Susceptibility of Listeria monocytogenes and Staphylococcus aureus Grown Under High Salt Conditions to X-ray Irradiation

Among ionizing radiations, X-ray irradiation has been proposed as an alternative approach to inactivate various pathogens. However, erroneous conclusions and misinterpretations of results may adversely affect its application if the factors affecting X-ray resistance of foodborne pathogens are not considered during X-ray irradiation studies. This study investigated the effect of prior exposure to high salt conditions on the susceptibility of two gram-positive pathogenic bacteria, Listeria monocytogenes and Staphylococcus aureus, to X-ray irradiation. Accordingly, a bacterial cocktail containing three strains of each pathogen was exposed to osmotic stress by suspension in a saturated 15% w/v NaCl solution (water activity = 0.88) for 0 (control), 4, 12, and 24 h. Subsequent X-ray irradiation at 0.8 kGy resulted in reduced counts of both L. monocytogenes (1.55, 2.32, 3.06, and 3.38 log CFU/mL, respectively) and S. aureus (2.44, 2.98, 3.58, and 4.07 log CFU/mL, respectively) held in NaCl solution for 0, 4, 12, and 24 h. The results of this study revealed that exposure to high salt conditions decreased the resistance of L. monocytogenes and S. aureus to X-ray irradiation. Mechanistic studies further identified the damage associated with reactive oxygen species and interrupted peptidoglycan synthesis as major factors contributing to the changes in sensitivity to X-ray irradiation under high salt conditions. Thus, X-ray irradiation may be beneficial for reducing the viability of gram-positive pathogens that are highly resistant to NaCl without heterogeneous stress adaptation.