Complete Lipopolysaccharide of Is Required for Full Virulence in the Intraperitoneally Challenged Atlantic Salmon, , Model.

Bacterial cell envelopes play a critical role in host-pathogen interactions. Macromolecular components of these structures have been closely linked to the virulence of pathogens. is a relevant salmonid pathogen with a worldwide distribution. This bacterium is the etiological agent of piscirickettsiosis, a septicemic disease that causes a high economic burden, especially for the Chilean salmon farming industry. Although has been discovered long ago, its pathogenicity and virulence mechanisms are not completely understood. In this work, we present a genetic approach for producing in-frame deletion mutants on genes related to the biosynthesis of membrane-associated polysaccharides. We provide a detailed phenotype description of knock-out mutants on and genes, which encode predicted lipopolysaccharide (LPS) flippase and undecaprenyl-phosphate glucose phosphotransferase enzymes, respectively. We exhibit evidence that the mutant strain carries a defect in the probably most external LPS moiety, while the mutant proved to be highly susceptible to the bactericidal action of serum but retained the ability of biofilm production. Beyond that, we demonstrate that the deletion of , but not , impairs the virulence of in an intraperitoneally infected Atlantic salmon, , model of piscirickettsiosis. Our findings support a role for LPS in the virulence of during the onset of piscirickettsiosis.