Listeria monocytogenes was detected in 23/132 RTE samples.

Starters with mayonnaise showed a very high prevalence (56.7 %).

49 isolates belonged to two serogroups IIb and IIa and were related to CC288-ST330 and CC121-ST717.

The results of the challenge test showed a limited ability to grow in the selected RTE product.

This study investigated Listeria monocytogenes prevalence and count in 132 ready-to-eat (RTE) delicatessen samples belonging to different categories (starters with/without mayonnaise pasta/rice-based courses, meat/fish-based main courses) produced by an Italian industry. Whole Genome Sequencing characterized the isolates to map the pathogen circulation. Moreover, the growth potential of L. monocytogenes in the most contaminated product was investigated by a challenge test.

L. monocytogenes was detected in 23 samples, giving an estimated prevalence of 17.4 %. Starters with mayonnaise showed a very high prevalence (56.7 %), showing the role of the sauce in the diffusion of the pathogen within the plant. A total of 49 isolates were obtained; they belonged to two different serogroups, IIb and IIa, and were related to two clonal complexes (CCs) and sequence types (STs) (CC288-ST330 and CC121-ST717), suggesting the possible persistence and circulation of the pathogen within the plant.

The results of the challenge test showed a limited ability to grow in the selected product thanks to the presence of lactic microflora.

Growth potential

Listeria monocytogenes

Delicatessen

Ready-to-eat

Whole genome sequencing

The genome assemblies of the strains were deposited at DDBJ/ENA/GenBank under the BioProjects PRJNA982763 (https://www.ncbi.nlm.nih.gov/bioproject/?term=PRJNA982763).

© 2023 The Authors. Published by Elsevier B.V.