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Gene Fragments Distinguishing an Epidemic-Associated Strain from a Virulent Prototype Strain ofListeria monocytogenes Belong to a Distinct Functional Subset of Genes and Partially Cross-Hybridize with OtherListeria Species

ABSTRACT Most major food-borne outbreaks of listeriosis in Europe and in the United States have been caused by genetically closely relatedListeria monocytogenes strains of serotype 4b. In order to assess whether genomic loci exist that could underlie this increased epidemic potential, we subtracted the genome of the virulent prototypeL. monocytogenes strain EGD from a prototype epidemic strain. A total of 39 DNA fragments corresponding to 20% of an estimated total of 150 to 190 kb of differential genome material were isolated. For 21 of these fragments, no function on the basis of homology could be predicted. Of the remaining 18 fragments, 15 had homologies to bacterial surface proteins, some of which have been implicated in virulence mechanisms such as cell invasion, adhesion, or immune escape. Southern hybridization of arrays containing the epidemic-clone-specific DNA segments with genomic DNA of differentL. monocytogenes strains was consistent with the current lineage division. Surprisingly, however, some of the fragments hybridized in a mosaic-like fashion to genomes of two otherListeria species, the animal pathogen L. ivanovii and the nonpathogen L. innocua. Taken together, our results provide a starting point for the identification of epidemic-trait-associated genes.