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

Abstract

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.

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

“…The amount of genes that were equal between CC1 and CC9 was the same as that between CC218 and CC9. These results were expected and consistent with previous studies describing hypervirulent and hypovirulent Lm clones [ 37 , 38 , 46 , 47 ]. Once identified the virulence gene profile for each strain and compared it between different CCs; Caco-2 cell line cells were used to evaluate their effective adhesion and invasion abilities and to investigate if the virulence gene profile of these Lm strains may predict their virulence in vitro.…”

“…As reported in previous studies, all the CC9 and CC121 strains carried a PMSC mutation in the inlA gene, leading to a truncated protein [ 37 ]. In contrast, atypical hemolytic L. innocua strains harboring a functional inlA gene have been described [ 55 ].…”

“…Four strains belonged to CC9, considered hypovirulent and infecting immunocompromised people [ 22 , 32 ]. The remaining three strains belonged to CC218 and in particular to the ST2801, recently reported by Guidi et al 2022 [ 37 ] for the first time and for which virulence potential was never studied before.…”

“…All the strains of this study were isolated from nonfood contact surfaces sampled during production and belonged to genomic clusters found to persist inside the plant. In more detail, two strains belonged to CC1-ST1 (serogroup IVb), four strains belong to CC9-ST9 (serogroup IIc), and the remaining three strains belonged to CC218-ST2801 and presented the atypical IVb-v1 serogroup profile [ 37 ]. CC1 and CC218 strains were selected for this study as they belonged to hypervirulent CCs, while CC9 strains were representatives of a hypovirulent CC.…”

Genomic and pathogenicity islands of Listeria monocytogenes—overview of selected aspects

“…Continued) Summary of selected papers (published in[2018][2019][2020][2021][2022] describing the frequency of genomic/pathogenicity islands among L. monocytogenes strains isolated from various sources.…”