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.

To assess the antimicrobial effect of a commercial UV-C system, knives inoculated with Escherichia coli and Staphylococcus aureus as well as naturally contaminated and collected from the wet and clean area of a slaughterhouse knives were examined. For inoculated knives, UVC treatment for 30 s reduced mean E. coli counts by 5.1 log CFU cm-2 and mean S. aureus counts by 4.5 log CFU cm-2. The presence of blood lowered mean reductions to 3.4 log CFU cm-2 for E. coli and to 2.5 log CFU cm-2 for S. aureus. The presence of fat had a greater negative impact on the efficacy of the UV-C treatment resulting in mean reductions <1.8 log CFU cm-2. For naturally contaminated knives from a slaughterhouse, total viable counts (TVC) before UV-C treatment varied considerably (wet area: 2.0-6.0 log CFU cm-2, clean area: 1.0–3.0 log CFU cm-2). UV-C treatment for 30s reduced mean TVC by 0.8 log CFU cm-2 (wet area) and 0.6 log CFU cm-2 (clean area), but the effect varied greatly between individual knives. Thus, under commercial conditions, the antibacterial effect of UV-C for the decontamination of knives is affected by the presence of additional contaminations like blood or fat. The adequate cleaning of the knives prior to UV-C decontamination is therefore of central importance.

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