Effective inactivation of food pathogens Listeria monocytogenes and Salmonella enterica by combined treatment of hypericin-based photosensitization and high power pulsed light

Abstract

Aims: The aim of this study was to evaluate the inactivation efficiency of Listeria monocytogenes ATC(L3) C 7644 and Salmonella enterica serovar Typhimurium strain DS88 by combined treatment of hypericin (Hyp)-based photosensitization and high power pulsed light (HPPL).

Methods and results: Cells were incubated with Hyp (1 × 10(-5) or 1 × 10(-7) mol l(-1)) in PBS and illuminated with a light λ = 585 nm. For the combined treatment, bacteria were, after photosensitization, exposed to 350 pulses of HPPL (UV light dose = 0·023 J cm(-2)). Fluorescence measurements were performed to evaluate optimal time for cell-Hyp interaction. Results indicate that Hyp tends to bind both Listeria and Salmonella. After photosensitization treatment, Listeria population was reduced 7 log, whereas Salmonella was inactivated just 1 log. Electron photomicrograps of Salmonella and Listeria confirmed that photosensitization induced total collapse of the Listeria cell wall, but not that of Salmonella. After combined photosensitization-HPPL treatment, the population of Listeria was diminished by 7 log and Salmonella by 6·7 log.

Conclusions: Listeria can be effectively inactivated by Hyp-based photosensitization (7 log), whereas Salmonella is more resistant to photosensitization and can be inactivated just by 1 log in vitro. Combined treatment of photosensitization and pulsed light inactivates effectively (6·7-7 log) both the Gram-positive and the more resistant to photosensitization Gram-negative bacteria.

Significance and impact of the study: A new approach to combat Gram-positive and Gram-negative bacteria is proposed, combining photosensitization with high power pulsed light.