PervasiveListeria monocytogenesare common in the Norwegian food system and associated with increased prevalence of stress survival and resistance determinants

To investigate the diversity, distribution, persistence, and prevalence of stress survival and resistance genes of Listeria monocytogenes clones dominating in food processing environments in Norway, genome sequences from 769 L. monocytogenes isolates from food industry environments, foods, and raw materials (of which 512 were sequenced in the present study) were subjected to wgMLST, SNP, and comparative genomic analyses. The dataset comprised isolates from nine meat and six salmon processing facilities in Norway collected over a period of three decades. The most prevalent clonal complex (CC) was CC121, found in ten factories, followed by CC7, CC8, and CC9, found in seven factories each. Overall, 72% of the isolates were classified as persistent, showing 20 or fewer wgMLST allelic differences towards an isolate found in the same factory in a different calendar year. Moreover, over half of the isolates (56%) showed this level of genetic similarity towards an isolate collected from a different food processing facility. These were designated as pervasive strains, defined as clusters with the same level of genetic similarity as persistent strains but isolated from different factories. The prevalence of genetic determinants associated with increased survival in food processing environments, including heavy metal and biocide resistance determinants, stress response genes and inlA truncation mutations, showed a highly significant increase among pervasive isolates, but not among persistent isolates. Furthermore, these genes were significantly more prevalent among the isolates from food processing environments compared to in isolates from natural and rural environments (n=218) and clinical isolates (n=111) from Norway. Importance Listeria monocytogenes can persist in food processing environments for months to decades and spread through the food system by e.g., contaminated raw materials. Knowledge about the distribution and diversity of L. monocytogenes is of importance in outbreak investigations and essential to effectively track and control this pathogen in the food system. The current study presents a comprehensive overview of the prevalence of persistent clones and of the diversity of L. monocytogenes in Norwegian food processing facilities. The results demonstrate extensive spread of highly similar strains throughout the Norwegian food system, in that 56% of the 769 collected isolates from food processing factories belonged to clusters of L. monocytogenes identified in more than one facility. These strains were associated with an overall increase in the prevalence of plasmids and determinants of heavy metal and biocide resistance as well as other genetic elements associated with stress survival mechanisms and persistence.