Radiation resistance and injury of Yersinia enterocolitica

The D values of Yersinia enterocolitica strains IP134, IP107, and WA, irradiated at 25 degrees C in Trypticase soy broth, ranged from 9.7 to 11.8 krad. When irradiated in ground beef at 25 and -30 degrees C, the D value of strain IP107 was 19.5 and 38.8 krad, respectively. Cells suspended in Trypticase soy broth were more sensitive to storage at -20 degrees C than those mixed in ground beef. The percentages of inactivation and of injury (inability to form colonies in the presence of 3.0% NaCl) of cells stored in ground beef for 10 days at -20 degrees C were 70 and 23%, respectively. Prior irradiation did not alter the cell's sensitivity to storage at -20 degrees C, nor did storage at -20 degrees C alter the cell's resistance to irradiation at 25 degrees C. Added NaCl concentrations of up to 4.0% in Trypticase soy agar (TSA) (which contains 0.5% NaCl) had little effect on colony formation at 36 degrees C of unirradiated Y. enterocolitica. With added 4.0% NaCl, 79% of the cells formed colonies at 36 degrees C; with 5.0% NaCl added, no colonies were formed. Although 2.5% NaCl added to ground beef did not sensitize Y. enterocolitica cells to irradiation, when added to TSA it reduced the number of apparent radiation survivors. Cells uninjured by irradiation formed colonies on TSA when incubated at either 36 or 5 degrees C. More survivors of an exposure to 60 krad were capable of recovery and forming colonies on TSA when incubated at 36 degrees C for 1 day than at 5 degrees C for 14 days. This difference in count was considered a manifestation of injury to certain survivors of irradiation.