The presence of the internalin gene in natural atypically hemolytic Listeria innocua strains suggests descent from L. monocytogenes.

The atypical hemolytic Listeria innocua strains PRL/NW 15B95 and J1-023 were previously shown to contain gene clusters analogous to the pathogenicity island (LIPI-1) present in the related foodborne gram-positive facultative intracellular pathogen Listeria monocytogenes, which causes listeriosis. LIPI-1 includes the hemolysin gene, thus explaining the hemolytic activity of the atypical L. innocua strains. No other L. monocytogenes-specific virulence genes were found to be present. In order to investigate whether any other specific L. monocytogenes genes could be identified, a global approach using a Listeria biodiversity DNA array was applied. According to the hybridization results, the isolates were defined as L. innocua strains containing LIPI-1. Surprisingly, evidence for the presence of the L. monocytogenes-specific inlA gene, previously thought to be absent, was obtained. The inlA gene codes for the InlA protein which enables bacterial entry into some nonprofessional phagocytic cells. PCR and sequence analysis of this region revealed that the flanking genes of the inlA gene at the upstream, 5'-end region were similar to genes found in L. monocytogenes serotype 4b isolates, whereas the organization of the downstream, 3'-end region was similar to that typical of L. innocua. Sequencing of the inlA region identified a small stretch reminiscent of the inlB gene of L. monocytogenes. The presence of two clusters of L. monocytogenes-specific genes makes it unlikely that PRL/NW 15B95 and J1-023 are L. innocua strains altered by horizontal transfer. It is more likely that they are distinct relics of the evolution of L. innocua from an ancestral L. monocytogenes, as postulated by others.