Structural, Biosynthetic, and Serological Cross-Reactive Elucidation of Capsular Polysaccharides from Streptococcus pneumoniae Serogroup 16

Diversity of sugar-diphospholipid-utilizing glycosyltransferase families

Peptidoglycan polymerases, enterobacterial common antigen polymerases, O-antigen ligases, and other bacterial polysaccharide polymerases (BP-Pols) are glycosyltransferases (GTs) that build bacterial surface polysaccharides. These integral membrane enzymes share the particularity of using diphospholipid-activated sugars and were previously missing in the carbohydrate-active enzymes database (CAZy; www.cazy.org ). While the first three classes formed well-defined families of similar proteins, the sequences of BP-Pols were so diverse that a single family could not be built. To address this, we developed a new clustering method using a combination of a sequence similarity network and hidden Markov model comparisons. Overall, we have defined 17 new GT families including 14 of BP-Pols. We find that the reaction stereochemistry appears to be conserved in each of the defined BP-Pol families, and that the BP-Pols within the families transfer similar sugars even across Gram-negative and Gram-positive bacteria. Comparison of the new GT families reveals three clans of distantly related families, which also conserve the reaction stereochemistry.

First Report on the Streptococcus gallolyticus (S. bovis Biotype I) DSM 13808 Exopolysaccharide Structure

Streptococcus gallolyticus subspecies gallolyticus, known as Streptococcus bovis biotype I, is a facultative pathogen causing bacteraemia, infective endocarditis and sepsis that has been linked with colorectal cancer (CRC), but this correlation is still unclear. Bacterial surface structures, such as the major sugar antigens exposed to the outside of the microorganism, are potential virulence factors. One of the primary sugar antigens loosely attached to the cell surface is the biofilm component, exopolysaccharide (EPS). EPSs of S. bovis are poorly characterized molecules. Until now, only one S. macedonicus Sc136 EPS structure was known to the entire S. bovis group. The S. gallolyticus DSM 13808 EPS was investigated by chemical analysis, mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. The hexasaccharide repeating unit of the EPS, containing four Glc, two Rha residues and one phosphate group, has been described “ →6)-α-d-Glcp-(1→3)-β-l-Rhap-(1→4)-β-d-Glcp-(1→3)-[β-d-Glcp-(1→2)]-α-l-Rhap-(1→2)-α-d-Glcp-(1→P→”.

Full NMR assignment, revised structure and biosynthetic analysis for the capsular polysaccharide from Streptococcus Pneumoniae serotype 15F

Upon investigation of Streptococcus pneumoniae serotype 15F capsular polysaccharide (CPS), we discovered that it had a different phosphorylation substituent, namely glycerol-2-phosphate like the other serogroup 15 CPS rather than the originally reported 0.2 equivalent of phosphate or phosphocholine. Furthermore, we also determined the locations of the two previously unassigned O-acetyl groups present in the repeating unit of the 15F CPS, and carried out full NMR assignments of the 15F as well as 15A CPS. Lastly, a biosynthetic analysis of serotypes 15F and 15A was performed and used to make a prediction for the structure of the recently discovered serotype 15D.

Structural, Genetic, and Serological Elucidation of Streptococcus pneumoniae Serogroup 24 Serotypes: Discovery of a New Serotype, 24C, with a Variable Capsule Structure

Pneumococcal capsules are important in pneumococcal pathogenesis and vaccine development. Although conjugate vaccines have brought about a significant reduction in invasive pneumococcal disease (IPD) caused by vaccine serotypes, the relative serotype prevalence has shifted with the dramatic emergence of serotype 24F in some countries. Here, we describe 14 isolates (13 IPD and 1 non-IPD) expressing a new capsule type, 24C, which resembles 24F but has a novel serological profile. We also describe the antigenic, biochemical, and genetic basis of 24F and 24C and the related serotypes 24A and 24B. Structural studies show that 24B, 24C, and 24F have identical polysaccharide backbones [β-Ribf-(1→4)-α-Rhap-(1→3)-β-GlcpNAc-(1→4)-β-Rhap-(1→4)-β-Glcp] but with different side chains, as follows: 24F has arabinitol-phosphate and 24B has ribitol-phosphate. 24C has a mixture of 24F and 24B repeating units, with the ratio of ribitol to arabinitol being strain dependent. In contrast, the 24A capsule has a backbone without β-Ribf but with arabinitol-phosphate and phosphocholine side chains. These structures indicate that factor-sera 24d and 24e recognize arabinitol and ribitol, respectively, which explains the serology of serogroup 24, including those of 24C. The structures can be genetically described by the bispecificity of wcxG, which is capable of transferring arabinitol or ribitol when arabinitol is limiting. Arabinitol is likely not produced in 24B but is produced in reduced amounts in 24C due to various mutations in abpA or abpB genes. Our findings demonstrate how pneumococci modulate their capsule structure and immunologic properties with small genetic changes, thereby evading host immune responses. Our findings also suggest a potential for new capsule types within serogroup 24.

Structural, biosynthetic and serological cross-reactive elucidation of capsular polysaccharides from Streptococcus pneumoniae serogroup 28

Capsular polysaccharides (CPS) are the key virulent factors in the pathogenesis of Streptococcus pneumoniae. The previously unknown CPS structures of the pneumococcal serotype 28F and 28A were thoroughly characterized by NMR spectroscopy, chemical analysis and AF4-MALS-dRI. The following repeat unit structures were determined: -4)[α-l-Rhap-[4-P-2-Gro]]-(1-3)-α-d-Sug-[6-P-Cho]-(1-3)-β-l-Rhap-[2-OAc]-(1-4)-β-d-Glcp-(1-; 28F: Sug = Glcp, Mw: 540.5 kDa; 28A: Sug = GlcpNAc, Mw: 421.9 kDa; The correlation of CPS structures with biosynthesis showed that glycosyltransferase WciU in serotypes 28F and 28A had different sugar donor specificity toward α-d-Glcp and α-d-GlcNAcp, respectively. Furthermore, latex agglutination tests of de-OAc and de-PO₄ CPS were conducted to understand cross-reactions between serogroup 28 with factor antiserum 23d. Interestingly, the de-OAc 28F and 28A CPS can still weakly react with factor antiserum 23d, while de-PO₄ CPS did not react with factor antiserum 23d. This indicated that OAc group could affect the affinity and P-2-Gro was crucial for cross-reacting with factor antiserum 23d.

Decoding capsule synthesis in Streptococcus pneumoniae

Streptococcus pneumoniae synthesizes more than one hundred types of capsular polysaccharides (CPS). While the diversity of the enzymes and transporters involved is enormous, it is not limitless. In this review, we summarized the recent progress on elucidating the structure-function relationships of CPS, the mechanisms by which they are synthesized, how their synthesis is regulated, the host immune response against them, and the development of novel pneumococcal vaccines. Based on the genetic and structural information available, we generated provisional models of the CPS repeating units that remain unsolved. In addition, to facilitate cross-species comparisons and assignment of glycosyltransferases, we illustrated the biosynthetic pathways of the known CPS in a standardized format. Studying the intricate steps of pneumococcal CPS assembly promises to provide novel insights for drug and vaccine development as well as improve our understanding of related pathways in other species.