Greater high-pressure resistance of bacteria in oysters than in buffer

High-pressure (HP) treatment is currently being investigated as a process for extending the shelf life of oysters through microbial inactivation. The aim of this study was to compare baroresistance in oysters and phosphate-buffered saline (PBS) of the Gram-negative bacteria, Vibrio mimicus 9583 and Escherichia coli O157:H45, and the Gram-positive bacteria, Listeria innocua MP2418 and Listeria monocytogenes LO28. A novel injection method was developed to allow reproducible, high numbers (10⁶-10⁸) of bacteria to be obtained in oysters. At a pressure >400 MPa, inactivation of all bacteria studied was considerably less in oysters than in PBS. This difference in the level of HP-induced inactivation of bacteria between oysters and PBS increased with treatment pressure, culminating in a 5-log difference at 480-600 MPa for all bacteria studied. E. coli and L. innocua were the most baroresistant of the species studied, with only a ∼3-log inactivation of both bacteria observed in oysters after treatment at 700 MPa. The influence of salt content, one of the main differences between oysters and PBS, on baroresistance of bacteria in tryptone soya yeast extract broth (TSBYE) containing 3.5% salt and 0.5% salt was subsequently investigated; all bacteria were considerably more resistant at higher salt concentrations.