Tracing Listeria monocytogenes in the Atlantic salmon (Salmo salar L.) production chain - from feed to primary processing

Listeria monocytogenes is a cold tolerant bacterial pathogen that thrives in many food processing environments. In the Atlantic salmon industry, it is a persisting food safety issue, and considerable efforts are made to keep it under control. However, there is sparse knowledge on the prevalence and potential transmission of L. monocytogenes in the production phase of salmon upstream of the primary processing facilities. A risk-based sampling plan for tracing L. monocytogenes in the Atlantic salmon ( Salmo salar L.) production chain, ranging from salmon feed production to processed salmon, was generated based on a simplified hazard analysis. A total of 1819 samples from the production chain were collected over almost one year. L. monocytogenes were detected by a qualitative real-time PCR method with culture-based confirmation and a subset of isolates were submitted to whole genome sequencing. The overall prevalence of L. monocytogenes was 2.7%. It was detected in samples from the salmon feed production (8%), in the sea phase production (1%), and in the primary processing facility (3%). A total of ten sequence types (STs) of L. monocytogenes were identified. The diversity was considerably higher in the feed production chain (eight STs) compared to the sea phase (two STs) and in the salmon processing facility (two STs). Noteworthy, L. monocytogenes of ST8 and ST37, displaying high degree of genetic relatedness in both core genome multi locus sequence typing (cgMLST) and single nucleotide polymorphism (SNP) based analyses, were found both in the feed production and in the processing facility, indicating a potential link between these two parts of the production chain. • Tracing of L. monocytogenes throughout the Atlantic salmon production chain. • Risk-based sampling scheme to target high-risk points in the production chain. • Highest prevalence and ST diversity of L. monocytogenes in feed production. • Low prevalence of L. monocytogenes (1%) and low ST diversity in the sea phase. • ST8 and ST37 were detected both in the feed and primary processing environments. • The role of feed as a source of L. monocytogenes must be further explored.