Kelley P. Knight

A combined discrete–continuous model describing the lag phase of Listeria monocytogenes☆

Food microbiologists generally use continuous sigmoidal functions such as the empirical Gompertz equation to obtain the kinetic parameters specific growth rate (mu) and lag phase duration (lambda) from bacterial growth curves. This approach yields reliable information on mu; however, values for lambda are difficult to determine accurately due, in part, to our poor understanding of the physiological events taking place during adaptation of cells to new environments. Existing models also assume a homogeneous population of cells, thus there is a need to develop discrete event models which can account for the behavior of individual cells. Time to detection (t(d)) values were determined for Listeria monocytogenes using an automated turbidimetric instrument, and used to calculate mu. Mean individual cell lag times (tL) were calculated as the difference between the observed t(d) and the theoretical value estimated using mu. Variability in tL for individual cells in replicate wells was estimated using serial dilutions. A discrete stochastic model was applied to the individual cells, and combined with a deterministic population-level growth model. This discrete-continuous model incorporating tL and the variability in tL (expressed as standard deviation; S.D.(L)) predicted a reduced variability between wells with increased number of cells per well, in agreement with experimental findings. By combining the discrete adaptation step with a continuous growth function it was possible to generate a model which accurately described the transition from lag to exponential phase. This new model may serve as a useful tool for describing individual cell behavior, and thus increasing our knowledge of events occurring during the lag phase.

Growth and survival of various strains of enterohemorrhagic Escherichia coli in hydrochloric and acetic acid

Nineteen strains of enterohemorrhagic Escherichia coli isolated from humans and foods were examined for their ability to grow and survive at low pH in organic (acetic) and mineral (HCl) acids. Strains were subcultured in tryptic soy broth adjusted to various pH values (3.75 to 4.75 for HCl and 4.75 to 5.75 for acetic acid) and incubated for 72 h at 37 degrees C to determine the minimum growth pH value. Minimum pH values for growth of 4.25 and 5.5 were found for HCl and acetic acid, respectively. Strains were also exposed to pH 2.0 (HCl) and pH 4.0 (acetic acid) for up to 24 h at 37 degrees C to assess their ability to survive. HCl was a more effective inhibitor after 6 h of exposure, whereas acetic acid was more effective after 24 h. Outbreak strains survived acid treatment significantly (P < or = 0.05) better than strains isolated from fermented or high-pH foods or animal or human isolates. Significant (P < or = 0.05) differences among serotypes and between O157:H7 and other serotypes were apparent after 3 or 6 h of exposure to acids.

Nisin reduces the thermal resistance of Listeria monocytogenes Scott A in liquid whole egg

D-values (decimal reduction times) and z-values (increase in temperature required for a 1-log change in D-value) for Listeria monocytogenes Scott A were determined in liquid whole egg with nisin (0 or 10 microg ml(-1)) and NaCl (0 or 10%) by a submerged glass ampoule procedure. Samples were plated onto nonselective agar at appropriate intervals, and D-values were determined using a modified biphasic logistic equation. Addition of NaCl increased D-values at all temperatures tested. The addition of nisin to unsalted liquid whole egg resulted in a rapid 4-log reduction in viable counts within the first hour. Nisin significantly (P < or = 0.05) decreased D-values at lower (< 58 degrees C) temperatures in both unsalted and salted liquid whole egg but had little effect on the D-values at current minimum U.S. and Canadian pasteurization temperatures (60 degrees C without NaCl; 63 degrees C with NaCl). However, when nisin was added 2 h prior to heat treatment, D-values were significantly (P < or = 0.05) reduced at these temperatures. Inhibitory levels of nisin were detected in the liquid whole egg postpasteurization. Nisin could have a favorable impact on the control of L. monocytogenes in pasteurized liquid egg products.