Abstract
Streptococcus pneumoniae clinical isolates were recently described that produced capsular polysaccharide with properties of both serotypes 6A and 6B. Their hybrid serological property correlated with mutations affecting the glycosyltransferase WciP, which links rhamnose to ribitol by an α(1-3) linkage for serotypes 6A and 6C and an α(1-4) linkage for serotypes 6B and 6D. The isolates had mutations in the triad residues of WciP that have been correlated with enzyme specificity. The canonical triad residues of WciP are Ala192-Ser195-Arg254 for serotypes 6A and 6C and Ser192-Asn195-Gly254 for serotypes 6B and 6D. To prove that the mutations in the triad residues are responsible for the hybrid serotype, we introduced the previously described Ala192-Cys195-Arg254 triad into a 6A strain and found that the change made WciP bispecific, resulting in 6A and 6B repeat unit expression, although 6B repeat unit production was favored over production of 6A repeat units. Likewise, this triad permitted a 6C strain to express 6C and 6D repeat units. With reported bispecificity in WciN, which adds either glucose or galactose as the second sugar in the serogroup 6 repeat unit, the possibility exists for a strain to simultaneously produce all four serogroup 6 repeat units; however, when genes encoding both bispecific enzymes were introduced into a 6A strain, only 6A, 6B, and 6D repeat units were detected serologically. Nonetheless, this may be the first example of a bacterial polysaccharide with three different repeat units. This strategy of expressing multiple repeat units in a single polymer is a novel approach to broadening vaccine coverage by eliminating the need for multiple polysaccharide sources to cover multiple serogroup members.