Through whole genome sequencing (WGS) analysis, a recent study has revealed pervasive strains of Listeria monocytogenes to be a common issue throughout the Norwegian food system. The study also associated the pervasive strains with an overall increase in the pathogen’s persistence and stress survival.

Researchers subjected 769 L. monocytogenes isolates from food industry environments, foods, and raw materials to WGS, single-nucleotide polymorphism, and comparative genomic analyses in an attempt to better understand the pathogen’s diversity, distribution, persistence, and prevalence. The data set included isolates from nine meat and six salmon processing facilities in Norway over a period of three decades.

The analyses identified one clonal complex (CC) as the most prevalent throughout Norway, being isolated from surfaces in 10 food production facilities. Three additional CCs were found in seven facilities each. The researchers classified 72 percent of the isolates as “persistent,” with WGS revealing 20 or fewer allelic differences toward an isolate found in the same factory during a different calendar year. Additionally, 56 percent of isolates showed genetic similarity toward another isolate collected from a different food processing facility; the researchers designated these strains as “pervasive,” defined as clusters with the same level of genetic similarity as persistent strains but isolated from different factories, which may have enhanced ability to spread or migrate to new geographical locations. The researchers note that defining strains as “pervasive” rather than “persistent” may be more useful in identifying strains that carry adaptations responsible for increased ability to survive and multiply in food processing environments.

The study found an increase of genetic determinants associated with increased survival in food processing environments among pervasive isolates, but not persistent isolates. Such genetic determinants included resistance to biocides and heavy metals, stress response genes, and inlA truncation mutations that may facilitate increased adhesion and biofilm formation, as well as increased tolerance to desiccation. The genetic determinants were also significantly more prevalent among the isolates from food processing environments than the isolates from natural and rural environments and clinical isolates, suggesting that genetics play a role in which strains survive in food environments. The researchers hypothesize that there is not one single genetic determinant responsible for survival in food processing environments, but rather an accumulation of stress resistance genes, genetic proclivity to biofilm formation, and inlA mutations.

The study also identified several problem areas within facilities that harbored persistent L. monocytogenes strains, including floors, drains, slicers, and tables. Additionally, isolation of persistent strains on conveyors and in fish gutting machines was observed in salmon processing plants.

The results of the study demonstrate the extensive spread of highly similar L. monocytogenes strains throughout the Norwegian food system. The researchers postulate that the occurrence of pervasive strains in the Norwegian food industry may be significantly higher than in other countries, perhaps due to a particularly complex and interconnected Norwegian meat supply chain. Additionally, in light of the study’s findings indicating that L. monocytogenes strains in food industry environments have increased genetic stress survival, the researchers suggest that facilities switch to a disinfectant with another mechanism of action when facing challenges with eliminating the pathogen, such as oxidative disinfectants, QAC-based disinfectants, or citric acid.