"Epidemic clones" of Listeria monocytogenes are widespread and ancient clonal groups

Abstract

The food-borne pathogen Listeria monocytogenes is genetically heterogeneous. Although some clonal groups have been implicated in multiple outbreaks, there is currently no consensus on how "epidemic clones" should be defined. The objectives of this work were to compare the patterns of sequence diversity on two sets of genes that have been widely used to define L. monocytogenes clonal groups: multilocus sequence typing (MLST) and multi-virulence-locus sequence typing (MvLST). Further, we evaluated the diversity within clonal groups by pulsed-field gel electrophoresis (PFGE). Based on 125 isolates of diverse temporal, geographical, and source origins, MLST and MvLST genes (i) had similar patterns of sequence polymorphisms, recombination, and selection, (ii) provided concordant phylogenetic clustering, and (iii) had similar discriminatory power, which was not improved when we combined both data sets. Inclusion of representative strains of previous outbreaks demonstrated the correspondence of epidemic clones with previously recognized MLST clonal complexes. PFGE analysis demonstrated heterogeneity within major clones, most of which were isolated decades before their involvement in outbreaks. We conclude that the "epidemic clone" denominations represent a redundant but largely incomplete nomenclature system for MLST-defined clones, which must be regarded as successful genetic groups that are widely distributed across time and space.

Fig 1

Minimum spanning tree of MLST data for 863 L. monocytogenes isolates. Each circle represents one ST, the size of which is related to the number of isolates with this ST, as indicated (111 represents ST1, the central ST of CC1). Red sectors represent the 125 isolates or reference strains included in this work, whereas white sectors represent previously published strains. Links between circles are represented according to the number of allelic mismatches between STs, as indicated. Gray zones surrounding groups of STs represent CC. Most CC numbers are indicated; those CCs that correspond to “epidemic clones” are written in bold with the corresponding EC number in parentheses. Laboratory reference strains are indicated in blue. STs of outbreak reference strains are indicated by colored triangles as shown on the right. Top, lineage II (starting with CC37); bottom, lineage I (starting with CC195).

Fig 2

Compared phylogenies based on MLST (left) and MvLST (right) gene sequences of the 125 study isolates. Phylogenies were obtained based on concatenated gene sequences by using the neighbor-joining method based on uncorrected p-distances. Branches corresponding to clonal complexes are colored according to the central legend. The positions of reference strains are indicated.