Phenotypic and genotypic characterization of persistent and transient L. monocytogenes strains to elucidate mechanisms of survival in the food processing plant environment and impact on virulence

Listeria monocytogenes is an important human pathogen responsible for a range illness of febrile gastroenteritis to meningitis affecting the most vulnerable populations (Pathogen Regulation Directorate, 2011; FDA, 2012; CDC, 2016). These bacteria are ranked as a major foodborne pathogen responsible for 1,591 illness, 1,455 and 255 deaths (Scallan et al., 2011). Listeria monocytogenes are ubiquitous in nature and have been associated with a number of food products; however, L. monocytogenes has been deemed an environmental pathogen suggesting the mode of contamination is from the processing environment. L. monocytogenes possess many factors that contribute to its ability to survive in the food processing environment, such as biofilm formation, sanitizer tolerance and temperature endurance. Specific strains of Listeria seem to be persistently associated within a food processing plant environment residing in harborage site, while other strains are transient in the same facility. The objective of this study is to determine the relationship between phenotypic and genotypic characteristics and the ability for L. monocytogenes to survive and persist in the food processing environment via in vitro assays and whole genome sequencing. Previously isolated strains were characterized persistent or transient strains were determined by a longitudinal study (Brandt, 2014). Strains were subjected to biomass formation assay, commercial sanitizer tolerance assay, Caco-2 invasion assay and genome sequencing and analysis. Subsequent data were recorded and analyzed. Biomass formation was greater for transient strains (P<0.05) compared to persistent strains overall; however, there was a difference (P<0.05) between persistent and transient strains at 30oC and not at 12oC (P>0.05). There was not enough evidence to show a difference (P>0.05) between persistent and transient strains in regard to sanitizer tolerance and Caco-2 cell invasion. There was no notable difference (P>0.05) between L. monocytogenes and non-L. monocytogenes strains biomass formation, sanitizer tolerance, and Caco-2 cell invasion rates. Overall, there was little difference between isolate sanitizer tolerance. Only two isolates exhibited greater invasion rates (P<0.05) as compared to the control (10403S). Some phenotypic variation was determined among the 30 Listeria isolates during the assessment of biomass production and Caco-2 cell invasion; however, there little variation in sanitizer tolerance across the isolates. Genomic analysis revealed seven L. monocytogenes isolates within lineage II and the remaining four L. monocytogenes isolates within lineage I. Genomics data also revealed little core changes in the genomic of persistent strains of Listeria spp. from a particular harborage site. It is imperative to consider the ability for Listeria spp. to persist in a harborage site in the food production environment. Listeria spp. can share genes that encourage persistence; therefore, evaluating the capacity of Listeria to resist harsh conditions yields information that can be used to prevent future outbreaks. These data provided a snapshot of Listerial behavior against common pressures in a food processing environment and demonstrate the benefit of characterizing environmental pathogens from a food processing perspective.