Increased resistance of Escherichia coli O157:H7 to electron beam following repetitive irradiation at sub-lethal doses

One way that food processors in the United States have been controlling food-borne pathogens in a non-thermal manner is the application of electron beam (e-beam) radiation. The development of an increased resistance of Escherichia coli O157:H7 to various stressors such to pH, temperature, ionic strength, and antibiotics has been demonstrated. The objective of this study was to determine if the D10-value for E. coli O157:H7 (E. coli) in ground beef increases due to repetitive exposure to e-beam at sub-lethal levels. Ground beef samples were inoculated with E. coli and incubated to approximately 109 CFU/g followed by e-beam processing. Survivors were enumerated using a standard spread-plating technique. Colonies of E. coli survivors from the highest e-beam dose were isolated and grown for the next cycle of inoculation in ground beef and e-beam processing. Five such consecutive cycles of isolation and e-beam processing were performed. The D10-values for E. coli survivors following each cycle of e-beam processing were calculated from survivor curves. The D10-values increased (P < 0.05) with subsequent cycles of e-beam processing, starting at 0.24 ± 0.03 kGy for E. coli ATCC strain 35150 and reaching 0.63 ± 0.02 kGy for E. coli isolate L3. Following four cycles of e-beam processing, the isolate L3 increased (P < 0.05) its radio-resistance and survived an e-beam dose of 3.0 kGy. Therefore, our data demonstrates that e-beam can efficiently inactivate E. coli in food products; however, similar to other inactivation techniques, E. coli has a capability to develop increased resistance to e-beam if the same populations of E. coli in food products are repetitively subjected to e-beam processing. Although the exact mechanism for the development of increased radio-resistance of E. coli to e-beam is unclear at the moment, based on the available literature regarding increased resistance of E. coli to various stressors, it is likely that some genetic mechanism in involved. Therefore, we are currently investigating this hypothesis with micro-arrays.