Photodynamic Inactivation of Food Pathogen Listeria monocytogenes

The aim of this study is to examine the possibility to inactivate food pathogen Listeria monocytogenes by nonthermal antimicrobial treatment – photosensitization. L. monocytogenes was incubated with 5-aminolevulinic acid (ALA) (7.5 mM) for 0–2 h to produce endogenous photosensitizers and then illuminated with visible light. The LED-based light source used for the illumination of L. monocytogenes emitted light at l=400 nm with energy density of 20 mW/cm. The illumination time varied from 0 to 20 min, and a total energy dose reached 0–24 J/cm. The obtained results reveal that L. monocytogenes can effectively produce endogenous porphyrins after incubation with 7.5 mMALA. Subsequent illumination of cells with visible light significantly decreased their viability in vitro (4 log). After adhesion of Listeria to the surface of packaging material and following photosensitization, the surface-attached bacterial population was inactivated by 3.7 log. In addition, most resistant Listeria biofilms are susceptible to this treatment. Their inactivation reached 3.1 log under certain experimental conditions. The cells and biofilms of Gram-positive bacteria L. monocytogenes ATCL3C 7644 could be effectively inactivated by ALA-based photosensitization in the solution as well as adhered onto the surface of packaging material in a nonthermal way.