title: Radioresistance development of DNA repair deficient Escherichia coli DH5α in ground beef subjected to electron beam at sub-lethal doses.
abstract: OBJECTIVE
  Electron beam (e-beam) efficiently and non-thermally inactivates microorganisms in food by lethal DNA changes (direct effects) and free radicals from water radiolysis (in-direct effects). Non-pathogenic Escherichia coli DH5α (α substrain of DH5 described by Hanahan 1985 , 'DH' stands for Douglas Hanahan) is a microorganism that lacks DNA repair capability, resulting in high radiosensitivity. Studying microbial inactivation of E. coli DH5α repeatedly subjected to sub-lethal e-beam in ground beef may enhance understanding of microbial radioresistance. The objective of this study was to determine if repetitive processing with e-beam at sub-lethal doses increases D-value (e-beam dose required to inactivate one log of microbial population) of E. coli DH5α in ground beef.
  
  
  METHODS
  Survivors from the highest e-beam dose were isolated and incubated in ground beef for the next cycle of e-beam processing. Five cycles were conducted. To acclimatise E. coli DH5α, first two cycles used low doses. D-values were determined following the third cycle.
  
  
  RESULTS
  D-values increased (p < 0.05) significantly with each cycle. Thus, E. coli DH5α has a capability to develop greater radioresistance under these experimental conditions. Following the third cycle D-values were 0.32 ± 0.006 and 0.32 ± 0.002 kGy for survivors enumerated on non-selective and selective media, respectively; the fourth cycle 0.39 ± 0.007 and 0.40 ± 0.019 kGy; and the fifth cycle 0.46 ± 0.006 and 0.46 ± 0.020 kGy. D-values on non-selective and selective media were similar (p > 0.05) indicating absence of cell recovery in E. coli DH5α.
  
  
  CONCLUSIONS
  E. coli DH5α increases radioresistance to e-beam as a result of repetitive exposure to sub-lethal doses despite its DNA repair deficiency.
authors.affiliation: Division of Animal and Nutritional Sciences, West Virginia University, Morgantown, WV 26508, USA.
authors.forename: Adiam T
authors.initials: AT
authors.lastname: Tesfai
authors.forename: Sarah K
authors.initials: SK
authors.lastname: Beamer
authors.forename: Kristen E
authors.initials: KE
authors.lastname: Matak
authors.forename: Jacek
authors.initials: J
authors.lastname: Jaczynski
mesh_terms: D000818:Animals; D004260:DNA Repair / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D004583:Electrons; D004926:Escherichia coli / Q000378:metabolism; D019453:Escherichia coli O157 / Q000254:growth & development / Q000528:radiation effects*; D005506:Food Contamination; D005511:Food Handling / Q000379:methods; D005514:Food Irradiation / Q000379:methods; D005516:Food Microbiology; D005609:Free Radicals; D007700:Kinetics; D008460:Meat; D011836:Radiation Tolerance; D014867:Water / Q000737:chemistry
chemical_list: D005609:Free Radicals; D014867:Water