RECOVERY OF FOODBORNE MICROORGANISMS FROM POTENTIALLY LETHAL RADIATION-DAMAGE

A two-stage recovery protocol was examined for microorganisms followin g gamma irradiation in phosphate buffer at 0 degrees C, In the first s tage, survivors were recovered on basal yeast extract agar and held at various temperatures suboptimal for their growth for 20 h (resuscitat ion protocol). In the second stage the survivors were incubated for an additional 24 h, but in this case at their optimum temperature for gr owth. Controls consisted of survivors which were not subjected to the resuscitation protocol (direct incubation at their optimum growth temp erature). The ratio of survivors enumerated with and without the resus citation protocol (control) at each specified temperature was used to formulate a recovery factor (RF). An RF was determined for each treatm ent dose. Results of this study indicated that the number of Escherich ia coli, Salmonella serotype typhimurium and Brochothrix thermosphacta survivors increased following a resuscitation protocol (RF > 2.0). Ov erall, optimum resuscitation temperatures ranged from 14 to 22 degrees C, The extent of recovery also appeared dose dependent, with larger t reatment doses giving rise to a higher RF. S. serotype typhimurium irr adiated at 1.5 kGy exhibited the highest RF. 161, when resuscitated at 22 degrees C, Listeria monocytogenes, Yersinia enterocolitica, Staphy lococcus aureus, Aeromonas hydrophila and Saccharomyces cerevisiae exh ibited 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 re spect to the enumeration of E. coli and salmonellae survivors in irrad iated foods such as poultry.