Inactivation of Salmonella Typhimurium and Listeria monocytogenes using high-pressure treatments:: destruction or sublethal stress?

Aims: To investigate potential resuscitation of Listeria monocytogenes and Salmonella Typhimurium after high hydrostatic pressure treatments. Methods and Results: Pressure treatments were applied at room temperature for 10 min on bacterial suspensions in buffers at pH 7 and 5.6. Total bacterial inactivation (8 log(10) CFU ml(-1) of bacterial reduction) obtained by conventional plating was achieved regarding both micro-organisms. Treatments at 400 MPa in pH 5.6 and 600 MPa in pH 7 for L. monocytogenes and at 350 MPa in pH 5.6 and 400 MPa in pH 7 for S. Typhimurium were required respectively. A 'direct viable count' method detected some viable cells in the apparently totally inactivated population. Resuscitation was observed for the two micro-organisms during storage (at 4 and 20 degrees C) after almost all treatments. In the S. Typhimurium population, 600 MPa, 10 min, was considered as the treatment achieving total destruction because no resuscitation was observed under these storage conditions. Conclusions: We suggest a delay before performing counts in treated samples in order to avoid the under-evaluation of surviving cells. Significance and impact of Study: The resuscitation of pathogen bacteria after physical treatments like high hydrostatic pressure has to be considered from the food safety point of view. Further studies should be performed in food products to study this resuscitation phenomenon.