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 Actinobacillus suis 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)-b-D- glucan as both K- and O-chain polysaccharides and were serologically classified as K:1/O:1. Here, we show that representa- tive 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-b-D-GlcpNAc-(1→3)-(b-D-Fruf-(2→2))-a-D-Galp-(1→PO4 - →). In addition, the serotype O:2 lipopolysaccharide was shown to express a sialylated O-chain (→3)-b-D-Galp-(1→4)-(Neu5Ac-(2→3)-a-D-Galp-(1→6))-b-D-Glcp-(1→6)-b-D- GlcpNAc-(1→). As (1→6)-b-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)-b-D-glucan as the K antigen and, in the case of K:2/O:2, the presence of sialic acid in the lipopoly- saccharide, a nonulosonic acid known to promote evasion of host recognition.