Epidemic clones of Listeria monocytogenes: detection, transmission and virulence

Although many different serotypes of Listeria monocytogenes have been isolated from foods, only a few such as 1/2a, 1/2b, and 4b account for the vast majority of clinical cases, and in particular most outbreaks of listeriosis have involved a small number of closely related clones in serotypes 1/2a and 4b (1). An epidemic clone (EC) of L. monocytogenes has been defined as groups of isolates that are genetically related and presumably of a common ancestor, but are implicated in different, geographically and temporally unrelated outbreaks (1–3). Since the introduction of this concept in 2002, four ECs have been recognized: ECI, ECII, and ECIV in serotype 4b and ECIII in serotype 1/2a (1,2). Most recently, in 2011, a novel serotype 1/2a EC that had been causing disease in Canada for the past twenty years was detected and designated as ECV (4). As L. monocytogenes continues to spread throughout the world and cause many different outbreaks, it is reasonable to assume that other novel ECs could be identified in the future. In order to control L. monocytogenes, determining the routes by which ECs and outbreak clones (OCs) are transmitted to foods will be extremely helpful. Consequently, the availability of subtyping assays capable of discerning closely related ECs and OCs is extremely important. In the past few years, the number and quality of subtyping assays has increased thanks to the development of DNA-based methods such as pulsed-field gel electrophoresis (PFGE), considered the gold-standard for its high discriminatory power (i.e. the ability to correctly differentiate unrelated strains). PFGE has proven to be a very accurate and reproducible method in subtyping L. monocytogenes (5). However, new sequence-based techniques, such as multilocus-sequence typing (MLST) and single nucleotide polymorphism (SNP) typing, are also currently being used (2,6,7). In particular, multi-virulence-locus sequence typing (MVLST) has been able to accurately identify and differentiate all five ECs of L. monocytogenes (2,4). This technique also proved very useful in the identification of ECs, when PFGE failed to identify strains belonging to the same ECs due to the presence of different PFGE profiles (4,8). Therefore, the use of novel DNA-sequence-based molecular subtyping strategies able to correctly determine the clonal relationship among different isolates is essential to accurately recognize outbreaks and epidemics and identify their routes of transmission. This chapter will review the five currently known ECs of L. monocytogenes, including listeriosis outbreaks caused by these ECs during the past few years, and the latest molecular subtyping methods developed to identify them.