Unravelling Listeria monocytogenes persistence in frozen potato processing: Insights from whole genome sequencing analysis

Listeria monocytogenes persistence remains an issue in the food industry, including the frozen vegetable/potato processing industry. This study demonstrates the added value of whole genome sequencing (WGS) for persistence and transmission routes within food businesses. A total of 79 isolates from environmental samples (including employees' shoes), collected over three years (2020−2022), along with two intermediate product samples, and one end product sample, and submitted for WGS. The isolates were taken from in and around the freezing tunnel of a potato processing plant after blanching and frying. Of the 82 isolates, 56 were confirmed as L. monocytogenes, 17 as L. innocua, two as Enterococcus faecalis, and seven had unclear/mixed results using both TYGS (Type Strain Genome Server) and FastANI (Fast Average Nucleotide Identity). Clusters were identified using a 99.5 % ANI value cutoff. A single large L. monocytogenes cluster encompassed 55 of the 56 confirmed isolates and exhibited fewer than six allelic differences in pairwise comparisons based on cgMLST results. Two L. innocua clusters comprised 12 and five isolates, respectively. The results indicated L. monocytogenes persistence for at least three years and identified employees' shoes and a tool cart's wheels as potential transmission vehicles. The isolates of intermediate product samples belonged to the L. monocytogenes cluster, indicating post-contamination via the freezing tunnel. However, the end product isolate did not belong to the L. monocytogenes cluster, suggesting other high-risk areas within the facility. Various genes related to virulence, stress response, biofilm formation, and disinfectant resistance were identified in both L. monocytogenes and L. innocua clusters and in the L. monocytogenes end product isolate.