Recovery of Escherichia coli from potentially lethal radiation damage: characterization of a recovery phenomenon

The recovery of Escherichia coli ATCC 11775 following irradiation in phosphate buffer at 0C was assessed using a two-stage incubation protocol. In the first stage survivors were recovered on basal yeast extract agar (BYEA) plates and incubated at 18C (18C-resuscitation) for specified time intervals (0 to 48 h). In the second stage, the incubation temperature was increased to 37C and the survivors were incubated for an additional 24 h to promote colony development. Controls consisted of survivors which were directly incubated at 37C on BYEA for time intervals equivalent to the total times used in the two-stage incubation protocol. The ratio of colony-forming units obtained with and without (control) the 18C-resuscitation protocol was used to calculate recovery factors (RF). Depending on the applied dose, RFs ranged from ca. 2.5 (0.44 kGy) to 15-18 (0.76 kGy) following an optimal resuscitation time of 24-28 h. Irreparable damage, incurred by incubating survivors directly at 37C (37C-damage) for 10 min prior to their recovery using the two-stage protocol, was minimal regardless of dose; however, extending 37C-damage to 60 min decreased RFs by ca. two-thirds. RFs for cells increased from 1.6 to 375 following irradiation at 0.25 and 0.53 kGy, respectively. In ground beef, RFs for cells treated at the highest dose (1.50 kGy) increased by ca. 100-fold following 18C-resuscitation. In contrast, the 18C-resuscitation protocol exhibited minimal effects when E. coli was irradiated in trypticase soy broth. Survivor levels for E. coli following either gamma or electron beam irradiation appeared similar; however, both appeared higher following the 18C-resuscitation protocol