Characterization of the growth variability of multiple Listeria monocytogenes strains on smoked salmon

Listeria monocytogenes poses a significant food safety risk in ready-to-eat products like smoked salmon, which support bacterial growth during refrigerated storage and has been linked to several listeriosis outbreaks. Product manufacturers rely on challenge tests and predictive microbiology (PM) to ensure product safety and strain variability is an important consideration for both of these approaches. In this study, we reported a detailed analysis of strain variability in challenge tests and compared results to the outputs of common PM tools. Detailed growth data were generated on a commercial smoked salmon product stored at refrigeration (4.1°C), temperature abuse (9.6°C) and room temperature (19.8°C) with nine individual L. monocytogenes strains. Strains were selected based on differences in lineage, strain origin, cold/salt tolerance in laboratory media and included strains recommended in the European Union Reference Laboratory for L. monocytogenes (EURL-Lm) technical guidance document. The growth rates were compared to predictions from well-established PM tools (ComBase, Food Spoilage and Safety Predictor, Pathogen Modeling Program) as well as a smoked salmon specific secondary model. Growth rates of all strains were below predictions of tertiary models at 9.6°C and 19.8°C but some strains had higher estimated growth rates than predicted rates at 4.1°C. Cluster analysis also demonstrated no significant correlation between growth rates and strain origin or salt/cold tolerance although strains from lineage II were clearly associated with higher growth rates at 4.1°C and 19.8°C. Overall, this study presents detailed strain-specific growth rate data of genetically diverse L. monocytogenes strains on a smoked salmon product and highlights the importance of considering strain variability when designing challenge tests and interpreting predictive microbiology outputs.