Introduction: Haemophilus parasuis (Hps) is a Gram-negative bacterium responsible for Glässer’s Disease in pigs. Little is known regarding the role of Hps capsular polysaccharide (CP) in serotype specificity and virulence.
Methods: CPs were purified from Hps serotypes 4, 5, and 9 by standard methods. Transmission electron microscopy (TEM) and immunofluorescence (IF), ELISA, and latex agglutination assays with rabbit antiserum to CPs were used to detect CP and antigenic reactivity. Bactericidal assay was used to assess serum resistance. qRT-PCR of the Hps 5 CP locus was used to examine CP regulation. Lipooligosaccharides (LOSs) were examined by electrophoretic analysis and immunoblotting.
Results: CP was produced from broth-grown Hps by each strain, but not by any Hps strain grown on agar medium. CP production in broth was enhanced by the addition of sodium bicarbonate. The CP was visible on broth-grown Hps by IF and TEM, but not on agar-grown cells or strains of a different serotype (IF). Weaker cross-reactivity with CPs of some heterologous serotypes occurred by ELISA. Agar-grown Hps cells were serum-susceptible, but broth-grown Hps were serum-resistant unless homologous anti-CP serum was added. CP genes were upregulated when Hps cells were broth-grown compared to agar-grown. The LOS profiles of each serotype exhibited a similar electrophoretic profile, and all reacted with homologous and heterologous antisera.
Conclusions: Hps CP production was upregulated during growth in broth and in the presence of bicarbonate, which is unusual for a CP. Each CP, but not LOS, was immunodominant, indicating that the CP is the serotype-specific antigen. CP was required for serum resistance, and thus protects the bacterium from innate host defenses. Killing of Hps by antibodies to CP indicate this antigen is important to protective host immunity. The CP may be an important antigen for development of improved diagnostic tests and vaccines for Hps.