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 pulsedfield gel electrophoresis (PFGE) patterns. Multilocus sequence typing (MLST) and multi-virulence-locus sequence typing (MV-LST) 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.

“Epidemic Clones” of Listeria monocytogenes Are Widespread and Ancient Clonal Groups

“…CC8 comprised the 2008 Canada ready-toeat meat products outbreak, consistent with a previous report (3). Strains of this outbreak were recently proposed to represent ECV (19). Clone ECVII (involved in the 2011 U.S. cantaloupe outbreak) was shown to belong to CC7 (4).…”

“…Given the close correspondence between MLST CCs and ECs, the identification methods developed previously for specific ECs (5,15,19,(53)(54)(55)(56) should apply readily to the distinction of MLST clones. Unlike MvLST, MLST data are publicly available for comparison through a dedicated website, providing a continuously growing overview of L. monocytogenes diversity.…”

“…Second, there is no consensual definition of an epidemic clone. For example, authors either accept gene sequence variation within clones (12) or they do not (4,13,17,19,20). In particular, ECIV to ECVII were defined based on complete identity at the six MvLST genes.…”

“…It has been suggested that the virulence-associated genes included in MvLST studies evolve faster that MLST genes, which code for proteins involved in central, housekeeping functions of the cell (28). To test this hypothesis, the 125 study isolates were sequenced at eight MvLST genes used for epidemic clone definition (4,17,19). The genetic variation recorded at MvLST genes (Table 2) represented 6.62% of nucleotide sites on average, ranging from 3.06% for inlC to 12.6% for the gene dal.…”

“…As a matter of fact, the discovery of genetically similar isolates involved in either geographically and temporally distant outbreaks or in large, single outbreaks led to the definition of L. monocytogenes epidemic clones (ECs) (6,8,(16)(17)(18). Although the initial ECs were defined primarily based on PFGE, multilocus enzyme electrophoresis, and ribotyping (6,18), subsequent ECs have been mostly defined on the basis of multi-virulence-locus sequence typing (MvLST) (4,17,19). MvLST, which is based on the analysis of six to eight genes, has also been used to redefine previously described epidemic clones (17,20).…”