Polymorphism of actA gene is not related to in vitro virulence of Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen which is able to cause serious disease both in humans and inanimals. 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. Inparticular, invasiveness and growth ability have been typically investigated using HeLa cell line. This study aimedto provide further insights on the virulence potential aswell as on themolecular and phenotypic characteristics ofL.monocytogenes isolated both fromfood sources and food environments. Thirty-eight isolateswere tested for cellinvasion and intracellular growth. Among the latter, 15 strains exhibited a high invasion index (I.I.); 18 strainsshowed intermediate II and 5 isolates revealed a low II. Regarding intracellular growth, all tested isolates had areplication time between 2 and 6 h. 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 allstrains. Interestingly, a polymorphismwas observed in the actA gene. However, the polymorphism could not berelated to a different level of invasion or intracellular growth. In conclusion, data presented in this study haverevealed considerable differences in the ability of L. monocytogenes strains to invade host cells and suggest thepresence of additional factors thatmay contribute to adhesion and invasion. Virulence of L. monocytogenes is stillnot fully understood in some respects. Further studies focused on the mechanisms of L. monocytogenespathogenicity togetherwith the development ofmore reliable and efficientmethods for virulence determinationin this species are still required.[...]