Molecular Tracing to Find Source of Protracted Invasive Listeriosis Outbreak, Southern Germany, 2012-2016

Abstract

We investigated 543 Listeria monocytogenes isolates from food having a temporal and spatial distribution compatible with that of the invasive listeriosis outbreak occurring 2012-2016 in southern Germany. Using forensic microbiology, we identified several products from 1 manufacturer contaminated with the outbreak genotype. Continuous molecular surveillance of food isolates could prevent such outbreaks.

Keywords: Germany; Listeria monocytogenes; bacteria; food safety; foodborne disease; forensic microbiology; listeriosis; molecular tracing; molecular typing; outbreak; pulsed-field gel electrophoresis; whole-genome sequencing.

Figure 1

ApaI restriction enzyme analysis of Listeria monocytogenes outbreak strain and isolates with >90% similarity to the outbreak strain, southern Germany, 2012–2016. We performed molecular subtyping in line with the PulseNet standardized PFGE protocol for L. monocytogenes (8) and the standard operating procedures of the European Union Reference Laboratory for L. monocytogenes (9) to ensure interlaboratory comparability of the results. We analyzed PFGE patterns using BioNumerics software version 7.5 (Applied Maths, Sint-Martens-Latem, Belgium). Dendrogram indicates percentage similarity between the ApaI PFGE pattern of the outbreak strain and that of the other closely related isolates. Outbreak strain PFGE pattern is labeled with the number 54. PFGE, pulsed-field gel electrophoresis.

Figure 2

Minimum spanning tree estimating the phylogenetic relationships among outbreak and nonoutbreak Listeria monocytogenes isolates from humans and from food products, southern Germany, 2012–2016. We conducted bioinformatics analyses using the Ridom SeqSphere+ software version 3.1.0-2016-01 (Ridom GmbH, Münster, Germany). The core-genome multilocus sequence typing scheme for whole-genome sequencing–based typing of L. monocytogenes relies on a set of 1,701 target genes (alleles) that are present in >99% of the known genomes of the species (7). Each circle represents an allelic profile. The numbers on the connecting lines illustrate the number of differing alleles in a pairwise comparison. Closely related genotypes (<10 allele difference) designated a cluster type and are indicated with bold red numbers. CT, cluster type; PFGE, pulsed-field gel electrophoresis.