Listeria monocytogenes is a foodborne pathogen which is able to cause serious disease both in humans and in animals. Several studies have demonstrated variations in the levels of virulence among L. monocytogenes strains. Invasion and growth ability of L. monocytogenes into cultured cells have been used to evaluate its pathogenicity. In particular, invasiveness and growth ability have been typically investigated using HeLa cell line. This study aimed to provide further insights on the virulence potential as well as on the molecular and phenotypic characteristics of L. monocytogenes isolated both from food sources and food environments. Thirty-eight isolates were tested for cell invasion and intracellular growth. Among the latter, 15 strains exhibited a high invasion index (I.I.); 18 strains showed intermediate II and 5 isolates revealed a low II. Regarding intracellular growth, all tested isolates had a replication time between 2 and 6h. Furthermore, nine virulence-associated genes (hlyA, actA, inlA, inlB, iap, plcA, plcB, mpl, prfA) were investigated by the multiplex PCR assay. All tested virulence genes were detected in all strains. Interestingly, a polymorphism was observed in the actA gene. However, the polymorphism could not be related to a different level of invasion or intracellular growth. In conclusion, data presented in this study have revealed considerable differences in the ability of L. monocytogenes strains to invade host cells and suggest the presence of additional factors that may contribute to adhesion and invasion. Virulence of L. monocytogenes is still not fully understood in some respects. Further studies focused on the mechanisms of L. monocytogenes pathogenicity together with the development of more reliable and efficient methods for virulence determination in this species are still required.
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