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Enhancement in Photoinactivation of Food-borne Pathogenic Bacteria with the Use of Curcumin and Surfactants

Photosensitizers and UV or visible light could photoinactivate bacteria. Curcumin was utilized as a food-grade photosensitizer, and an enhancement of its water-dispersibility, chemical stability, and antimicrobial activity when introduced into surfactant solutions was observed. Stock curcumin-surfactant solutions were prepared by titrating curcumin dissolved in ethanol into either Surfynol 465 or Tween 80 solutions (5 mM sodium citrate buffer). Stock curcumin surfactant solutions with different surfactant concentrations were diluted to 1 µM prior to irradiation. The resulting solutions also had surfactant concentrations below, near, and above their critical micelle concentrations (CMCs) of the respective surfactants. The antimicrobial activity of the curcumin-loaded surfactant solutions was evaluated against Escherichia coli O157: H7 and Listeria innocua after 5 min irradiation with UV-A light (λ= 365 nm). The photoinactivation efficiency of the curcumin surfactant solution depended on the pH of the solution (low > high) and the amount of surfactant present (below CMC ≥ near CMC > above CMC = unencapsulated curcumin). Synergistic antimicrobial activity was observed when Surfynol 465 was present below or near the CMC with curcumin at pH 3.5, which was attributed due to a more effective interaction of the Surfynol 465 with the cell membrane causing it to be permeable. This may have allowed the localization of curcumin inside the cell membrane in ways that promoted photoinactivation of the bacteria. In addition, loosely-bound and unbound extracellular curcumin had significant roles in microbial photoinactivation as the surfactants increased the concentration of curcumin adjacent to the cells.