Environmental dissemination of pathogenic Listeria monocytogenes in flowing surface waters in Switzerland

Abstract

Listeria monocytogenes is an opportunistic pathogen that is widely distributed in the environment. The aquatic environment may represent a potential source for the transmission of L. monocytogenes to animals and the food chain. The present study assessed the occurrence of L. monocytogenes in 191 surface water samples from rivers, streams and inland canals throughout Switzerland. Twenty-five (13%) of the surface water samples contained L. monocytogenes. Whole genome sequence (WGS) data were used to characterize the 25 isolates. The isolates belonged to major lineages I and II, with the majority assigned to either serotype 1/2a (48%), or 4b (44%). The predominant CCs identified were the hypervirulent serotype 4b clones CC1 and CC4, and the serotype CC412; all three have been implicated in listeriosis outbreaks and sporadic cases of human and animal infection worldwide. Two (8%) of the isolates belonged to CC6 which is an emerging hypervirulent clone. All isolates contained intact genes associated with invasion and infection, including inlA/B and prfA. The four CC4 isolates all harbored Listeria pathogenicity island 4 (LIPI-4), which confers hypervirulence. The occurrence of L. monocytogenes in river ecosystems may contribute to the dissemination and introduction of clinically highly relevant strains to the food chain.

Figure 1

Map of Switzerland showing surface waters, sampling locations, and sites of strain isolation. The map was created using the open source geographic information system (GIS) software QGIS version 3.10 (https://qgis.org).

Figure 2

Minimum-spanning tree based on cgMLST allelic profiles of 25 Listeria monocytogenes isolated from surface waters. Each circle represents an allelic profile based on sequence analysis of 1,701 cgMLST target genes. The numbers on connecting lines represent the number of allelic differences between two strains. Each circle contains the strain ID, and CCs are color coded. Lineages and serotypes are indicated.

Figure 3

Minimum-spanning trees based on cgMLST allelic profiles of 25 Listeria monocytogenes isolated from surface water and 368 genome-sequenced clinical L. monocytogenes isolated during 2011–2020 available in the database of the Swiss National Reference Centre for Enteropathogenic Bacteria and Listeria (NENT) in Switzerland. Strains are grouped by clonal complex (CC). Each circle represents an allelic profile based on sequence analysis of 1,701 cgMLST target genes. The numbers on connecting lines represent the number of allelic differences between two strains. Clusters were defined as isolates containing ≤ 10 different alleles between a pair of isolates. Strains from this study are indicated with a pink star.