Effect of mild heat-stress at 42 °C on X-ray resistance in Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes, and analysis of cross-protection mechanism

초록

Unintentional exposure to sublethal mild heat stress prior to disinfection processing may increase bacterial resistance, potentially reducing the effectiveness of subsequent microbial inactivation. In this study, we investigated the changes in resistance to X-ray irradiation observed in three major foodborne pathogens, namely Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes, in tryptic soy broth without dextrose (TSB w/o D) as well as in commercial pasteurized milk, a real food matrix, after mild heat stress was applied at 42 degrees C for 30, 60, and 90 min. At the X-ray dose of 0.8 kGy in TSB w/o D, reduction in E. coli O157:H7 viability was lower by 1.11 log CFU/mL in the mild heat-stressed group (90 min) than that in the non-stressed group, although no significant difference (P > 0.05) was observed in this regard for S. Typhimurium and L. monocytogenes between the non-stressed and mildly heat-stressed groups. Thus, we have inferred that mildly heat-stressed E. coli O157:H7 exhibited higher X-ray resistance than that of the non-stressed specimens. Mechanistic analysis showed that mild heat stress at 42 degrees C elicited X-ray resistance by reducing intracellular reactive oxygen species generation and the subsequent DNA damage in E. coli O157:H7. The significant increase (P < 0.05) in X-ray resistance of E. coli O157:H7 was also observed in commercial pasteurized milk, which showed results similar to those obtained from TSB w/o D, demonstrating the potential risk of mild heat-induced X-ray resistance in real food systems.

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