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Background  Currently, food regulatory authorities consider all  Listeria monocytogenes  isolates as equally virulent. However, an increasing number of studies demonstrate extensive variations in virulence and pathogenicity of  L. monocytogenes  strains. Up to now, there is no comprehensive overview of the population genetic structure of  L. monocytogenes  taking into account virulence level. We have previously demonstrated that different low-virulence strains exhibit the same mutations in virulence genes suggesting that they could have common evolutionary pathways. New low-virulence strains were identified and assigned to phenotypic and genotypic Groups using cluster analysis. Pulsed-field gel electrophoresis, virulence gene sequencing and multi-locus sequence typing analyses were performed to study the genetic relatedness and the population structure between the studied low-virulence isolates and virulent strains.    Results  These methods showed that low-virulence strains are widely distributed in the two major lineages, but some are also clustered according to their genetic mutations. These analyses showed that low-virulence strains initially grouped according to their lineage, then to their serotypes and after which, they lost their virulence suggesting a relatively recent emergence.    Conclusions  Loss of virulence in lineage II strains was related to point mutation in a few virulence genes ( prfA, inlA, inlB, plcA ). These strains thus form a tightly clustered, monophyletic group with limited diversity. In contrast, low-virulence strains of lineage I were more dispersed among the virulence strains and the origin of their loss of virulence has not been identified yet, even if some strains exhibited different mutations in  prfA  or  inlA .

Polyphasic characterization and genetic relatedness of low-virulence and virulent Listeria monocytogenes isolates