Worldwide Distribution of Major Clones of Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen that can cause listeriosis, a severe invasive infection in humans with a particularly high case-fatality rate. Listeriosis is a major public health concern in all world regions, with an increasing incidence in Europe, especially among elderly persons (1,2). L. monocytogenes is genetically heterogeneous (3–5). To help epidemiologic investigation and to define clones, i.e., groups of genetically similar isolates descending from a common ancestor, a variety of typing methods have been used, including pulsed-field gel electrophoresis (5,6), single nucleotide polymorphism typing (7), and multiple housekeeping and virulence gene sequencing (8,9). Some clones implicated in multiple outbreaks have been defined as epidemic clones (EC) (3,5,9–11). ECI and ECIV have been described in several countries (3,5), but because of the lack of standardization of genotyping, a definition of clones is not widely accepted, and current knowledge on the global distribution of L. monocytogenes clones is virtually absent. Multilocus sequence typing (MLST) is a reference method for global epidemiology and population biology of bacteria, and its application to L. monocytogenes (12) effectively allows isolate comparisons across laboratories (www.pasteur.fr/mlst). The aim of this study was to investigate the global distribution of L. monocytogenes MLST-defined clones.