Molecular grouping and pathogenic analysis of Listeria monocytogenes of clinical and food origin

The phylogenesis and pathogenesis of ten Chinese Listeria monocytogenes isolates of clinical (n = 2) and food (n = 8) origin have been investigated in this study. The 597 bp nucleotide sequence at 3′ terminal of actA gene of these L. monocytogenes isolates were amplified and sequenced. Compared with those of published sequences of the corresponding region within the same gene, the phylogenetic tree was constructed using MEGA2.1 software. It was shown that one clinical strain 1579 and food strains YZ5, YZ7, YZ8 belong to lineage I, while the other clinical strain 1191, and food strains YZ1, YZ2, YZ3, YZ4, YZ6 belong to lineage II. The virulence of these strains based on experimental infection model of SPF chick embryos were determined, and compared between L. monocytogenes isolates of two lineages and their hemolytic activity was assessed according to the development of zones of hemolysis around the colonies. Isolates of lineage I showed high virulence while isolates of lineage II (except clinical isolates 1191) exhibited low virulence, since 100% mortality rate and relative short time to death for embryos were observed for strains of lineage I, while 20–30% mortality rate and long time to death for embryos were observed for isolates of lineage II. Hyper-virulent isolate 1579, YZ8 and lower-virulent isolates YZ2, YZ3, YZ4 and YZ6 show strong hemolytic activity while hyper-virulent isolate 1191, YZ5, YZ7 and lower-virulent isolate YZ1 show weak hemolytic activity. The discrepancy between virulence and hemolytic activity indicates that L. monocytogenes hemolytic activity is not directly proportional to their virulence and thus it can not be used as a reliable criteria for assessment of the virulence of L. monocytogenes.