Genetic diversity of Listeria monocytogenes strains contaminating food and food producing environment as single based sample in Italy (retrospective study)

Human listeriosis outbreaks are often associated with food products, which could be contaminated, at the same time, also by different clones of Listeria monocytogenes. This emphasize the need to type more than one L. monocytogenes isolate found in a single food or environmental sample.The purpose of the present study was to evaluate the presence of different L. monocytogenes strains in food and food production environment in order to understand if there is need to type more isolates from the same sample in case of presence of L. monocytogenes. Between 2011 and 2015, at the Italian National Reference Laboratory for L. monocytogenes, for each positive sample, from two to twenty-three isolates of L. monocytogenes were collected. All the isolates were characterized by conventional serotyping and pulsed field gel electrophoresis (PFGE). Moreover, isolates from the same sample, having indistinguishable PFGE profile, were subjected to whole genome sequencing in order to perform core genome Multi Locus Sequence Typing (cgMLST).Within each sample, more than one serotype and one pulsotype were found in 11.9% and 27.5%, respectively. For indistinguishable PFGE patterns the cgMLST analysis showed 96.2% of concordance demonstrating the added value of new sequencing technologies.This study has demonstrated the need to select and type more than one L. monocytogenes colony in one food or food environmental sample to detect the diversity of L. monocytogenes strains and facilitate downstream investigations and effective source attribution in foodborne outbreak inquiry.