M. Ritz et al., Morphological and physiological characterization of Listeria monocytogenessubjected to high hydrostatic pressure, APPL ENVIR, 67(5), 2001, pp. 2240-2247
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.