Backtracking and forward checking of human listeriosis clusters identified a multiclonal outbreak linked to Listeria monocytogenes in meat products of a single producer

Abstract

Due to its high case fatality rate, foodborne listeriosis is considered a major public health concern worldwide. We describe one of the largest listeriosis outbreaks in Germany with 83 cases of invasive listeriosis between 2013 and 2018. As part of the outbreak investigation, we identified a highly diverse Listeria monocytogenes population at a single producer of ready-to-eat meat products. Strikingly, the extensive sampling after identification of a first match between a cluster of clinical isolates and a food isolate allowed for a linkage between this producer and a second, previously unmatched cluster of clinical isolates. Bacterial persistence in the processing plant and indications of cross-contamination events explained long-term contamination of food that led to the protracted outbreak. Based on screening for virulence factors, a pathogenic phenotype could not be ruled out for other strains circulating in the plant, suggesting that the outbreak could have been even larger. As most isolates were sensitive to common biocides used in the plant, hard to clean niches in the production line may have played a major role in the consolidation of the contamination. Our study demonstrates how important it is to search for the origin of infection when cases of illness have occurred (backtracking), but also clearly highlights that it is equally important to check whether a contamination at food or production level has caused disease (forward checking). Only through this two-sided control strategy, foodborne disease outbreaks such as listeriosis can be minimized, which could be a real improvement for public health.

Keywords: Listeriosis; disinfection; outbreak; source tracking; whole genome sequencing.

Figure 1.

Epidemiological curve of the listeriosis outbreak under investigation. The outbreak comprised 83 cases between 2013 and 2018 and consisted of two distinct core genome MLST clusters (cluster 1, blue, and cluster 2, orange). During the outbreak, a shift from mainly male cases (dark shading) to a balanced ratio between male and female cases (light shading) occurred.

Figure 2.

Core genome (cg) MLST-based single linkage clustering of 312 Listeria monocytogenes isolates from clinical cases and a single food producer. Colored rings indicate clustering results and metadata of isolates. From inside to outside: cgMLST cluster number, source and sampling year.

Figure 3.

Heatmap of in silico detected virulence genes. Black: gene present; white: gene absent. Isolates are sorted by source and by cgMLST cluster number. A set of 26 virulence genes was present in all 312 study isolates. Genes positively associated with the outbreak clusters are marked by an asterisk. CgMLST clusters 9, 10, and 12–16 had the same virulence factor composition as outbreak cluster 1. Isolates in the clusters 8 and 11 were lacking the vip gene and were hence identical to outbreak cluster 2.

Figure 4.

Heatmap of in silico detected antimicrobial resistance genes. Black: gene present; white: gene absent. Isolates are sorted by source and by cgMLST cluster number (for legend see Figure 3). All 312 study isolates contained the lin gene (lincomycin resistance) and the fosX gene (fosfomycin resistance). Only 48 isolates of three different cgMLST clusters (cluster 3, 9 and 15) additionally contained the benzalkonium chloride tolerance genes bcrB and bcrC.