Recovery of Foodborne Microorganisms from Potentially Lethal Radiation Damage

Abstract

A two-stage recovery protocol was examined for microorganisms following gamma irradiation in phosphate buffer at 0°C. In the first stage, survivors were recovered on basal yeast extract agar and held at various temperatures suboptimal for their growth for 20 h (resuscitation protocol). In the second stage the survivors were incubated for an additional 24 h, but in this case at their optimum temperature for growth. Controls consisted of survivors which were not subjected to the resuscitation protocol (direct incubation at their optimum growth temperature). The ratio of survivors enumerated with and without the resuscitation protocol (control) at each specified temperature was used to formulate a recovery factor (RF). An RF was determined for each treatment dose. Results of this study indicated that the number of Escherichia coli, Salmonella serotype typhimurium and Brochothrix thermosphacta survivors increased following a resuscitation protocol (RF > 2.0). Overall, optimum resuscitation temperatures ranged from 14 to 22°C. The extent of recovery also appeared dose dependent, with larger treatment doses giving rise to a higher RF. S. serotype typhimurium irradiated at 1.5 kGy exhibited the highest RF, 161, when resuscitated at 22°C. Listeria monocytogenes, Yersinia enterocolitica, Staphylococcus aureus, Aeromonas hydrophila and Saccharomyces cerevisiae exhibited an RF < 2.0 regardless of resuscitation temperature. Results of this study indicated that the use of suboptimal holding temperatures as part of a recovery protocol may have advantages, especially with respect to the enumeration of E. coli and salmonellae survivors in irradiated foods such as poultry.