R-060. Population Structure and Recombination in Listeria monocytogenes

H. C. den Bakker1, X. Didelot2, E. Fortes1, K. Nightingale3, R. H. Orsi1, M. Wiedmann1;
1Cornell Univ., Ithaca, NY, 2Univ. of Warwick, Coventry, UNITED KINGDOM, 3Colorado State Univ., Fort Collins, CO.

In order to elucidate the population structure of Listeria monocytogenes we studied the the patterns of genetic diversity in a sample of 184 isolates of this species from the state of New York (US). The sources of the isolates were human and animal sporadic clinical cases, foods and urban, farm and natural environments. The loci sequenced in this study were fragments from four house-keeping genes (gap, prs, purM and ribC), one stress response gene (sigB) and two virulence genes (actA and inlA). Most population genetic summary statistics of the before-mentioned loci were similar to previously reported values, with the exception of prs. This locus displays an exceptionally low diversity in the two main lineages of L. monocytogenes (5 haplotypes in lineage I and 4 haplotypes in lineage II). A strongly negative Tajima’s D (-2.02, P< 0.05) indicates that prs is has been subject to a selective sweep or that the population went through a bottleneck. Recombination is most prevalent in lineage II and is most frequent in two house-keeping genes (ribC and purM) and the two virulence genes (actA and inlA). Bayesian phylogenetics (ClonalFrame) and population genetics (STRUCTURE) analyses suggest Lineage I is a species in its own right, with a relatively low recombination rate (ρ/θ = 0.16), and that most of the recombination is intra-lineage. Lineage II seems to represent a so-called fuzzy species, with a relatively high recombination rate (ρ/θ = 0.71) and imports coming from lineage II and lineage III. While lineage I has traditionally been seen as less divergent and clonal, our analyses clearly show that high diversity in lineage II is due to recombination. Though lineage I and lineage II may be considered separate species from theoretical point of view, they do not seem to differ in their ability to cause listeriosis and therefore it is not recommended to change these lineages to the rank of species.