Whole Genome Sequence–Based Surveillance of Human Clinical Listeria monocytogenes Isolates From Switzerland, 2019–2024

Abstract Background Listeria monocytogenes is a foodborne pathogen that poses a significant public health risk due to the high case-fatality rate of listeriosis. Whole genome sequencing (WGS) is critical for identifying outbreak clusters and enables high-resolution characterization of isolates. Methods We collected 335 L monocytogenes isolates from human cases of listeriosis occurring in Switzerland between 2019 and 2024. WGS was used to identify disease clusters and characterize the phylogenetic relatedness, virulence profiles, and antimicrobial resistance genes of the isolates. Results The majority (77%) of the cases involved patients aged ≥65 years and was associated with bacteremia. The 5 major clonal complexes (CCs) were CC1, CC4, CC6, CC8, and CC388. The isolates belonged to 35 infection clusters, including 3 large outbreak clusters of 21 to 37 isolates per cluster. Nine smaller clusters included isolates highly related to food isolates obtained up to 4 years earlier. Multiple singly occurring isolates were linked to international outbreaks. WGS identified variations within key virulence factors including premature stop codons in InlA, length polymorphisms of ActA, and amino acid substitutions within PrfA. Hypervirulence associated with Listeria pathogenicity island (LIPI)-3 was found predominantly among CC1, CC4, and CC6 isolates. LIPI-4 was present in CC4 and CC388, and in infrequently occurring ST32, ST213, ST217, ST220, ST382, and ST1460. Conclusions High-resolution typing of L monocytogenes is effective for the detection of disease clusters and for linking cases to international outbreaks. The identification of rare hypervirulent lineages demonstrates the importance of investigating the phylogenetic relatedness and virulence profiles of clinical L monocytogenes.