The potential virulence of Listeria monocytogenes strains isolated from fresh produce processing facilities as determined by an invertebrate Galleria mellonella model

Listeria monocytogenes, a bacterium responsible for listeriosis, is an environmental and food-borne pathogen that poses a particular risk to pregnant women and the elderly. While traditionally associated with animal products, ready-to-eat salads are increasingly recognised as a source of Listeria outbreaks. However, little is known about the potential virulence of Listeria isolates from the fresh produce environment. This study assessed the virulence potential of nine L. monocytogenes strains from the fresh produce chain using the Galleria mellonella invertebrate infection model. Larvae were infected with 106 CFU of each strain via their circulatory system and compared to a reference strain L. monocytogenes (EGD-e) and Listeria ivanovii. Virulence was evaluated by measuring mortality rates, health index score of larvae, viable bacterial counts in the larvae, and the larvae's immune. Significant differences in larval mortality were observed among strains. Strains NLmo4 and NLmo5 caused the highest mortality rates (98.8% and 96.7%, respectively at 7 days post-infection), while strain NLmo20 had a significantly lower mortality rate of 65% at the same time point (p<0.05). Six isolates that caused varied mortality rates were then selected and tested for their ability to replicate both in vitro and in vivo and their impact on larval haemocyte density. In vitro growth rates were not significantly different among L. monocytogenes strains or compared to Listeria ivanovii. However, L. monocytogenes strains persisted and replicated in larvae up to 7d days post-infection, whereas Listeria ivanovii was reduced by 5 logs CFU by day 7. The presence of these L. monocytogenes strains caused organ damage in larvae, indicated by increased melanisation and subsequent larval death. Haemocyte density showed insignificant fluctuations following infection. In conclusion, the results of this study suggest L. monocytogenes strains from fresh produce food chain have varying pathogenicity levels and can pose potential risk to human health.