Evaluation of Listeria monocytogenes growth control by biopreservation using a washed-rind soft cheese model.

Abstract

This work was carried out in the context of the microbiological safety of washed-rind soft cheeses, and more specifically Herve cheese, an emblematic product of Walloon heritage with a Protected Designation of Origin. Due to its physico-chemical properties (moderate pH, high humidity, Aw > 0.94) and its artisanal ripening process with no final heat treatment, this type of cheese is an ideal breeding ground for pathogenic micro-organisms such as Listeria monocytogenes. Biopreservation, a natural preservation method, represents a promising alternative for reinforcing the microbiological safety of food. It relies on the use of beneficial microorganisms or their metabolites to inhibit pathogens. Among the agents studied are lactic acid bacteria, bacteriocins, positive biofilms, phages, certain plant extracts and enzyme systems such as lactoperoxidase. These natural levers make it possible to limit the use of chemical additives and intensive heat treatments, while preserving the organoleptic qualities of products. In this work, the focus is on the natural flora of Herve cheese, in particular lactic acid bacteria (Lactococcus, Brevibacterium) and a still undescribed strain of the genus Fusobacterium, detected by metagenetic analyses from the cheese rind of a traditional cheese produced by the « Vieux Moulin » farm. Several previous studies have highlighted its recurrent presence and possible involvement in the reduction of Listeria monocytogenes. Although its isolation in pure culture has yet to be performed, specific PCR tools have been developed for its monitoring. Implantation trials in experimental cheese models have enabled us to assess its behavior, without however providing definitive proof of its antimicrobial activity mechanisms. The main aim of this work is to study the effect of weekly washing of the rind of soft washed-rind cheese (Herve type) on the growth of inoculated Listeria innocua, while monitoring the evolution of the Fusobacterium population using targeted PCR and metagenetics. Our hypothesis is that the Fusobacterium population has a bactericidal effect on Listeria innocua, a hypothesis that has been observed experimentally but needs to be confirmed. The aim of this project is to reconcile microbiological innovation with the enhancement of traditional cheese-making know-how.