Listeria monocytogenes Strains Persisting in a Meat Processing Plant in Central Italy: Use of Whole Genome Sequencing and In Vitro Adhesion and Invasion Assays to Decipher Their Virulence Potential.

In this study, we used both a WGS and an in vitro approach to study the virulence potential of nine Listeria monocytogenes (Lm) strains belonging to genetic clusters persisting in a meat processing plant in Central Italy. The studied clusters belonged to CC1-ST1, CC9-ST9, and CC218-ST2801. All the CC1 and CC218 strains presented the same accessory virulence genes (LIPI-3, gltA, gltB, and aut_IVb). CC1 and CC9 strains presented a gene profile similarity of 22.6% as well as CC9 and CC218 isolates. CC1 and CC218 showed a similarity of 45.2% of the same virulence profile. The hypervirulent strains of lineage I (CC1 and CC218) presented a greater ability to adhere and invade Caco-2 cells than hypovirulent ones (CC9). CC1 strains were significantly more adhesive and invasive compared with CC9 and CC218 strains, although these last CCs presented the same accessory virulence genes. No statistically significant difference was found comparing CC218 with CC9 strains. This study provided for the first time data on the in vitro adhesiveness and invasiveness of CC218-ST2801 and added more data on the virulence characteristics of CC1 and CC9. What we observed confirmed that the ability of Lm to adhere to and invade human cells in vitro is not always decipherable from its virulence gene profile.

Atypical Serogroup IVb-v1 of Listeria monocytogenes Assigned to New ST2801, Widely Spread and Persistent in the Environment of a Pork-Meat Producing Plant of Central Italy.

In this study, we characterized 84 Listeria monocytogenes (Lm) strains having an atypical IVb-v1 profile and isolated in a meat producing plant of Central Italy. They were assigned to the new MLST type ST2801 (CC218). The new ST was widespread in the food-producing environment where it was able to persist for over a year even after cleaning and sanitation. Cluster analysis identified three main clusters genetically close to each other (0-22 allelic differences and 0-28 SNPs) from two different cgMLST types, suggesting a common source. The coexistence of closely related clusters over time could be the result of a different evolution path starting from a common ancestor first introduced in the plant and/or the consequence of the repetitive reintroduction of closely related clones probably by raw materials. All the strains presented several determinants for heavy metals resistance, stress response, biofilm production, and multidrug efflux pumps with no significant differences among the clusters. A total of 53 strains carried pLI100 and the j1776 plasmids, while in one strain, the pLM33 was found in addition to pLI100. Only the strains carrying plasmids presented cadA and cadC for cadmium resistance and the mco gene encoding a multicopper oxidase and gerN for an additional Na+/H+-K+ antiporter. All the strains presented a virulence profile including a full-length inlA gene and the additional LIPI-3. The isolation of a new ST with a large pattern of stress-adaptation genes and able to persist is an important contribution to deepening the current knowledge on the uncommon IVb-v1 and in general on the genomic diversity of Lm.

Hyper-Virulent Listeria monocytogenes Strains Associated With Respiratory Infections in Central Italy.

Listeria monocytogenes (Lm) is a foodborne pathogen causing listeriosis. Invasive forms of the disease mainly manifest as septicaemia, meningitis and maternal-neonatal infections. Lm-associated respiratory infections are very rare and little known. We reported two Lm respiratory infection cases occurred in Central Italy during the summer of 2020, in the midst of the SARS-CoV2 pandemic. In addition to collect the epidemiological and clinical characteristics of the patients, we used Whole Genome Sequencing to study the genomes of the Lm isolates investigating their virulence and antimicrobial profiles and the presence of genetic mobile elements. Both the strains belonged to hypervirulent MLST clonal complexes (CC). In addition to the Listeria Pathogenicity Island 1 (LIPI-1), the CC1 strain also carried LIPI-3 and the CC4 both LIPI-3 and LIPI-4. Genetic determinants for antimicrobial and disinfectants resistance were found. The CC1 genome presented prophage sequences but they did not interrupt the comK gene, involved in the phagosomal escape of Lm. None of the strains carried plasmids. Lm is an important, although rare, opportunistic pathogen for respiratory tract and lung infections. To avoid dangerous diagnostic delays of these severe clinical forms, it is important to sensitize hospital laboratories to this rare manifestation of listeriosis considering Lm in the differential diagnosis of respiratory infections.