Evolution and Molecular Phylogeny ofListeria monocytogenesIsolated from Human and Animal Listeriosis Cases and Foods

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Abstract
To probe the evolution and phylogeny ofListeria monocytogenesfrom defined host species and environments,L. monocytogenesisolates from human (n= 60) and animal (n= 30) listeriosis cases and food samples (n= 30) were randomly selected from a larger collection of isolates (n= 354) obtained in New York State between 1999 and 2001. Partial sequencing of four housekeeping genes (gap,prs,purM, andribC), one stress response gene (sigB), and two virulence genes (actAandinlA) revealed between 11 (gap) and 33 (inlA) allelic types as well as 52 sequence types (unique combination of allelic types).actA,ribC, andpurMdemonstrated the highest levels of nucleotide diversity (π > 0.05).actAandinlAas well asprsand the hypervariable housekeeping genesribCandpurMshowed evidence of horizontal gene transfer and recombination.actAandinlAalso showed evidence of positive selection at specific amino acid sites. Maximum likelihood phylogenies for all seven genes confirmed thatL. monocytogenescontains two deeply separated evolutionary lineages. Lineage I was found to be highly clonal, while lineage II showed greater diversity and evidence of horizontal gene transfer. Allelic types were exclusive to lineages, except for a singlegapallele, and nucleotide distance within lineages was much lower than that between lineages, suggesting that genetic exchange between lineages is rare. Our data show that (i)L. monocytogenesis a highly diverse species with at least two distinct phylogenetic lineages differing in their evolutionary history and population structure and (ii) horizontal gene transfer as well as positive selection contributed to the evolution ofL. monocytogenes.