Polysaccharide biosynthesis protein CapD is a novel pathogenicity-associated determinant of Haemophilus parasuis involved in serum-resistance ability.

The capD gene, encoding a polysaccharide biosynthesis protein, was identified previously as a differential gene between Haemophilus parasuis virulent strain Nagasaki and avirulent strain SW114; however, the characteristics of this gene associating with the pathogenicity of H. parasuis remain unclear. Here, the capD deletion mutant (ΔcapD) and its complement strain (C-capD) were generated in H. parasuis virulent strain SH0165. The deletion of capD gene significantly attenuated the pathogenicity of the SH0165 strain, while the complementation of this gene largely recovered the pathogenicity to piglets. Additionally, the ΔcapD strain could not be recovered from piglets after challenge, while both SH0165 and C-capD strains were recovered from most systemic sites. Moreover, the ΔcapD strain exhibited an extreme sensitivity to the complement-mediated killing compared with SH0165 strain, while its serum-resistance ability largely restored with the capD gene complementation. These data present the evidence that the capD gene is a novel pathogenicity-associated determinant and involved in serum-resistance ability of H. parasuis.