Inactivation and injury of Yersinia enterocolitica by radiation and freezing

Cell inactivation and cell injury by irradiation and freezing of the potentially enteropathogenic, food-borne gram-negative rod Yersinia enterocolitica strain WA was investigated. The radiation dose necessary to kill 90% of the initial population, i.e., one D-value, was 10.0, 14,3, and 24.0 krad when irradiation was carried out at 2 to 0, -18, and -75 degrees C, respectively. On the other hand, cell injury, i.e., inability to form colonies in agar containing 2.5% NaCl, was 32, 42 and 54% when cells were irradiated to one D-value at 2 to 0, -18, and -75 degrees C, respectively. Freezing alone (without irradiation) at -18 and -75 degrees C for 1 h resulted in 7 and 42% cell inactivation and 55 and 83% cell injury, respectively. These data show that given the same extent of cell inactivation, freezing caused substantially greater cell injury than radiation. For purposes of radiation sterilization, doses of 100 and 150 krad would be sufficient to inactivate 10 log cycles of Y. enterocolitica strain WA if irradiated at 2 to 0 and -18 degrees C, respectively. Presence of 2.5% NaCl may result in a further 50% reduction of the dose required to achieve sterility.