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

Abstract

Increased Resistance of Escherichia coli O157:H7 to Electron Beam Following Repetitive Irradiation at Sub-lethal Doses and an Analysis of Genes of Escherichia coli Resistance Leah Levanduski One way that food processors in the United States control food-borne pathogens in a non-thermal manner is with the application of electron beam (e-beam) radiation. Increased resistance of Escherichia coli O157:H7 to various stressors such as pH, temperature, ionic strength, and antibiotics has been demonstrated; therefore, the objective of this study was to determine if the D 10-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 an ATCC strain of E. coli and incubated to approximately 10 9 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. Four such consecutive cycles of isolation and e-beam processing were performed. The D 10-values for E. coli survivors following each cycle of e-beam processing were calculated from survivor curves. The D 10-values increased (P < 0.05) with each subsequent cycle 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, which is the result of three cycles of e-beam exposure. Following four cycles of e-beam processing, radio-resistance increased for isolate L4 (P < 0.05), resulting in the survival of this strain to an e-beam dose of 3.0 kGy. The data demonstrates that e-beam can efficiently inactivate E. coli in food products; however, the organism demonstrated increased resistance when repeatedly subjected to sublethal e-beam processing. Although the exact mechanism 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 is involved. Therefore, we are currently investigating this hypothesis through genome-wide expression analysis using micro-array technology.