Listeria monocytogenes consists of at least 4 evolutionary lineages (I, II, III, and IV) with different but overlapping ecological niches. Most L. monocytogenes isolates seem to belong to lineages I and II, which harbor the serotypes more commonly associated with human clinical cases, including serotype 1/2a (lineage II) and serotypes 1/2b and 4b (lineage I). Lineage II strains are common in foods, seem to be widespread in the natural and farm environments, and are also commonly isolated from animal listeriosis cases and sporadic human clinical cases. Most human listeriosis outbreaks are associated with lineage I isolates though. In addition, a number of studies indicate that, in many countries, lineage I strains are overrepresented among human isolates, as compared to lineage II strains. Lineage III and IV strains on the other hand are rare and predominantly isolated from animal sources. The apparent differences in the distribution of strains representing the L. monocytogenes lineages has lead to a number of studies aimed at identifying phenotypic differences among the different lineages. Interestingly, lineage II isolates seem to carry more plasmids than lineage I isolates and these plasmids often confer resistance to toxic metals and possibly other compounds that may be found in the environment. Moreover, lineage II isolates seem to be more resistant to bacteriocins than lineage I isolates, which probably confers an advantage in environments where bacteriocin-producing organisms are abundant. A large number of lineage II isolates and strains have been shown to be virulence-attenuated due to premature stop codon mutations in inlA and mutations in prfA. A subset of lineage I isolates carry a listeriolysin S hemolysin, which is not present in isolates belonging to lineages II, III, or IV. While lineage II isolates also show higher recombination rates than lineage I isolates, possibly facilitating adaptation of lineage II strains to diverse environments, lineage I isolates are clonal and show a low prevalence of plasmids and IS elements, suggesting that lineage I isolates may have mechanisms that limit the acquisition of foreign DNA by horizontal gene transfer. Diversifying selection has also been shown to have played an important role during evolution of the L. monocytogenes lineages and during divergence of L. monocytogenes from the non-pathogenic species L. innocua. Overall evidence thus suggests that the 4 L. monocytogenes lineages identified so far represent distinct ecologic, genetic, and phenotypic characteristics, which appear to affect their ability to be transmitted through foods and to cause human disease. Further insights into the ecology, evolution, and characteristics of these lineages will thus not only provide an improved understanding of the evolution of this foodborne pathogen, but may also facilitate improved control of foodborne listeriosis.
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