Synthetic Pseudaminic-Acid-Based Antibacterial Vaccine Confers Effective Protection against Acinetobacter baumannii Infection

KpsS1 Mediates the Glycosylation of Pseudaminic Acid in Acinetobacter Baumannii

Pseudaminic acid (Pse) is found in the polysaccharide structures of the cell surface of various Gram-negative pathogenic bacteria including Acinetobacter baumannii and considered as an important component of cell surface glycans including oligosaccharides and glycoproteins. However, the glycosyltransferase that is responsible for the Pse glycosylation in A. baumannii remains unknown yet. In this study, through comparative genomics analysis of Pse-positive and negative A. baumannii clinical isolates, we identified a potential glycosyltransferase, KpsS1, located right downstream of the Pse biosynthesis genetic locus. Deletion of this gene in an Pse-positive A. baumannii strain, Ab8, impaired the glycosylation of Pse to the surface CPS and proteins, while the gene knockout strain, Ab8ΔkpsS1, could still produce Pse with 2.86 folds higher amount than that of Ab8. Furthermore, impairment of Pse glycosylation affected the morphology and virulence potential of A. baumannii, suggesting the important role of this protein. This study will provide insights into the further understanding of Pse in bacterial physiology and pathogenesis.

Current treatment strategies for targeting virulence factors and biofilm formation in Acinetobacter baumannii

A higher prevalence of Acinetobacter baumannii infections and mortality rate has been reported recently in hospital-acquired infections (HAI). The biofilm-forming capability of A. baumannii makes it an extremely dangerous pathogen, especially in device-associated hospital-acquired infections (DA-HAI), thereby it resists the penetration of antibiotics. Further, the transmission of the SARS-CoV-2 virus was exacerbated in DA-HAI during the epidemic. This review specifically examines the complex interconnections between several components and genes that play a role in the biofilm formation and the development of infections. The current review provides insights into innovative treatments and therapeutic approaches to combat A. baumannii biofilm-related infections, thereby ultimately improving patient outcomes and reducing the burden of HAI.

Bacteria-surface interactions: role of impacting bacteria-laden droplets

Understanding the interactions of pathogenic droplets with surfaces is crucial to biomedical applications. In this study, using E. coli as the model microbe, we investigate the impact of a bacteria-laden droplet on different substrates, both bare and antimicrobial. In doing so, we unveil the significance of kinetic energy and spreading parameters of the impacting droplet in determining the microbes' proliferation capabilities. Our results indicate an inverse relationship between the impact Weber number and the bacterial ability to proliferate. We reveal that the mechanical stress generated during impact impedes the capabilities of microbes present inside the droplet to create their progeny. Following an order analysis of the mechanical stress generated, we argue that the impact does not induce lysis-driven cell death of the bacteria; rather, it promotes a stress-driven transition of viable bacteria to a viable-but-non-culturable (VBNC) state. Furthermore, variations in the concentration of particles on the antimicrobial surfaces revealed the role of the post-impact spreading behaviour in dictating bacterial proliferation capabilities. Contrary to the conventional notion, we demonstrate that during the early stages of interaction, a bare substrate may outperform an antibacterial substrate in the inactivation of the bacterial load. Finally, we present an interaction map illustrating the complex relationship between bacterial colony-forming units, bactericide concentration on the surface, and the impact Weber number. We believe that the inferences of the study, highlighting the effect of mechanical stresses on the soft cell wall of microbes, could be a useful design consideration for the development of antimicrobial surfaces.

Investigation on the substrate specificity and N-substitution tolerance of PseF in catalytic transformation of pseudaminic acids to CMP-Pse derivatives

Pseudaminic acid (Pse) belongs to a class of bacterial non-2-ulosonic acids, and has been implicated in bacterial infection and immune evasion. Various Pse structures with diverse N-substitutions have been identified in pathogenic bacterial strains like Pseudomonas aeruginosa, Campylobacter jejuni, and Acinetobacter baumannii. In this study, we successfully synthesized three new Pse species, including Pse5Ac7Fo, Pse5Ac7(3RHb) and Pse7Fo5(3RHb) using chemical methods. Furthermore, we investigated the substrate specificity of cytidine 5'-monophosphate (CMP)-Pse synthetase (PseF), resulting in the production of N-modified CMP-Pse derivatives (CMP-Pses). It was found that PseF was promiscuous with the Pse substrate and could tolerate different modifications at the two nitrogen atoms. This study provides valuable insights into the incorporation of variable N-substitutions in the Pse biosynthetic pathway.

Discovery of a Monoclonal Antibody That Targets Cell-Surface Pseudaminic Acid of Acinetobacter baumannii with Direct Bactericidal Effect

The therapeutic effects of antibodies include neutralization of pathogens, activation of the host complement system, and facilitation of phagocytosis of pathogens. However, antibody alone has never been shown to exhibit bactericidal activity. In this study, we developed a monoclonal antibody that targets the bacterial cell surface component Pseudaminic acid (Pse). This monoclonal antibody, Pse-MAB1, exhibited direct bactericidal activity on Acinetobacter baumannii strains, even in the absence of the host complements or other immune factors, and was able to confer a protective effect against A. baumannii infections in mice. This study provides new insight into the potential of developing monoclonal antibody-based antimicrobial therapy of multidrug resistant bacterial infections, especially those which occurred among immunocompromised patients.

Stereoselective Synthesis of the O-antigen of A. baumannii ATCC 17961 Using Long-Range Levulinoyl Group Participation

Herein, we described the first synthesis of the pentasaccharide and decasaccharide of the A. baumannii ATCC 17961 O-antigen for developing a synthetic carbohydrate-based vaccine against A. baumannii infection. The efficient synthesis of the rare sugar 2,3-diacetamido-glucuronate was achieved using our recently introduced organocatalytic glycosylation method. We found, for the first time, that long-range levulinoyl group participation via a hydrogen bond can result in a significantly improved β-selectivity in glycosylations. This solves the stereoselectivity problem of highly branched galactose acceptors. The proposed mechanism was supported by control experiments and DFT computations. Benefiting from the long-range levulinoyl group participation strategy, the pentasaccharide donor and acceptor were obtained via an efficient [2+1+2] one-pot glycosylation method and were used for the target decasaccharide synthesis.

Nanotherapeutics with immunoregulatory functions for the treatment of bacterial infection

The advent of drug-resistant pathogens results in the occurrence of stubborn bacterial infections that cannot be treated with traditional antibiotics. Antibacterial immunotherapy by reviving or activating the body's immune system to eliminate pathogenic bacteria has confirmed promising therapeutic strategies in controlling bacterial infections. Subsequent studies found that antimicrobial immunotherapy has its own benefits and limitations, such as avoiding recurrence of infection and autoimmunity-induced side effects. Current studies indicate that the various antibacterial therapeutic strategies inducing immune regulation can achieve superior therapeutic efficacy compared with monotherapy alone. Therefore, summarizing the recent advances in nanomedicine with immunomodulatory functions for combating bacterial infections is necessary. Herein, we briefly introduce the crisis caused by drug-resistant bacteria and the opportunity for antibacterial immunotherapy. Then, immune-involved multimodal antibacterial therapy for the treatment of infectious diseases was systematically summarized. Finally, the prospects and challenges of immune-involved combinational therapy are discussed.

Structural diversity among Acinetobacter baumannii K-antigens and its implication in the in silico serotyping

Acinetobacter baumannii is an emerging opportunistic pathogen. It exhibits multi-, extreme-, and pan-drug resistance against several classes of antibiotics. Capsular polysaccharide (CPS or K-antigen) is one of the major virulence factors which aids A. baumannii in evading the host immune system. K-antigens of A. baumannii exploit the Wzx/Wzy-dependent pathway that involves 13 different proteins for its assembly and transport onto the outer membrane. A total of 64 (out of 237 K-locus(KL) types) known K-antigen sugar repeating structures are discussed here and are classified into seven groups based on their initial sugars, QuiNAc4NAc, GalNAc, GlcNAc, Gal, QuiNAc/FucNAc, FucNAc, and GlcNAc along with Leg5Ac7Ac/Leg5Ac7R. Thus, the corresponding seven initializing glycosyltransferases (ItrA1, ItrA2, ItrA3, ItrA4, ItrB1, ItrB3, and ItrA3 along with ItrB2) exhibit serotype specificity. The modeled 3D-structural repository of the 64 K-antigens can be accessed at https://project.iith.ac.in/ABSD/k_antigen.html. The topology of K-antigens further reveals the presence of 2-6 and 0-4 sugar monomers in the main and side chains, respectively. The presence of negatively (predominant) or neutrally charged K-antigens is observed in A. baumannii. Such diversity in the K-antigen sugar composition provides the K-typing specificity (viz., 18-69% in terms of reliability) for Wza, Wzb, Wzc, Wzx, and Wzy proteins involved in the Wzx/Wzy-dependent pathway. Interestingly, the degree of uniqueness of these proteins among different K-types is estimated to be 76.79%, considering the 237 reference sequences. This article summarizes the A. baumannii K-antigen structural diversity and creation of a K-antigen digital repository and provides a systematic analysis of the K-antigen assembly and transportation marker proteins.

Guanidine Additive Enabled Intermolecular ortho-Phthalaldehyde-Amine-Thiol Three-Component Reactions for Modular Constructions

Recently, ortho-phthalaldehyde (OPA) is experiencing a renascence for the modification of proteins and peptides through OPA-amine two-component reactions for bioconjugation and intramolecular OPA-amine-thiol three-component reactions for cyclization. Historically, small thiol molecules were used in large excess to allow for the intermolecular OPA-amine-thiol reaction forming 1-thio-isoindole derivatives. In this study, we discovered that guanidine could serve as an effective additive to switch the intermolecular OPA-amine-thiol three-component reaction to a stoichiometric process and enable the modular construction of peptide-peptide, and peptide-drug conjugate structures. Thus, 12 model peptide-peptide conjugates have been synthesized from unprotected peptides featuring all proteinogenic residues. Besides, 6 peptide-drug conjugates have been prepared in one step, with excellent conversions and isolated yields. In addition, a conjugate product has been further functionalized by utilizing a premodified OPA derivative, demonstrating the versatility and flexibility of this reaction.

Stereoisomer-specific reprogramming of a bacterial flagellin sialyltransferase

Glycosylation of surface structures diversifies cells chemically and physically. Nucleotide-activated sialic acids commonly serve as glycosyl donors, particularly pseudaminic acid (Pse) and its stereoisomer legionaminic acid (Leg), which decorate eubacterial and archaeal surface layers or protein appendages. FlmG, a recently identified protein sialyltransferase, O-glycosylates flagellins, the subunits of the flagellar filament. We show that flagellin glycosylation and motility in Caulobacter crescentus and Brevundimonas subvibrioides is conferred by functionally insulated Pse and Leg biosynthesis pathways, respectively, and by specialized FlmG orthologs. We established a genetic glyco-profiling platform for the classification of Pse or Leg biosynthesis pathways, discovered a signature determinant of eubacterial and archaeal Leg biosynthesis, and validated it by reconstitution experiments in a heterologous host. Finally, by rewiring FlmG glycosylation using chimeras, we defined two modular determinants that govern flagellin glycosyltransferase specificity: a glycosyltransferase domain that either donates Leg or Pse and a specialized flagellin-binding domain that identifies the acceptor.

Development in the Concept of Bacterial Polysaccharide Repeating Unit-Based Antibacterial Conjugate Vaccines

The surface of cells is coated with a dense layer of glycans, known as the cell glycocalyx. The complex glycans in the glycocalyx are involved in various biological events, such as bacterial pathogenesis, protection of bacteria from environmental stresses, etc. Polysaccharides on the bacterial cell surface are highly conserved and accessible molecules, and thus they are excellent immunological targets. Consequently, bacterial polysaccharides and their repeating units have been extensively studied as antigens for the development of antibacterial vaccines. This Review surveys the recent developments in the synthetic and immunological investigations of bacterial polysaccharide repeating unit-based conjugate vaccines against several human pathogenic bacteria. The major challenges associated with the development of functional carbohydrate-based antibacterial conjugate vaccines are also considered.

Carriers and Antigens: New Developments in Glycoconjugate Vaccines

Glycoconjugate vaccines have proven their worth in the protection and prevention of infectious diseases. The introduction of the Haemophilus influenzae type b vaccine is the prime example, followed by other glycoconjugate vaccines. Glycoconjugate vaccines consist of two components: the carrier protein and the carbohydrate antigen. Current carrier proteins are tetanus toxoid, diphtheria toxoid, CRM197, Haemophilus protein D and the outer membrane protein complex of serogroup B meningococcus. Carbohydrate antigens have been produced mainly by extraction and purification from the original host. However, current efforts show great advances in the development of synthetically produced oligosaccharides and bioconjugation. This review evaluates the advances of glycoconjugate vaccines in the last five years. We focus on developments regarding both new carriers and antigens. Innovative developments regarding carriers are outer membrane vesicles, glycoengineered proteins, new carrier proteins, virus-like particles, protein nanocages and peptides. With regard to conjugated antigens, we describe recent developments in the field of antimicrobial resistance (AMR) and ESKAPE pathogens.

Glycoconjugate vaccines against antimicrobial resistant pathogens

INTRODUCTION

Antimicrobial resistance (AMR) is responsible for the death of millions worldwide and stands as a major threat to our healthcare systems, which are heavily reliant on antibiotics to fight bacterial infections. The development of vaccines against the main pathogens involved is urgently required as prevention remains essential against the rise of AMR.

AREAS COVERED

A systematic research review was conducted on MEDLINE database focusing on the six AMR pathogens defined as ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Escherichia coli), which are considered critical or high priority pathogens by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC). The analysis was intersecated with the terms carbohydrate, glycoconjugate, bioconjugate, glyconanoparticle, and multiple presenting antigen system vaccines.

EXPERT OPINION

Glycoconjugate vaccines have been successful in preventing meningitis and pneumoniae, and there are high expectations that they will play a key role in fighting AMR. We herein discuss the recent technological, preclinical, and clinical advances, as well as the challenges associated with the development of carbohydrate-based vaccines against leading AMR bacteria, with focus on the ESKAPE pathogens. The need of innovative clinical and regulatory approaches to tackle these targets is also highlighted.

Immune Response to Conjugates of Fragments of the Type K9 Capsular Polysaccharide of Acinetobacter baumannii with Carrier Proteins

The clonal bacterial species Acinetobacter baumannii is an emerging multidrug-resistant pathogen which causes high-lethality infections. Cells of A. baumannii are surrounded by the type-specific capsular polysaccharide (CPS), which provides resistance to the protective mechanisms of the host and is considered a target for immunization. The conjugates of three inert carrier proteins and A. baumannii type K9 CPS fragments, which contained various numbers of oligosaccharide repeats (K-units), were synthesized by periodate oxidation and squaric acid chemistry. The conjugates were applied to immunize mice, and chemical synthesis by squaric acid was shown to significantly improve the immunogenic properties of glycoconjugate. In BALB/c mice, IgG antibodies were predominant among type K9 CPS reactive antibodies, and their total content was several times higher than that of IgM. Immune sera were characterized by their opsonization ability during practically the entire lives of the experimental mice. The sera were cross-reactive, but the highest specificity was observed against the antigen (type K9 CPS) used for immunization. The immunization of BALB/c and ICR-1 mice with a glycoconjugate without adjuvants led to varying degrees of stimulation of IL-10, IL-17A, and TNF-α production, but not IL-4 production in the ICR-1 mice. This is in contrast to the BALB/c mice, in which γ-IFN production was also activated. The protective effectiveness of the glycoconjugates obtained by squaric acid chemistry was demonstrated by experiments that involved challenging immunized and nonimmunized animals with a lethal dose of A. baumannii K9. IMPORTANCE Immunization by glycoconjugates with A. baumannii type K9 CPS fragments induced a high level of antibodies (predominantly IgG) in sera, which reacted specifically with the CPS of A. baumannii type K9, as well as a long immunological memory. The sera of immunized animals efficiently opsonized A. baumannii type K9. Immunization resulted in the balanced production of pro/anti-inflammatory lymphokines and protective antibodies to ensure the survival of the mice infected with A. baumannii. The level of specific antibodies was sufficient to provide protective immunity against the challenge by A. baumannii, making this approach applicable in the development of vaccine preparations.

Semi-Synthetic Glycoconjugate Vaccine Lead Against Acinetobacter baumannii 17978

Acinetobacter baumannii is a opportunistic bacterial pathogen responsible for serious nosocomial infections that is becoming increasingly resistant against antibiotics. Capsular polysaccharides (CPS) that cover A. baumannii are a major virulence factor that play an important role in pathogenesis, are used to assign serotypes and provide the basis for vaccine development. Synthetic oligosaccharides resembling the CPS of A. baumannii 17978 were printed onto microarray slides and used to screen sera from patients infected with A. baumannii as well as a monoclonal mouse antibody (mAb C8). A synthetic oligosaccharide emerged from glycan array screening as lead for the development of a vaccine against A. baumannii 17978. Tetrasaccharide 20 is a key epitope for recognition by an antibody and is a vaccine lead.

Cross reacting material (CRM197) as a carrier protein for carbohydrate conjugate vaccines targeted at bacterial and fungal pathogens

This paper gives an overview of conjugate glycovaccines which contain recombinant diphtheria toxoid CRM197 as a carrier protein. A special focus is given to synthetic methods used for preparation of neoglycoconjugates of CRM197 with oligosaccharide epitopes of cell surface carbohydrates of pathogenic bacteria and fungi. Syntheses of commercial vaccines and laboratory specimen on the basis of CRM197 are outlined briefly.

The K218 capsular polysaccharide produced by Acinetobacter baumannii isolate 52-249 includes 5,7-di-N-acetylpseudaminic acid linked by a KpsS3 glycosyltransferase

Two acylated forms of the higher sugar, 5,7-diamino-3,5,7,9-tetradeoxy-l-glycero-l-manno-non-2-ulosonic acid called pseudaminic acid, Pse5Ac7Ac and Pse5Ac7RHb where R indicates (R)-3-hydroxybutanoyl, have been found to occur in many capsular polysaccharide (CPS) types produced by isolates of an important human pathogen, Acinetobacter baumannii. The presence of either a psaABCEDF or psaABCGHF gene module at the K locus (KL) for CPS biosynthesis determines the type of the variant produced. Here, an A. baumannii clinical isolate 52-249, recovered in 2015 in Moscow, Russia, was found to include a novel psaABCIJF gene module in the KL218 sequence at the K locus. The CPS from 52-249 was extracted and studied by sugar analysis and partial acid hydrolysis along with one- and two-dimensional ¹H and ¹³C NMR spectroscopy. A branched tetrasaccharide repeating unit was identified, which included a →3)-α-d-Galp-(1→6)-α-d-GlcpNAc-(1→3)-β-d-GalpNAc-(1→ main chain and Pse5Ac7Ac attached as a side branch, indicating that the psaABCIJF gene module is associated with synthesis of this variant. The K218 CPS was found to be structurally related to the K46 CPS of A. baumannii, and a comparison of the two structures enabled the assignment of glycosyltransferases. A KpsS3 protein for the α-(2→6) linkage of the Pse5Ac7Ac residue to D-Galp in K218 was identified.

Syntheses of Legionaminic Acid, Pseudaminic Acid, Acetaminic Acid, 8-epi-Acetaminic Acid, and 8-epi-Legionaminic Acid Glycosyl Donors from N-Acetylneuraminic Acid by Side Chain Exchange

Metaperiodate cleavage of the glycerol side chain from an N-acetyl neuraminic acid-derived thioglycoside and condensation with the two enantiomers of the Ellman sulfinamide afford two diastereomeric N-sulfinylimines from which bacterial sialic acid donors with the legionaminic and acetaminic acid configurations and their 8-epi-isomers are obtained by samarium iodide-mediated coupling with acetaldehyde and subsequent manipulations. A variation on the theme, with inversion of the configuration at C5, similarly provides two differentially protected pseudaminic acid donors.

Association of capsular polysaccharide locus 2 with prognosis of Acinetobacter baumannii bacteraemia

Acinetobacter baumannii causes healthcare-associated infections worldwide. Capsular polysaccharide (CPS) is shown an important virulence factor of A. baumannii both in vitro and in vivo. Capsule locus 2 (KL2) for CPS is the most common KL type and is associated with carbapenem resistance. It is unclear whether KL2 is related to the clinical outcome of invasive A. baumannii infection. Here we had followed patients with A. baumannii bacteraemia prospectively between 2009 and 2014. One-third of the unduplicated blood isolates were randomly selected each year for microbiological and clinical studies. The KL2 gene cluster was identified using polymerase chain reaction. A total of 148 patients were enrolled randomly. Eighteen isolates (12.2%) carried KL2, and 130 isolates (87.8%) didn’t. Compared with non-KL2 isolates, KL2 isolates had significantly higher resistance to imipenem, sulbactam, and tigecycline. Compared with the non-KL group, in the KL2 group, the hospital stay before development of bacteraemia was longer (P < 0.001), a higher percentage had pneumonia (P = 0.004), and the white blood cell count was lower (P = 0.03). Infection with KL2 A. baumannii predicted mortality (adjusted hazard ratio [aHR], 2.03; 95% confidence interval [CI], 1.09–3.78; P = 0.03), independently of the Pitt bacteraemia score (aHR, 1.34; 95% CI, 1.23–1.46; P < 0.001) and leucopenia (aHR, 2.16; 95% CI, 1.30–3.57; P = 0.003). Thrombocytopenia contributed to the effect of KL2 on mortality in bacteraemia (Sobel test P = 0.01). Large-scale studies are warranted to confirm these findings and the underlying mechanisms deserve further investigation.