High Pressure Processing Effects on Listeria monocytogenes and Listeria innocua: Evidence for Variability in Inactivation Behaviour

Introduction: Listeriosis is an important public health issue with considerable negative economic impact. Although it is a disease with relative low incidence, reported case fatality rate is the highest (15.6%) of all the foodborne diseases under European Union surveillance. The use of nonthermal technologies, such as High Hydrostatic Pressure (HHP), has emerged as a new preservation method to control, slow, and prevent the growth of foodborne pathogens; thereby, extending shelf-life with high energy efficiency and minimal food processing.

Purpose: The effect of HHP on the survival of 14 strains of Listeria monocytogenes, from food or clinical origins, and two strains of Listeria innocua (2030c PHLS and NCTC 11288) was evaluated.

Methods:  Stationary phase cells were exposed to 300, 400, and 500 MPa at 10°C for 5 min. The pressure-treated samples and controls were serially diluted, without a prior repair incubation period. Dilutions were plated in triplicate in TSAYE and PALCAM agar. Plates were incubated at 37°C for 48 h and colony forming units (CFU)/ml determined.

Results: Two L. monocytogenes of food origin were the most sensitive strains at 300 and 400 MPa, showing significantly higher log reductions in comparison with other strains (P < 0.05). Strains of L. monocytogenes resistant to one or more antibiotics exhibited significantly higher levels of survival after high pressure treatment at 400 MPa. No correlation was found between strain origin or thermal tolerance and resistance to HHP. Selective enumeration of Listeria cells in PALCAM yielded higher log reduction values compared to enumeration with TSAYE, reflecting cellular damage caused by HHP treatment and being statistically significant at 400 MPa (P < 0.05). Listeria innocua exhibited significantly higher sensitivity to HHP than observed for some L. monocytogenes.

Significance: The data obtained underlines the importance of strain selection for studies aiming to evaluate HHP efficacy to ensure safety of HHP-treated foods.