Edinburgh Napier University

  The virulence potential of 9 L. monocytogenes strains isolated from food products at different stages of the fresh produce supply chain and a food processing facility was determined using G. mellonella. Strains were used to infect G. mellonella larvae through the haemocoel with a dose of 106 CFU, which were then compared to a L. monocytogenes reference EGD-e and also L. ivanovii. Virulence was assessed by evaluating mortality rates, larvae health index score (HIS), counting viable bacteria in larvae, and the larval immunological response to infection. Significant differences in ability to cause larval mortality were observed between strains. L. monocytogenes strains NLmo4 and NLmo5 caused the most mortality rates, 98.8% and 96.7%, respectively, at 7 d after infection, whilst NLmo20 induced a mortality rate of 65% at the same time point, significantly differing from NLmo4&5 (p<0.5). Six isolates that caused the least to most mortality rates we selected and tested for ability to replicate in vitro, in vivo and their effect on larvae haemocyte density. Growth rates in vitro were significantly not different amongst L. monocytogenes strains as well as when compared to L. ivanovii. However, whilst L. monocytogenes strains replicated and persisted in larvae for up to 7 d after infection L. ivanovii was gradually cleared from larvae decreasing by 5 Logs CFU at the end of the 7 d time course. The persistence of these strain in larvae caused damage to larval organs as seen by increasing melanisation and consequently larval death. Insignificant fluctuations in haemocyte density was observed after larvae infection, however, increased expressions of the antimicrobial peptides galiomycin and gallerimycin were noted upon infection with NLmo4, EGD-e and L. ivanovii. Taken together, results of this study suggest L. monocytogenes strains found in fresh produce products have different pathogenic potentials and are potential hazards for human health.