Morphological and physiological characterization of Listeria monocytogenessubjected to high hydrostatic pressure

High hydrostatic pressure is a new food preservation technology known for i ts capacity to inactivate spoilage and pathogenic microorganisms, That inac tivation is usually assessed by the number of colonies growing on solid med ia after treatment. Under normal conditions the method does nut permit reco very of damaged cells and may underestimate the number of cells that will r emain viable and growth after a few days in high-pressure-processed foodstu ffs. This study investigated the damage inflicted on Listeria monocytogenes cells treated by high pressure for 10 min at 400 MPa in pH 5.6 citrate buf fer. Under these conditions, no cell growth occurred after 48 h on plate co unt agar. Scanning electron microscopy, light scattering by flow cytometry, and cell volume measurements were compared to evaluate the morphological c hanges in cells after pressurization,,ill these methods revealed that cellu lar morphology was not really affected. Esterase activity, as assessed eith er by enzymatic activity assays or qv carboxy fluorescein diacetate fluores cence monitored by flow cytometry, was dramatically lowered. but not totall y. obliterated, under the effects of treatment, The measurement of propidiu m iodide uptake followed by flow cytometry demonstrated that membrane integ rity was preserved in a small part of the population, although the membrane potential measured by analytical methods or evaluated by oxonol uptake was reduced from -86 to -5 mV. These results showed that such combined methods as fluorescent dyes monitored by flow cytometry and physiological activity measurements provide valuable indications of cellular viability.