Discovery and characterization of a fructosylated capsule polysaccharide and sialylated lipopolysaccharide in a virulent strain of Actinobacillus suis.

We are developing a serotyping system for based on its capsule (K) and lipopolysaccharide O-chain (O) structures. Previously, we have shown that less virulent strains of this swine pathogen express a (1→6)-β- D-glucan as both K- and O-chain polysaccharides and were serologically classified as K:1/O:1. Here, we show that representative A. suis strains with a high (H91-0380; serotype K:2/O:2) and intermediate (C84; serotype K:2/O:1) degree of virulence possess a capsule polysaccharide (K:2) composed of an O-acetylated diglycosyl phosphate repeat decorated with fructose: [→4)-3- O-Ac-β- D-Glc pNAc-(1→3)-[β- D-Fru f-(2→2)]-α- D-Gal p-(1→PO4-→]. In addition, the serotype O:2 lipopolysaccharide was shown to express a sialylated O-chain [→3)-β-D-Gal p-(1→4)-[Neu5Ac-(2→3)-α- D-Gal p-(1→6)]-β- D-Glc p-(1→6)-β- D-Glc pNAc-(1→]. As (1→6)-β- D-glucan is ubiquitous in the environment, low levels of antibodies in the animals are predicted to prevent disease by K:1/O:1 strains. The greater potential associated with K:2/O:2 and K:2/O:1 strains is most likely due to the absence of (1→6)-β- D-glucan as the K antigen and, in the case of K:2/O:2, the presence of sialic acid in the lipopolysaccharide, a nonulosonic acid known to promote evasion of host recognition. [ABSTRACT FROM AUTHOR]