Non‐Thermal Inactivation Models for Listeria monocytogenes

Abstract

The effects of temperature, lactic acid (or pH), sodium chloride, and sodium nitrite on the non-thermal inactivation of a three strain mixture of Listeriu monocytogenes were examined in brain heart infusion broth. A total of 249 survivor curves representing 157 combinations of the four variables were generated. The survivor curves were described mathematically by fitting data using linear and nonlinear primary models. Supplemental studies demonstrated that (1) preculturing the microorganism in an acidic environment or in media containing glucose increased acid tolerance, (2) survivor curve tailing was not due to the presence of a more resistant subpopulation, and (3) the rate of nonthermal inactivation was independent of initial population density. Response surface models were developed for predicting the effects and interactions of the four independent variables on the inactivation of List&u monocytogenes under adverse environmental conditions. INTRODUCTION LIKE OTHER foodborne pathogens, when Listetiu monocytogenes is placed in an adverse environment, it will be inactivated at a rate that is dependent on the severity of the conditions and the physiological characteristics of the species. In general, the environmental and cultural factors that influence the growth of bacteria in foods also influence their inactivation. This includes well recognized parameters such as storage temperature, pH, acidulant identity, water activity, humectant identity, animicrobials, etc. With Listeria monocytogenes, the rate of nonthermal inactivation is dependent on most of these factors (Ahamad and Marth, 1989;El-Shenawy and Marth, 1989;Sorrells et al., 1989;Sorrells and Enigl, 1990;Cole et al., 1990; Buchanan et al., 1993).While the kinetics of microbial inactivation in response to thermal processing has been studied extensively, there has been relatively little reported quantitative data on nonthermal inactivation. Using Lisferia monocytogenes as an example of a relatively hardy, vegetative foodborne pathogen, the objective of our study was to examine quantitatively the effects and interactions of temperature, sodium chloride (water activity), lactic acid (or pH), and sodium nitrite on aerobic inactivation. These data were then used to (1) evaluate the effectiveness of two primary models for depicting survivor curves, and (2) assess the feasibility of developing secondary models for predicting the impact of the same variables on the rate of L. monocytogenes inactivation.

General Model, Based on Two Mixed Weibull Distributions of Bacterial Resistance, for Describing Various Shapes of Inactivation Curves

“…A bacterial strain can produce different shapes of survival curves. Frequently, concave curves may become convex or sigmoidal when the intensity of the stress varies (6,7,10,19,24,38,45,47,48). The patterns of survival curves may also vary with the physiological state of the cells and are dependent on the phase of growth (exponential or stationary phase) and also on the conditions of adaptation before the stress (18,25,36).…”

“…Whiting's model involves a sum of two logistic models corresponding to the two subpopulations which are characterized by the difference in their levels of resistance to stress (47). It was used to describe the nonthermal inactivation of Salmonella, Listeria monocytogenes, and Staphylococcus aureus in brain heart infusion (BHI) broth (6,47,48). The main advantage of this model is that it can describe many shapes of inactivation curves often observed in nonthermal inactivation studies.…”

Isolation of Highly Heat-Resistant Listeria monocytogenes Variants by Use of a Kinetic Modeling-Based Sampling Scheme

“…1). Furthermore, the high frequency of revertance to a wild-type phenotype of some of the isolated resistant variants might explain the difficulty of isolating resistant L. monocytogenes variants, as reported before by various research groups (5,16).…”

“…The occurrence of stress-resistant variants in monoclonal populations of bacteria remains an important subject of debate, as some research groups have reported the isolation of such variants while others failed to demonstrate their presence and questioned their existence (1,3,5,16,17,19,24,29,31). In this study, we used a new strategy involving a kinetic modeling-based sampling scheme and an optimized stress challenge cycle method for the isolation of stress-resistant variants of L. monocytogenes.…”

“…Further characterization of the LO28 HHP-resistant variants without mutations in ctsR also revealed increased resistance to heat for most of these variants (31). In addition, thermal inactivation of L. monocytogenes was previously fitted with a biphasic model, indicating the presence of a heat-resistant fraction (1,3,5). However, so far these variants could not be isolated after heat inactivation.…”