A microbial subtyping approach for the identification of food potentially involved in listeriosis in Northern Italy

Objectives: Listeriosis is a relatively rare but life threatening foodborne disease. One general method for attribution of the human disease burden of foodborne infections to specific sources is “microbial subtyping approach”. This approach is enabled by the identification of strong associations between some of the dominant subtypes and a specific food. The objective of this work was to attribute the burden of human listeriosis illness to the specific sources in Lombardy region, in order to identify and prioritize appropriate food safety interventions aimed at controlling foodborne disease. Materials and Methods: All Listeria monocytogenes isolates, from both human (n=73) and food (n=232) origin, were subtyped by Pulsed-Field Gel Electrophoresis (PFGE) according to the PulseNet protocol with AscI enzyme to detect molecular subtypes correlating in the 2012-2014 period. Subsequently, for all correlating human isolates (n=30) was performed Multi-locus Sequence Typing (MLST). Sequence Types (STs) were assigned in accordance to the Listeria MLST database (Pasteur Institute, France). Source attribution was performed by microbial subtyping approach that involves comparison of the subtypes of isolates from different sources with those isolated from humans. Results: The correlation between human and food isolates showed 60 AscI pulsotypes and 14 AscI clusters, including 3 to 77 isolates. MLST analyses revealed that the 14 AscI clusters belonged to 13 STs: ST1, ST5, ST7, ST8, ST9, ST37, ST38, ST101, ST121, ST218, ST288, ST325 and ST398. L. monocytogenes were isolated from meat products and preparations (43.2%), fishery products (24.9%), other ready-to-eat products (26.6%) and cheeses (5.3%). The source attribution revealed that the most frequent subtypes in human cases (ST8, n = 11; ST37, n = 3; ST38, n = 3) are distributed in different food categories, while less frequent subtypes seem to be related with a particular ecological niche, such as meat products (ST7, ST101, ST121 and ST218), fishery products (ST1 and ST288) and cheeses (ST325). Conclusions: This study identified the main genotypes that spread from the food supply chain to humans as well as the foods potentially involved in listeriosis cases in the Lombardy region. These knowledge could be useful to find out the association of the different genotypes with particular ecological niches and contribute to the epidemiological investigations of listeriosis, which are often challenging.