The effect of culture growth phase on induction of the heat shock responsein Yersinia enterocolitica and Listeria monocytogenes

The effect of culture growth phase on induction of the heat shock response in Yersinia enterocolitica and Listeria monocytogenes, was examined. Expone ntial or stationary preconditioned cultures were heat shocked and survivor numbers estimated using selective and overlay/resuscitation recovery techni ques. The results indicate that prior heat shock induced increased heat res istance in both micro-organisms to higher heat treatments. Heat-shocked cel ls of each micro-organism were able to survive much longer than non-heat-sh ocked cells when heated at 55 degrees C. The size of the change in heat res istance between heat-shocked and non-heat-shocked cells was greatest for ex ponential cultures (X:X). Results indicate that the overall relative therma l resistance of each pathogen was dependent on cell growth phase. Stationar y cultures (S:S) were significantly (P < 0.01) more thermotolerant than exp onential cultures (X:X) under identical processing conditions. Under most c onditions, the use of an overlay/resuscitation recovery medium resulted in higher D-values (P < 0.05) compared with a selective recovery medium.