Growth temperature influences the resistance of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium on lettuce to X-ray irradiation.

This study aimed to investigate the effect of different growth temperatures on the resistance of Escherichia coli O157:H7 and Salmonella Typhimurium to low-energy X-ray irradiation. Irradiation of contaminated phosphate-buffered saline with 0.6 kGy X-ray decreased the counts of E. coli O157:H7 cultured at 37 °C to below the detection limit (<1.0 colony-forming unit (CFU)/mL) and those of E. coli O157:H7 cultured at 25 and 15 °C by 4.82 and 4.45 log CFU/mL, respectively. The viable counts of S. Typhimurium cultured at 37, 25, and 15 °C in phosphate-buffered saline decreased by 3.56, 3.08, and 2.75 log CFU/mL, respectively, after irradiation with 0.6 kGy X-ray. Irradiation of contaminated lettuce with 0.4 kGy decreased the counts of E. coli O157:H7 cultured at 37, 25, and 15 °C by 3.97, 3.45, and 3.10 log CFU/cm2, respectively, and those of S. Typhimurium by 4.41, 3.84, and 3.40 log CFU/cm2, respectively. Growth temperature influenced pathogen resistance to X-ray irradiation by modulating cellular membrane and DNA integrity, intracellular enzyme activity, and efflux pump function. The results of this study suggest that the stress resistance status of pathogenic bacteria cultured at different growth temperatures should be considered for the application of X-ray irradiation for fresh produce sterilization.