Complex Population Structure and Virulence Differences among Serotype 2 XXXX Strains Belonging to Sequence Type 28.

@entity11769 @entity1707 @entity1 @entity281 @entity4334
UNASSIGNED: @entity11769 is a major @entity1707 pathogen and a zoonotic agent. Serotype 2 strains are the most frequently associated with disease. However, not all serotype 2 lineages are considered virulent. Indeed, sequence type (ST) 28 serotype 2 @entity11769 strains have been described as a homogeneous group of low virulence. However, ST28 strains are often isolated from diseased @entity1707 in some countries, and at least four @entity1 ST28 cases have been reported. Here, we used whole-genome sequencing and @entity281 models to test the hypothesis that the ST28 lineage comprises strains of different genetic backgrounds and different virulence. We used 50 @entity11769 ST28 strains isolated in Canada, the United States and Japan from diseased @entity1707 , and one ST28 strain from a @entity1 case isolated in Thailand. We report a complex population structure among the 51 ST28 strains. Diversity resulted from variable gene content, recombination events and numerous genome-wide polymorphisms not attributable to recombination. Phylogenetic analysis using core genome single- @entity4334 polymorphisms revealed four discrete clades with strong geographic structure, and a fifth clade formed by US, Thai and Japanese strains. When tested in experimental animal models, strains from this latter clade were significantly more virulent than a Canadian ST28 reference strain, and a closely related Canadian strain. Our results highlight the limitations of MLST for both phylogenetic analysis and virulence prediction and raise concerns about the possible emergence of ST28 strains in @entity1 clinical cases.
