The large majority of isolates clusters into two lineages, but two additional lineages have been identified. However, these lineages correspond more to different but overlapping MLN2238 order niches than to virulence-related clusters. We previously described low-virulence L. monocytogenes strains using a method that combines a plaque-forming (PF) assay with the subcutaneous (s.c.) inoculation of mice [3]. Using the results

of cell infection assays and phospholipase activities, the low-virulence strains were assigned to one of four groups by cluster analysis. Sequencing of virulence-related genes highlighted the molecular causes of low virulence. Group I included strains that exhibited two different types of mutation in the prfA gene: either a single amino acid substitution, PrfAK220T, or a truncated PrfA, PrfAΔ174-237 [7]. In Group III, strains exhibited the same mutations in the plcA, inlA and inlB genes that lead to a lack of InlA protein, an absence of PI-PLC activity and a mutated InlB [8]. The fact that numerous strains exhibit GS-4997 mw the same substitutions in virulence genes suggests that they could have common evolutionary pathways. In contrast, Ragon et al. reported that numerous L. monocytogenes strains exhibit

different mutations in the inlA gene due to convergent evolution [9].

These data emphasize the interest of providing a framework for the population study based on the virulence of this bacterium. The aim of this study was to assign the new low-virulence strains identified by different methods to phenotypic and genotypic Groups using cluster analysis, and to study their relatedness with virulent Listeria monocytogenes strains using pulsed-field gel GSK2399872A supplier electrophoresis CHIR-99021 cost and multi-locus sequence typing analyses Results Phenotypic characterisation of the low-virulence strains The combination of PF assays followed by s.c. injections of immunocompetent mice, allowed us through different studies, to collect 43 low-virulence strains mainly of serotypes 1/2a (51%) and 4b (28%), which are usually related to sporadic and epidemic human cases of listeriosis, respectively [4] (Table 1). In this study, a strain is considered a low-virulence strain when fewer than 4 mice out of 5 inoculated become infected with a mean number of bacteria in the spleen less than 3.45 ± 0.77 log [3]. Table 1 Characterization of the low-virulence L. monocytogenes strains Strains Sub-cutaneous test Phenotypic Groupc Mutations Genotypic Groupd MLST PFGE types Mean (log spleens) ± S.D.