Maximum growth rate of nine strains of Listeria monocytogenes as a crucial factor to determine the dynamic of pathogens in foods

Listeriosis poses a serious threat for human and animal health. It is caused mostly, but not exclusively, by Listeria monocytogenes. The symptoms are various, from a flu like fever, to meningitis, abortion, or death. The outbreaks are rare, but the hospitalization rate is very high, as well as mortality rate. The fundamental characteristic of Listeriosis is that it is almost exclusively transmitted via food. This requires a deep understanding and monitoring of the behaviour of L. monocytogenes in various food matrices. In this regard, challenge tests are the most valuable tool available to determine both maximum growth rate and growth potential of L. monocytogenes on a selected food matrix, as it allows FBO and authorities to monitor food safety all along the food production chain; it is possible to experimentally determine the maximum amount of CFU (and whether or not this amount is sufficient to lead to a serious infection) and the dynamic of the growth. Challenge tests are based on two main documents: EURL Lm Anses and the standard UNI EN ISO 20976-1:2019. In this thesis study, a challenge test was conducted to assess the maximum growth rate of nine L. monocytogenes strains combined with a strain of Pseudomonas fluorescens. All strains were originally retrieved on several food matrices. Initially, a reference L. monocytogenes strain was studied in a classical challenge test. Once the maximum growth rate was established, the nine strains were tested with P. fluorescens. This was meant to shed light on a controversial series of results from other authors, whether or not the co-presence of these two microorganisms will stunt or improve their growth. During the experimental campaign, pH and aW of the matrix was monitored. Finally, a statistical analysis was performed, in order to determine whether or not the difference was significant.