Controlling Listeria monocytogenes in the food processing environment: Lessons learned from a salmon processor associated with outbreaks

Listeria monocytogenes is a pathogenic bacterium responsible for listeriosis, often linked to ready-to-eat foods like cold-smoked salmon. This study investigates contamination sources and mitigation measures using a ‘Seek and Destroy’ process at a cold-smoked salmon producer linked to two listeriosis outbreaks caused by L. monocytogenes ST121. Over 11 weeks, 329 samples were analysed (140 food-contact sites, 41 non-food-contact, and 148 salmon/product samples). Processing line studies showed contamination occurring after skinning and/or salting. The skinning machine was identified as the primary contamination source: the outbreak strain persisted in two conveyor belts and an air-filled roller. Despite extensive cleaning and disinfection, hydrogen-peroxide whole-room treatment, and heat exposure of the skinning machine up to 80 °C for 66 h (excluding heat-sensitive belts/roller), contamination recurred. Replacing the skinning machine, combined with enhanced sanitation (including disassembly/boiling of brine-injector needles), was followed by no detection in 34 salmon samples and 53 environmental samples during Weeks 8–11, and during 8 months of follow-up, none of 220 tested product samples were positive. Whole genome sequencing (WGS) was performed on 37 isolates: 21 collected during the factory investigation (Weeks 1–11), nine historical isolates, and seven from follow-up experiments with the discarded skinning machine. All belonged to ST121, and one cluster matched the outbreak strain. The study highlights the importance of hygienic design in food processing equipment (especially belt and roller materials/interfaces) and the value of WGS in resolving persistent contamination in RTE seafood processing. • Outbreak strain L. monocytogenes ST121 persisted in a salmon skinning machine. • Extensive deep cleaning and disinfection failed to remove contamination. • Outbreak strain found in conveyor belts and an air-filled roller • Replacing the skinning machine prevented further detection of the outbreak strain. • Hygienic design and WGS are critical for controlling contamination.