Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: September 4, 2008
Publication Date: September 4, 2008
Citation: Ward, T.J. 2008. Evolutionary analyses of Listeria monocytogenes and application to molecular subtyping. Meeting Abstract. Technical Abstract: In order to test hypotheses regarding Listeria monocytogenes evolution, ecology, taxonomy, and lineage composition, a robust phylogenetic framework was developed based on allelic variation identified at more than 20 genes from across the genomes of more than 200 L. monocytogenes isolates. These analyses demonstrate that L. monocytogenes represents a single monophyletic group within the Listeria, and is divided into at least four major evolutionary lineages, that may represent distinct species. Detailed analyses of genetic variation within the three most common L. monocytogenes lineages revealed that the epidemic-associated lineage I strains have experienced a series of population bottlenecks that did not affect variation within other lineages. Phylogenetic analyses further demonstrated that serotype 4b isolates from lineage I form a monophyletic group most closely related to serotype 1/2b isolates, and that previously defined epidemic clones from lineage I do not form a distinct monophyletic clade within the 4b group. This detailed characterization of genetic diversity within L. monocytogenes was used to develop lineage-specific multilocus genotyping assays for rapid and accurate subtype determination based on single nucleotide polymorphisms. The potential utility of these assays for epidemiological investigations and outbreak detection is being investigated. In addition, application of these assays to a large panel of ready to eat (RTE) food product isolates demonstrated that lineage I isolates with a 4b serotype are rare in RTE food products, but are dominated by epidemic clones; that a variety of inlA truncation mutants are commonly (˜ 30%) found in RTE products; and that several of the PFGE types most commonly encountered in RTE foods are polymorphic for inlA truncations. These common PFGE types from RTE have a polyphyletic distribution when mapped onto the multigene phylogeny, indicating that widely distributed strains, which are apparently well-adapted to food processing environments, are genetically diverse and do not share a unique evolutionary history. These results suggest that MLGT has the potential to be of significant value in risk assessment as it provides direct information on subtypes associated with greater/lower public health risk and allows for accurate determination of strain relatedness so that risk assessment models can be applied to individual clonal lineages.