Molecular subtyping of human and food-environmental Listeria monocytogenes isolates by PFGE in northern Italy (2012-2014)

Introduction: Human invasive listeriosis is a relatively rare but life threatening foodborne disease, with high hospitalization and fatality rates in vulnerable populations. The main route of transmission to humans is through the consumption of contaminated food, but the heterogeneity of food products, the extended shelf life of many marketed products and the prolonged incubation periods prejudice the identification of common source outbreaks and food vehicles. Purpose: The aim of this study was to evaluate the molecular correlation between human and food-environmental Listeria monocytogenes isolates in the Lombardy region during the period 2012-2014, in order to improve the network of integrated surveillance and support epidemiological investigations. Methods: Seventy-three human isolates were collected by the regional reference laboratory of the Lombardy region at University of Milan. Simultaneously, 301 food-environmental isolates were detected from the food chain by the Department of Food Microbiology (IZSLER) in the same period and area under study. All L. monocytogenes isolates were subtyped by PFGE according to the PulseNet protocol with AscIenzyme and statistical analysis was performed using the chi-square test (p≤0.05). Results: Among the 73 human isolates were recognized 30 pulsotypes and 8 clusters, four of which were reported in previous years (Mammina C. et al., 2013). While 54 pulsotypes and 9 clusters were identified among the 301 food-environment isolates, the number of clusters observed in food-environment isolates was significatively lower than those observed in human isolates (p<0.01). The correlation between human and food-environmental isolates showed 65 pulsotypes and 15 clusters, in particular L. monocytogenes strains were isolated from meat products and preparations (43.2%, n=73), fishery products (24.9%, n=42), other ready-to-eat products (26.6%, n=45) and cheeses (5.3%, n=9). Significance: This study identified the main pulsotypes that spread from the food supply chain to humans as well as the ecological niches or foods potentially involved in listeriosis cases in Lombardy.