Abstract
The capsular polysaccharide (CPS) is a major virulence factor in many encapsulated pathogens, as it is the case for Streptococcus suis, an important swine pathogen and emerging zoonotic agent. Moreover, the CPS is the antigen at the origin of S. suis classification into serotypes. Hence, analyses of the CPS structure are an essential step to dissect its role in virulence and the serological relations between important serotypes. Here, the CPSs of serotypes 1 and 1/2 were purified and characterized for the first time. Chemical and spectroscopic data gave the following repeating unit sequences: [6)[Neu5Ac(α2-6)GalNAc(β1-4)GlcNAc(β1-3)]Gal(β1-3)Gal(β1-4)Glc(β1-]n (serotype 1) and [4)[Neu5Ac(α2-6)GalNAc(β1-4)GlcNAc(β1-3)]Gal(β1-4)[Gal(α1-3)]Rha(β1-4)Glc(β1-]n (serotype 1/2). The Sambucus nigra lectin, which recognizes the Neu5Ac(α2-6)Gal/GalNAc sequence, showed binding to both CPSs. Compared with previously characterized serotype 14 and 2 CPSs, N-acetylgalactosamine replaces galactose as the sugar bearing the sialic acid residue in the side chain. Serological analyses of the cross-reaction of serotype 1/2 with serotypes 1 and 2 and that between serotypes 1 and 14 suggested that the side chain, and more particularly the terminal sialic acid, constitutes one important epitope for serotypes 1/2 and 2. The side chain is also an important serological determinant for serotype 1, yet sialic acid seems to play a limited role. In contrast, the side chain does not seem to be part of a major epitope for serotype 14. These results contribute to the understanding of the relationship between S. suis serotypes and provide the basis for improving diagnostic tools.
Highlights
Streptococcus suis is one of the most important pathogens in the porcine industry causing septicemia, meningitis, and many other infections
The capsular polysaccharide (CPS) is the antigen at the origin of S. suis classification into serotypes
35 serotypes have been originally described based on capsular polysaccharide (CPS)3 antigenic diversity, and most of them have been isolated from diseased pigs [2,3,4,5]
Summary
Capsular Polysaccharide Isolation and Purification—S. suis serotype 1 strain 1178027, isolated from the meninges of a diseased pig in Canada, and serotype 1/2 strain 2651, isolated from a diseased pig in Denmark [2], were used in this study. Gel filtration fractions giving a differential refractometer signal but no UV absorption at 280 and 254 nm were considered to be CPS This was further confirmed by dot-blotting of all fractions with anti-S. suis serotype 1 or 2 serum [6]. The phase-sensitive gradient-enhanced two-dimensional heteronuclear single-quantum coherence (HSQC) using a z-filter and selection before t1 and the phasesensitive gradient-enhanced two-dimensional HSQC using echo-antiecho and adiabatic pulses for inversion and refocusing and Bloch-Siegert effects were optimized for 145 Hz. The phase-sensitive gradient-enhanced two-dimensional HSQCTOCSY with MLEV using echo-antiecho was performed with a delay optimized for a 140-Hz coupling constant and a mixing time of 80 or 100 ms. The reaction, which is done using formalin-fixed whole bacteria cultures with reference monospecific polyclonal rabbit sera against S. suis serotype 1, 1/2, 2, or 14, was performed as described previously [6]. Raw integration values were normalized using respective homologous signal values with native CPS
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