STRUCTURAL CHANGES IN LISTERIA MONOCYTOGENES TREATED WITH GAMMA RADIATION, PULSED ELECTRIC FIELD AND ULTRA-HIGH PRESSURE

Understanding the mode of action of gamma (gamma) radiation, pulsed electric field (PEF) and ultra-high pressure (UHP) against targeted pathogens should help improve implementation of these technologies. Listeria monocytogenes Scott A was treated with gamma radiation, PEF and UHP at doses that caused equal lethality (3-log inactivation). These doses were 0.57 kGy at 5C (gamma radiation), 30 kV/cm at 22C for 216 mu s (PEF) and 400 MPa at 24 +/- 1C for 6 min (UHP). When structural changes in cells of L. monocytogenes were compared using transmission electron microscopy (TEM), the nucleoid region of gamma-irradiated cells appeared to have spread throughout the cytoplasm. UHP-treated cells showed aggregated cytoplasm, indicating extensive protein denaturing. Structural differences between PEF-treated and untreated cells were minimal, as revealed by TEM. Electrophoresis of genomic DNA from gamma-irradiated L. monocytogenes showed considerable fragmentation. These DNA lesions were not detected when L. monocytogenes was treated with PEF or UHP. When irradiated cells were analyzed by pulsed field gel electrophoresis, the banding pattern changed into smearing. In conclusion, gamma radiation, PEF and UHP targeted different loci in the cell, and thus synergy between these treatments against L. monocytogenes is likely.