Sialylation of Streptococcus suis serotype 2 is essential for capsule expression but is not responsible for the main capsular epitope

The capsular polysaccharide is a critical virulence factor of the swine and zoonotic pathogen Streptococcus suis serotype 2. The capsule of this bacterium is composed of five different sugars, including terminal sialic acid. To evaluate the role of sialic acid in the pathogenesis of the infection, the neuC gene, encoding for an enzyme essential for sialic acid biosynthesis, was inactivated in a highly virulent S. suis serotype 2 strain. Using transmission electron microscopy, it was shown that inactivation of neuC resulted in loss of expression of the whole capsule. Compared to the parent strain, the ΔneuC mutant strain was more phagocytosed by macrophages and was also severely impaired in virulence in a mouse infection model. Both native and desialylated S. suis serotype 2 purified capsular polysaccharides were recognized by a polyclonal anti-whole cell S. suis serotype 2 serum and a monospecific polyclonal anti-capsule serotype 2 serum. In contrast, only the native capsular polysaccharide was recognized by a monoclonal antibody specific for the sialic acid moiety of the serotype 2 capsule. Together, our results infer that sialylation of S. suis serotype 2 may be essential for capsule expression, but that this sugar is not the main epitope of this serotype.