Radiation resistance and injury in starved Escherichia coli O157:H7 treated with electron-beam irradiation in 0.85% saline and in apple juice.

This study evaluated the influence of starvation on the radiation resistance and injury in Escherichia coli O157:H7, following electron beam irradiation in 0.85% (wt/vol) saline and in apple juice. Washed exponential-phase cells of E. coli O157:H7 that were grown in tryptic soy broth (TSB) at 35°C were starved in 0.85% saline (25°C) for 10 days. Exponential- or stationary-phase cells grown in TSB (35°C) served as controls. Samples of 0.85% saline or pasteurized apple juice, inoculated with control cells or cells starved for 8 days, were exposed to electron beam irradiation at doses ranging from 0.0 (control) to 0.70 kGy. The E. coli survivors were enumerated by plating diluted samples on tryptic soy agar or on Sorbitol McConkey agar and counting bacterial colonies after incubation (35°C) for 24 h. Starved cells consistently exhibited higher irradiation D-values than controls (p<0.05). The D-values for control and starved E. coli O157:H7 in 0.85% saline were 0.11 and 0.26 kGy, respectively; D-values in apple juice were 0.16, 0.19, and 0.33 kGy for exponential, stationary, and starved cells, respectively. Irradiation (0.70 kGy) of E. coli O157:H7 in apple juice reduced numbers of exponential- and stationary-phase cells by ∼4.32 and 3.74 logs, respectively, whereas starved cells were reduced by only 2.20 logs. Exponential-phase cells exhibited the lowest resistance to irradiation, and sublethal injury in survivors of this group was higher than that of stationary-phase or starved cells irradiated at 0.50 or 0.70 kGy (p<0.05). The results of this study indicate that starvation-induced stress cross-protects E. coli O157:H7 from ionizing radiation and should be considered an important factor when determining irradiation D-values for this pathogen.