Sequence typing confirms that a predominant Listeria monocytogenes clone caused human listeriosis cases and outbreaks in Canada from 1988 to 2010

Abstract

Human listeriosis outbreaks in Canada have been predominantly caused by serotype 1/2a isolates with highly similar pulsed-field gel electrophoresis (PFGE) patterns. Multilocus sequence typing (MLST) and multi-virulence-locus sequence typing (MVLST) each identified a diverse population of Listeria monocytogenes isolates, and within that, both methods had congruent subtypes that substantiated a predominant clone (clonal complex 8; virulence type 59; proposed epidemic clone 5 [ECV]) that has been causing human illness across Canada for more than 2 decades.

Fig 1

Minimum spanning tree analysis of Canadian human clinical L. monocytogenes isolates obtained between 1988 and 2010 based on the MLST method of Ragon et al. (10). The data set is based on isolates presented in Table S1 in the supplemental material. Circles correspond to sequence types (STs), while numbers on branches are the numbers of allele differences between connected STs. Clonal complexes (STs with single allele differences) are shaded gray, and clonal complex 8 (CC8) is indicated. The size of each circle is proportional to the number of isolates in each ST. The alignment and minimum spanning tree were created using BioNumerics v5.10.

Fig 2

Unrooted unweighted-pair group method using average linkages (UPGMA) tree based on MVLST with 6 virulence gene fragments that shows the placement of the proposed epidemic clone V (ECV) isolates among other ECs and non-ECs of L. monocytogenes. Non-ECV strains that were part of the 71 isolates analyzed in the present study are listed in bold with the NML of Canada isolate identification number. BL, isolates from Knabel's MVLST database that were not part of the 71 isolates analyzed in the present study but that were added for comparison purposes. Lineage information is shown as I, II, and III.