Biochemical, Genetic, and Serological Characterization of Two Capsule Subtypes among Streptococcus pneumoniae Serotype 20 Strains DISCOVERY OF A NEW PNEUMOCOCCAL SEROTYPE

The bacterial pathogen Streptococcus pneumoniae expresses one of over 90 structurally distinct polysaccharide (PS) capsule serotypes. Prior PS structural analyses of the vaccine-associated serotype 20 do not agree with reports describing the genes that mediate capsule synthesis. Furthermore, using immunized human sera-based assays, serological differences were recently noted among strains typed as serotype 20. We examined the capsule structures of two serologically dissimilar serotype 20 strains, 20 alpha and 20 beta, by extensive biochemical analysis. 20 alpha PS was composed of the previously described serotype 20 hexasaccharide repeat unit, whereas the 20 beta PS was composed of a novel heptasaccharide repeat unit containing an extra branching alpha-glucose residue. Genetic analysis of the subtypes revealed that 20 alpha may have arisen from a 20 beta progenitor following loss of function mutation to the glycosyltransferase gene whaF. Conventional serotyping methods using rabbit polyclonal or mouse monoclonal antibodies were unable to distinguish the subtypes. However, genetic analysis of multiple "serotype 20" clinical isolates revealed that all strains contain the 20 beta genotype. We propose naming bacteria that express the previously described 20 alpha capsule structure 20A and bacteria that express the novel 20 beta capsule structure 20B, a new pneumococcal serotype.