Stress induction and response, inactivation, and recovery of vegetative microorganisms by pulsed electric fields

Abstract

Electroporation phenomena can be distinguished by the reversibility of the membrane permeabilization. This definition, however, takes into account structural aspects only. It does not include the physiological state of the microorganisms, which is another key aspect in order to characterize the vitality of cells. Depending on the strain, matrix, and process intensity, sublethal injury, a state in-between alive and death, may occur. As many food applications of pulsed electric fields (PEF) rely on the complete inactivation of spoilage and pathogenic bacteria, sublethal injury is an important issue which has to be overcome. In order to detect sublethal injury or related aspects, a variety of different methods can be applied, e.g., differential plating, leakage of intracellular components, or staining. The most accurate and fast technique is the use of flow cytometry in combination with different dyes for the determination of structural and physiological aspects, i.e., membrane integrity and metabolic activity. Considering molecular stress responses, PEF induces similar pathways than oxidation stress; however the effects are not as pronounced as for heat shocks, as the main target of the PEF response is the expression of proteins related to membrane repair. After PEF treatment, sublethally injured cells might recover, depending on intrinsic, strainspecific factors, as well as the surrounding matrix properties, especially the pH, and storage time and temperature. Thus, knowledge of sublethal injury is crucial for the design of food-related PEF applications, in order to ensure high levels of food safety.