Polyphasic characterization and genetic relatedness of low-virulence and virulent Listeria monocytogenes isolates
1 INRA, UR 1282 Infectiologie Animale et Santé Publique, Agents transmissibles et Infectiologie, F-37380, Nouzilly and IFR, 136, France
2 Université de Tours, UMR1282 Infectiologie et Santé Publique, F-37000, Tours, France
3 ANSES LERQAP, Unité Caractérisation et Epidémiologie Bactérienne, F-94706, Maisons-Alfort, France
4 Institut Pasteur, French National Reference Center and WHO collaborating Center for Listeria, F-75724, Paris, France
5 INRA, UR 0341 Mathématiques et Informatique Appliquées, F-78352, Jouy-en-Josas, France
6 PHYLOGENE, F-30620, Bernis, France
7 Université François Rabelais de Tours, EA3854 « Bactéries et risque materno-fœtal », Tours, France and CHRU, F-37044, Tours, France
8 Current address: ANSES UHQPAP, Unité Hygiène et Qualité des Produits Avicoles et Porcins, F-22440, Ploufragan, France
9 Current address: Université Paris Sud Orsay, CNRS-UMR807, Unité Ecologie Systématique Evolution, F-91400, Orsay, France
10 Current address: Institut Pasteur, Centre de Ressources Biologiques de l’Institut Pasteur (CRBIP), F-75724, Paris, France
11 Current address: Laboratoire de Microbiologie-Hygiène, Centre Hospitalier de Versailles, Université Paris Sud, F-78150, Le Chesnay, France
BMC Microbiology 2012, 12:304 doi:10.1186/1471-2180-12-304Published: 26 December 2012
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.
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.
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.