Investigation of Listeria monocytogenes in Food in Northwestern Italy (2020–2024)

<i>Listeria monocytogenes</i> is a foodborne pathogen of significant public health concern due to its high environmental resilience and ability to cause severe infections in vulnerable populations. The objective of the present study is to characterize foodborne strains of <i>Listeria monocytogenes</i> isolated between 2020 and 2024 in northwestern Italy. <i>Lm</i> was detected through isolation, biochemical confirmation and molecular serogrouping. Next generation sequencing (NGS) analysis was used to characterize the strains in terms of virulence and antibiotic resistance. A total of 39 positive samples were identified from various food matrices, including meat products, fish, cheeses and ready-to-eat foods. The most frequently detected serogroups were IIc and IIa, with a notable presence of the highly virulent IVb group. Next-generation sequencing (NGS) was applied to all isolates, revealing the presence of virulence genes associated with the LIPI-1 island and internalins. In addition to pathogenicity islands, genes related to stress resistance (<i>clpCEP</i>, <i>Gad A</i>, <i>GadB</i>, <i>GadC</i>), biofilm production (<i>agrA</i>, <i>flaA</i>, <i>degU</i>, <i>hfq</i>) and sortase-mediated anchoring of surface protein (<i>strA</i>, <i>strB</i>) have been identified. The presence of antibiotic resistance genes was confirmed, with all isolates harboring the <i>fosX</i> gene. Moreover, four isolates exhibited resistance determinants against antibiotics belonging to two different classes: tetracyclines (<i>tetM</i>) and lincosamides (<i>lsa</i>(<i>A</i>)). Multilocus sequence typing (MLST) showed that clonal complex CC9 was the most prevalent among the isolates. Further, cgMLST and SNP analyses identified a principal cluster of closely related strains, which were isolated from meat products. These findings highlight the need for continuous surveillance of <i>L. monocytogenes</i>.