Genome organization and the evolution of the virulence gene locus in Listeria species.

The chromosomal region of Listeria monocytogenes harboring the gene cluster prfA-plcA-hly-mpl-actA-plcB (virulence gene cluster; vgc) harbors virulence genes critical for the survival of the bacteria following infection. Previous studies have implicated it as an ancestral pathogenicity island, derivatives of which are present in the species L. ivanovii and L. seeligeri, but absent in non-pathogenic species such as L. innocua. We cloned the corresponding region from L. innocua and L. welshimeri and compared its sequences to those from L. monocytogenes, L. ivanovii and L. seeligeri. The analysis allowed exact determination of delineation and size of the vgc and suggests that these genes may have been acquired by bacteriophage transduction. Thus, here we present an alternative view of the evolution of Listeria spp. and suggest that L. monocytogenes may be the primordial species of this genus.