Modelling the Effect of Acid and Salt Stress on the Survival and Diversity of Listeria monocytogenes in a Lactic Soft Cheese Stored at 4°C

Introduction: Listeria monocytogenes is the causative agent of the human listeriosis. As a ubiquitous organism, it is widely present in food processing plants where it causes product contamination. Continued stress exposure results in the selection of stress resistant variants with enhanced survival. Among dairy products, soft cheeses are leading causes of listeriosis outbreaks.

Purpose: The purpose of the study was to model the survival response of L. monocytogenes strains in a lactic soft cheese stored at 4°C after exposure to acid and salt stress and to evaluate the effect of lactate and diacetate salts on the inactivation kinetics.

Methods: Listeria monocytogenes strains T69, 159/10, 243 and ATCC19115 were subjected to acid and NaCl stress, then inoculated into soft cheese and stored at 4°C for 15 days. Survival data was fitted into four primary inactivation models. A secondary second order polynomial function was used to model the effect of sodium lactate and diacetate on the rate parameter of the Weibull model. GTG₅REP PCR fingerprinting was used to assess diversity of survivors.

Results: Stress treated cell inactivation was described by non-linear models (R² > 0.90). When unstressed, inactivation was best described by a convex double Weibull model (R² = 0.86). Stressed cells had significantly reduced (p < 0.05) inactivation rates. Addition of sodium lactate and diacetate (0.5 – 2.5% (m/v)) to the soft cheese resulted in an increase in the rate parameter of the Weibull model described by a second order polynomial function (R² = 0.84). Strain 159/10 remained the dominant strain out of 40 isolates representing survivors of acid and salt treated L. monocytogenes after 15 days.

Significance: Stressed L. monocytogenes have enhanced survival in acid soft cheese. Lactate and diacetate salts are not effective in controlling L. monocytogenes in lactic soft cheese. Recent food isolates survive better than laboratory strains in contaminated foods and could be more useful in predicting L. monocytogenes survival response.