component was decreased from 32.63 mJ/ m2 to 25.19 and 24.66 mJ/ m2 for treatments with erythrosine and rose bengal, respectively. The results obtained demonstrated that rose bengal and erythrosine present antimicrobial properties against E. coli upon illumination with LED light source. Moreover, it can be presumed that such activity may be related to changes on physicochemical properties of cell surface.