Submitted to: Journal of Bacteriology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/23/2004
Publication Date: 7/15/2004
Citation: Ward, T.J., Gorski, L.A., Borucki, M.K., Mandrell, R.E., Hutchins, J., Pupedis, K. 2004. Intraspecific phylogeny and lineage group identification based on the Prfa virulence gene cluster of Listeria monocytogenes. Journal of Bacteriology. 2004. 186(15):4994-5002. Interpretive Summary: Listeria monocytogenes is a foodborne bacterium that imposes a serious burden on human health and the agricultural economy. Listeria monocytogenes is responsible for over one-quarter of foodborne disease-related deaths linked to known pathogens, and L. monocytogenes contamination has been the leading cause of food recalls due to biological concerns in recent years. Differences in the ability of groups of strains (lineages) within L. monocytogenes to cause disease in humans have been suggested, and taxonomic changes have been proposed that could impact regulatory policy. However, the number and composition of lineages within L. monocytogenes remains unclear, and perceived differences in virulence or host specificity have not been evaluated with respect to relative frequencies of exposure. To address this problem, DNA sequence data obtained from six genes was used to demonstrate that L. monocytogenes comprises at least three evolutionary lineages. In addition, these data were used to develop a rapid test for lineage identification used in a survey of food products to demonstrate that the low frequency of association between lineage 3 isolates and human listeriosis cases reflects rarity of exposure and not reduced virulence for humans as has been previously suggested. These data also suggest that lineage 3 isolates may be better adapted to the animal production environment than the food processing environment, and indicate that the three L. monocytogenes lineages may represent distinct species. The results of the present study provide a better understanding of differences in the ecology and host specificity of L. monocytogenes lineages, provide evolutionary information regarding the taxonomic status of these lineages that will inform regulatory policy decisions, and provide molecular tools to enhance epidemiological investigations and facilitate additional studies of lineage prevalence in different environments.
Technical Abstract: Listeria monocytogenes is a serious foodborne pathogen which can cause invasive disease in humans and animals, and has been the leading cause of food recalls due to biological concerns in recent years. In order to test hypotheses regarding L. monocytogenes lineage composition, evolution, ecology, and taxonomy, a robust intraspecific phylogeny was developed based on prfA virulence gene cluster sequences from 113 L. monocytogenes isolates. The results of the multi-gene phylogenetic analyses confirm that L. monocytogenes comprises at least three evolutionary lineages, demonstrate that lineages most frequently (lineage 1) and least frequently (lineage 3) associated with human listeriosis are sister-groups, and reveal for the first time that the human epidemic associated serotype 4b is prevalent among strains from lineage 1 and lineage 3. In addition, a PCR-based test for lineage identification was developed and used in a survey of food products to demonstrate that the low frequency of association between lineage 3 isolates and human listeriosis cases likely reflects rarity of exposure and not reduced virulence for humans as has been previously suggested. However, prevalence data do suggest lineage 3 isolates may be better adapted to the animal production environment than the food processing environment. Finally, analyses of haplotype diversity indicate that lineage 1 has experienced a purge of genetic variation which was not observed in the other lineages, and it is suggested that the three L. monocytogenes lineages may represent distinct species within the framework of the cohesion species concept.