PplD is a de-N-acetylase of the cell wall linkage unit of streptococcal rhamnopolysaccharides

Structure and mechanism of biosynthesis of Streptococcus mutans cell wall polysaccharide

Streptococcus mutans, the causative agent of human dental caries, expresses a cell wall attached Serotype c- specific Carbohydrate (SCC) that is critical for cell viability. SCC consists of a repeating →3)α-Rha(1→2)α-Rha(1→ polyrhamnose backbone, with glucose (Glc) side-chains and glycerol phosphate (GroP) decorations. This study reveals that SCC has one major and two minor Glc modifications. The major Glc modification, α-Glc, attached to position 2 of 3-rhamnose, is installed by SccN and SccM glycosyltransferases and is the site of the GroP addition. The minor Glc modifications are β-Glc linked to position 4 of 3-rhamnose installed by SccP and SccQ glycosyltransferases, and α-Glc attached to position 4 of 2-rhamnose installed by SccN working in tandem with an unknown enzyme. Both the major and the minor β-Glc modifications control bacterial morphology, but only the GroP and major Glc modifications are critical for biofilm formation.

Disorder regulates homeostasis of extracytoplasmic proteins in streptococci

Proteins harboring intrinsically disordered regions (IDRs) that lack regular secondary or tertiary structure are abundant across three domains of life. Here, using a deep neural network (DNN)-based method we predict IDRs in the extracytoplasmic proteome of Streptococcus mutans , Streptococcus pyogenes and Streptococcus pneumoniae . We identify a subset of the serine/threonine-rich IDRs and demonstrate that they are O -glycosylated with glucose by a GtrB-like glucosyltransferase in S. pyogenes and S. pneumoniae , and N-acetylgalactosamine by a Pgf-dependent mechanism in S. mutans . Loss of glycosylation leads to a defect in biofilm formation under ethanol-stressed conditions in S. mutans . We link this phenotype to a C-terminal IDR of peptidyl-prolyl isomerase PrsA which is protected from proteolytic degradation by O -glycosylation. The IDR length attenuates the efficiency of glycosylation and expression of PrsA. Taken together, our data support a model in which extracytoplasmic IDRs function as dynamic switches of protein homeostasis in streptococci.

Cell wall polysaccharides of streptococci: A genetic and structural perspective

The Streptococcus genus comprises both commensal and pathogenic species. Additionally, Streptococcus thermophilus is exploited in fermented foods and in probiotic preparations. The ecological and metabolic diversity of members of this genus is matched by the complex range of cell wall polysaccharides that they present on their cell surfaces. These glycopolymers facilitate their interactions and environmental adaptation. Here, current knowledge on the genetic and compositional diversity of streptococcal cell wall polysaccharides including rhamnose-glucose polysaccharides, exopolysaccharides and teichoic acids is discussed. Furthermore, the species-specific cell wall polysaccharide combinations and specifically highlighting the presence of rhamnose-glucose polysaccharides in certain species, which are replaced by teichoic acids in other species. This review highlights model pathogenic and non-pathogenic species for which there is considerable information regarding cell wall polysaccharide composition, structure and genetic information. These serve as foundations to predict and focus research efforts in other streptococcal species for which such data currently does not exist.

Human oral lectin ZG16B acts as a cell wall polysaccharide probe to decode host-microbe interactions with oral commensals

The oral microbiome is critical to human health and disease, yet the role that host salivary proteins play in maintaining oral health is unclear. A highly expressed gene in human salivary glands encodes the lectin zymogen granule protein 16 homolog B (ZG16B). Despite the abundance of this protein, its interaction partners in the oral microbiome are unknown. ZG16B possesses a lectin fold, but whether it binds carbohydrates is unclear. We postulated that ZG16B would bind microbial glycans to mediate recognition of oral microbes. To this end, we developed a microbial glycan analysis probe (mGAP) strategy based on conjugating the recombinant protein to fluorescent or biotin reporter functionality. Applying the ZG16B-mGAP to dental plaque isolates revealed that ZG16B predominantly binds to a limited set of oral microbes, including Streptococcus mitis, Gemella haemolysans, and, most prominently, Streptococcus vestibularis. S. vestibularis is a commensal bacterium widely distributed in healthy individuals. ZG16B binds to S. vestibularis through the cell wall polysaccharides attached to the peptidoglycan, indicating that the protein is a lectin. ZG16B slows the growth of S. vestibularis with no cytotoxicity, suggesting that it regulates S. vestibularis abundance. The mGAP probes also revealed that ZG16B interacts with the salivary mucin MUC7. Analysis of S. vestibularis and MUC7 with ZG16B using super-resolution microscopy supports ternary complex formation that can promote microbe clustering. Together, our data suggest that ZG16B influences the compositional balance of the oral microbiome by capturing commensal microbes and regulating their growth using a mucin-assisted clearance mechanism.

Progress towards a glycoconjugate vaccine against Group A Streptococcus

The Group A Carbohydrate (GAC) is a defining feature of Group A Streptococcus (Strep A) or Streptococcus pyogenes. It is a conserved and simple polysaccharide, comprising a rhamnose backbone and GlcNAc side chains, further decorated with glycerol phosphate on approximately 40% GlcNAc residues. Its conservation, surface exposure and antigenicity have made it an interesting focus on Strep A vaccine design. Glycoconjugates containing this conserved carbohydrate should be a key approach towards the successful mission to build a universal Strep A vaccine candidate. In this review, a brief introduction to GAC, the main carbohydrate component of Strep A bacteria, and a variety of published carrier proteins and conjugation technologies are discussed. Components and technologies should be chosen carefully for building affordable Strep A vaccine candidates, particularly for low- and middle-income countries (LMICs). Towards this, novel technologies are discussed, such as the prospective use of bioconjugation with PglB for rhamnose polymer conjugation and generalised modules for membrane antigens (GMMA), particularly as low-cost solutions to vaccine production. Rational design of "double-hit" conjugates encompassing species specific glycan and protein components would be beneficial and production of a conserved vaccine to target Strep A colonisation without invoking an autoimmune response would be ideal.

The Cell Wall Deacetylases Spy1094 and Spy1370 Contribute to Streptococcus pyogenes Virulence

Streptococcus pyogenes, or Group A Streptococcus (GAS), is a strictly human pathogen that causes a wide range of diseases, including skin and soft tissue infections, toxic shock syndrome and acute rheumatic fever. We have recently reported that Spy1094 and Spy1370 of S. pyogenes serotype M1 are N-acetylglucosamine (GlcNAc) deacetylases. We have generated spy1094 and spy1370 gene deletion mutants in S. pyogenes and gain-of-function mutants in Lactococcus lactis. Similar to other cell wall deacetylases, our results show that Spy1094 and Spy1370 confer lysozyme-resistance. Furthermore, deletion of the genes decreased S. pyogenes virulence in a human whole blood killing assay and a Galleria mellonella (Greater wax moth) larvae infection model. Expression of the two genes in L. lactis resulted in increased lysozyme resistance and survival in whole human blood, and reduced survival of infected G. mellonella larvae. Deletion of the spy1370, but not the spy1094 gene, decreased resistance to the cationic antimicrobial peptide cecropin B, whereas both enzymes increased biofilm formation, probably resulting from the increase in positive charges due to deacetylation of the cell wall. In conclusion, Spy1094 and Spy1370 are important S. pyogenes virulence factors and might represent attractive targets for the development of antibacterial agents.

Primary Structure of Glycans by NMR Spectroscopy

Glycans, carbohydrate molecules in the realm of biology, are present as biomedically important glycoconjugates and a characteristic aspect is that their structures in many instances are branched. In determining the primary structure of a glycan, the sugar components including the absolute configuration and ring form, anomeric configuration, linkage(s), sequence, and substituents should be elucidated. Solution state NMR spectroscopy offers a unique opportunity to resolve all these aspects at atomic resolution. During the last two decades, advancement of both NMR experiments and spectrometer hardware have made it possible to unravel carbohydrate structure more efficiently. These developments applicable to glycans include, inter alia, NMR experiments that reduce spectral overlap, use selective excitations, record tilted projections of multidimensional spectra, acquire spectra by multiple receivers, utilize polarization by fast-pulsing techniques, concatenate pulse-sequence modules to acquire several spectra in a single measurement, acquire pure shift correlated spectra devoid of scalar couplings, employ stable isotope labeling to efficiently obtain homo- and/or heteronuclear correlations, as well as those that rely on dipolar cross-correlated interactions for sequential information. Refined computer programs for NMR spin simulation and chemical shift prediction aid the structural elucidation of glycans, which are notorious for their limited spectral dispersion. Hardware developments include cryogenically cold probes and dynamic nuclear polarization techniques, both resulting in enhanced sensitivity as well as ultrahigh field NMR spectrometers with a ¹H NMR resonance frequency higher than 1 GHz, thus improving resolution of resonances. Taken together, the developments have made and will in the future make it possible to elucidate carbohydrate structure in great detail, thereby forming the basis for understanding of how glycans interact with other molecules.

Structural variations and roles of rhamnose-rich cell wall polysaccharides in Gram-positive bacteria

Rhamnose-rich cell wall polysaccharides (Rha-CWPSs) have emerged as crucial cell wall components of numerous Gram-positive, ovoid-shaped bacteria—including streptococci, enterococci, and lactococci—of which many are of clinical or biotechnological importance. Rha-CWPS are composed of a conserved polyrhamnose backbone with side-chain substituents of variable size and structure. Because these substituents contain phosphate groups, Rha-CWPS can also be classified as polyanionic glycopolymers, similar to wall teichoic acids, of which they appear to be functional homologs. Recent advances have highlighted the critical role of these side-chain substituents in bacterial cell growth and division, as well as in specific interactions between bacteria and infecting bacteriophages or eukaryotic hosts. Here, we review the current state of knowledge on the structure and biosynthesis of Rha-CWPS in several ovoid-shaped bacterial species. We emphasize the role played by multicomponent transmembrane glycosylation systems in the addition of side-chain substituents of various sizes as extracytoplasmic modifications of the polyrhamnose backbone. We provide an overview of the contribution of Rha-CWPS to cell wall architecture and biogenesis and discuss current hypotheses regarding their importance in the cell division process. Finally, we sum up the critical roles that Rha-CWPS can play as bacteriophage receptors or in escaping host defenses, roles that are mediated mainly through their side-chain substituents. From an applied perspective, increased knowledge of Rha-CWPS can lead to advancements in strategies for preventing phage infection of lactococci and streptococci in food fermentation and for combating pathogenic streptococci and enterococci.

Antibiotics and Carbohydrate-Containing Drugs Targeting Bacterial Cell Envelopes: An Overview

Certain bacteria constitute a threat to humans due to their ability to escape host defenses as they easily develop drug resistance. Bacteria are classified into gram-positive and gram-negative according to the composition of the cell membrane structure. Gram-negative bacteria have an additional outer membrane (OM) that is not present in their gram-positive counterpart; the latter instead hold a thicker peptidoglycan (PG) layer. This review covers the main structural and functional properties of cell wall polysaccharides (CWPs) and PG. Drugs targeting CWPs are discussed, both noncarbohydrate-related (β-lactams, fosfomycin, and lipopeptides) and carbohydrate-related (glycopeptides and lipoglycopeptides). Bacterial resistance to these drugs continues to evolve, which calls for novel antibacterial approaches to be developed. The use of carbohydrate-based vaccines as a valid strategy to prevent bacterial infections is also addressed.