Streptococcal Lancefield polysaccharides are critical cell wall determinants for human Group IIA secreted phospholipase A2 to exert its bactericidal effects

Pangenome evaluation of gene essentiality in Streptococcus pyogenes

Bacterial species often consist of strains with variable gene content, collectively referred to as the pangenome. Variations in the genetic makeup of strains can alter bacterial physiology and fitness. To define biologically relevant genes of a genome, genome-wide transposon mutant libraries have been used to identify genes essential for survival or virulence in a given strain. Such phenotypic studies have been conducted in four different genotypes of the human pathogen Streptococcus pyogenes, yet challenges exist in comparing results across studies conducted in different genetic backgrounds and conditions. To advance genotype to phenotype inferences across different S. pyogenes strains, we built a pangenome database of 249 S. pyogenes reference genomes. We systematically re-analyzed publicly available transposon sequencing datasets from S. pyogenes using a transposon sequencing-specific analysis pipeline, Transit. Across four genetic backgrounds and nine phenotypic conditions, 355 genes were essential for survival, corresponding to ~24% of the core genome. Clusters of Orthologous Genes (COG) categories related to coenzyme and lipid transport and growth functions were overrepresented as essential. Finally, essential operons across S. pyogenes genotypes were defined, with an increased number of essential operons detected under in vivo conditions. This study provides an extendible database to which new studies can be added, and a searchable html-based resource to direct future investigations into S. pyogenes biology.IMPORTANCEStreptococcus pyogenes is a human-adapted pathogen occupying restricted ecological niches. Understanding the essentiality of genes across different strains and experimental conditions is important to direct research questions and efforts to prevent the large burden of disease caused by S. pyogenes. To this end we systematically reanalyzed transposon sequencing studies in S. pyogenes using transposon sequencing-specific methods, integrating them into an extendible meta-analysis framework. This provides a repository of gene essentiality in S. pyogenes which was used to highlight specific genes of interest and for the community to guide future phenotypic studies.

O-glycosylation of intrinsically disordered regions regulates homeostasis of membrane proteins in streptococci

Proteins harboring intrinsically disordered regions (IDRs) lacking stable secondary or tertiary structures are abundant across the three domains of life. These regions have not been systematically studied in prokaryotes. Our genome-wide analysis identifies extracytoplasmic serine/threonine-rich IDRs in several biologically important membrane proteins in streptococci. We demonstrate that these IDRs are O-glycosylated with glucose by glycosyltransferases GtrB and PgtC2 in Streptococcus pyogenes and Streptococcus pneumoniae, and with N-acetylgalactosamine by a Pgf-dependent mechanism in Streptococcus mutans. Absence of glycosylation leads to a defect in biofilm formation under ethanol-stressed conditions in S. mutans. We link this phenotype to the C-terminal IDR of a post-translocation secretion chaperone PrsA. O-glycosylation of the IDR protects this region from proteolytic degradation. The IDR length attenuates the efficiency of glycosylation and, consequently, the expression level of PrsA. Taken together, our data reveal that O-glycosylation of IDRs functions as a dynamic switch of protein homeostasis in streptococci.

Determinants of bacterial survival and proliferation in blood

Bloodstream infection is a major public health concern associated with high mortality and high healthcare costs worldwide. Bacteremia can trigger fatal sepsis whose prevention, diagnosis, and management have been recognized as a global health priority by the World Health Organization. Additionally, infection control is increasingly threatened by antimicrobial resistance, which is the focus of global action plans in the framework of a One Health response. In-depth knowledge of the infection process is needed to develop efficient preventive and therapeutic measures. The pathogenesis of bloodstream infection is a dynamic process resulting from the invasion of the vascular system by bacteria, which finely regulate their metabolic pathways and virulence factors to overcome the blood immune defenses and proliferate. In this review, we highlight our current understanding of determinants of bacterial survival and proliferation in the bloodstream and discuss their interactions with the molecular and cellular components of blood.

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.

Interference with Lipoprotein Maturation Sensitizes Methicillin-Resistant Staphylococcus aureus to Human Group IIA-Secreted Phospholipase A2 and Daptomycin

Methicillin-resistant Staphylococcus aureus (MRSA) has been classified as a high priority pathogen by the World Health Organization underlining the high demand for new therapeutics to treat infections. Human group IIA-secreted phospholipase A2 (hGIIA) is among the most potent bactericidal proteins against Gram-positive bacteria, including S. aureus. To determine hGIIA-resistance mechanisms of MRSA, we screened the Nebraska Transposon Mutant Library using a sublethal concentration of recombinant hGIIA. We identified and confirmed the role of lspA, encoding the lipoprotein signal peptidase LspA, as a new hGIIA resistance gene in both in vitro assays and an infection model in hGIIA-transgenic mice. Increased susceptibility of the lspA mutant was associated with enhanced activity of hGIIA on the cell membrane. Moreover, lspA deletion increased susceptibility to daptomycin, a last-resort antibiotic to treat MRSA infections. MRSA wild type could be sensitized to hGIIA and daptomycin killing through exposure to LspA-specific inhibitors globomycin and myxovirescin A1. Analysis of >26,000 S. aureus genomes showed that LspA is highly sequence-conserved, suggesting universal application of LspA inhibition. The role of LspA in hGIIA resistance was not restricted to MRSA since Streptococcus mutans and Enterococcus faecalis were also more hGIIA-susceptible after lspA deletion or LspA inhibition, respectively. Overall, our data suggest that pharmacological interference with LspA may disarm Gram-positive pathogens, including MRSA, to enhance clearance by innate host defense molecules and clinically applied antibiotics.

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.

Endolysins against Streptococci as an antibiotic alternative

Multi-drug resistance has called for a race to uncover alternatives to existing antibiotics. Phage therapy is one of the explored alternatives, including the use of endolysins, which are phage-encoded peptidoglycan hydrolases responsible for bacterial lysis. Endolysins have been extensively researched in different fields, including medicine, food, and agricultural applications. While the target specificity of various endolysins varies greatly between species, this current review focuses specifically on streptococcal endolysins. Streptococcus spp. causes numerous infections, from the common strep throat to much more serious life-threatening infections such as pneumonia and meningitis. It is reported as a major crisis in various industries, causing systemic infections associated with high mortality and morbidity, as well as economic losses, especially in the agricultural industry. This review highlights the types of catalytic and cell wall-binding domains found in streptococcal endolysins and gives a comprehensive account of the lytic ability of both native and engineered streptococcal endolysins studied thus far, as well as its potential application across different industries. Finally, it gives an overview of the advantages and limitations of these enzyme-based antibiotics, which has caused the term enzybiotics to be conferred to it.

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

The cell wall of the human bacterial pathogen Group A Streptococcus (GAS) consists of peptidoglycan decorated with the Lancefield group A carbohydrate (GAC). GAC is a promising target for the development of GAS vaccines. In this study, employing chemical, compositional, and NMR methods, we show that GAC is attached to peptidoglycan via glucosamine 1-phosphate. This structural feature makes the GAC-peptidoglycan linkage highly sensitive to cleavage by nitrous acid and resistant to mild acid conditions. Using this characteristic of the GAS cell wall, we identify PplD as a protein required for deacetylation of linkage N-acetylglucosamine (GlcNAc). X-ray structural analysis indicates that PplD performs catalysis via a modified acid/base mechanism. Genetic surveys in silico together with functional analysis indicate that PplD homologs deacetylate the polysaccharide linkage in many streptococcal species. We further demonstrate that introduction of positive charges to the cell wall by GlcNAc deacetylation protects GAS against host cationic antimicrobial proteins.

Immunobiology of the Classical Lancefield Group A Streptococcal Carbohydrate Antigen

Group A Streptococcus (GAS) is a preeminent human bacterial pathogen causing hundreds of millions of infections each year worldwide. In the clinical setting, the bacterium is easily identified by a rapid antigen test against the group A carbohydrate (GAC), a polysaccharide that comprises 30 to 50% of the GAS cell wall by weight. Originally described by Rebecca Lancefield in the 1930s, GAC consists of a polyrhamnose backbone and a N-acetylglucosamine (GlcNAc) side chain. This side chain, the species-defining immunodominant antigen, is potentially implicated in autoreactive immune responses against human heart or brain tissue in poststreptococcal rheumatic fever or rheumatic heart disease. The recent discovery of the genetic locus encoding GAC biosynthesis and new insights into its chemical structure have provided novel insights into the assembly of the polysaccharide, its contribution to immune evasion and virulence, and ideas for safely harnessing its natural immunogenicity in vaccine design. This minireview serves to summarize the emerging new literature on GAC, the eponymous cell well antigen that provides structural integrity to GAS and directly interfaces with host innate and adaptive immune responses.

Protocols for Tn-seq Analyses in the Group A Streptococcus

Transposon-sequencing (Tn-seq) has revolutionized forward-genetic analyses to study genotype-phenotype associations and interrogate bacterial cell physiology. The Tn-seq approach allows the en masse monitoring of highly complex mutant libraries, leveraging massive parallel DNA sequencing as a means to characterize the composition of these mutant pools on a genome-scale with unprecedented nucleotide-level high resolution. In this chapter, we present step-by-step protocols for Tn-seq analyses in the human pathogen Streptococcus pyogenes (Group A Streptococcus or GAS) using the mariner-based Krmit transposon. We detail how to generate highly complex Krmit mutant libraries in GAS and the en masse production of Krmit insertion tags for Illumina sequencing of the transposon-genome junctions for Tn-seq analyses. Most of the protocols presented here were developed and implemented using the S. pyogenes M1T1 serotype clinical isolate 5448, but they have been successfully applied to multiple GAS serotypes as well as other pathogenic Streptococci.

Type IIA Secreted Phospholipase A2 in Host Defense against Bacterial Infections

The enzyme type IIA secreted phospholipase A2 (sPLA2-IIA) is crucial for mammalian innate host defense against bacterial pathogens. Most studies have investigated the role of sPLA2-IIA in systemic bacterial infections, identifying molecular pathways of bacterial resistance against sPLA2-IIA-mediated killing, and providing insight into sPLA2-IIA mechanisms of action. Sensitization of (antibiotic-resistant) bacteria to sPLA2-IIA action by blocking bacterial resistance or by applying sPLA2-IIA to treat bacterial infections might represent a therapeutic option in the future. Because sPLA2-IIA is highly expressed at mucosal barriers, we also discuss how sPLA2-IIA is likely to be an important driver of microbiome composition; we anticipate that future research in this area may bring new insights into the role of sPLA2-IIA in health and disease.

Group A, B, C, and G Streptococcus Lancefield antigen biosynthesis is initiated by a conserved α-d-GlcNAc-β-1,4-l-rhamnosyltransferase

Group A carbohydrate (GAC) is a bacterial peptidoglycan-anchored surface rhamnose polysaccharide (RhaPS) that is essential for growth of Streptococcus pyogenes and contributes to its ability to infect the human host. In this study, using molecular and synthetic biology approaches, biochemistry, radiolabeling techniques, and NMR and MS analyses, we examined the role of GacB, encoded in the S. pyogenes GAC gene cluster, in the GAC biosynthesis pathway. We demonstrate that GacB is the first characterized α-d-GlcNAc-β-1,4-l-rhamnosyltransferase that synthesizes the committed step in the biosynthesis of the GAC virulence determinant. Importantly, the substitution of S. pyogenes gacB with the homologous gene from Streptococcus agalactiae (Group B Streptococcus), Streptococcus equi subsp. zooepidemicus (Group C Streptococcus), Streptococcus dysgalactiae subsp. equisimilis (Group G Streptococcus), or Streptococcus mutans complemented the GAC biosynthesis pathway. These results, combined with those from extensive in silico studies, reveal a common phylogenetic origin of the genes required for this priming step in >40 pathogenic species of the Streptococcus genus, including members from the Lancefield Groups B, C, D, E, G, and H. Importantly, this priming step appears to be unique to streptococcal ABC transporter-dependent RhaPS biosynthesis, whereas the Wzx/Wzy-dependent streptococcal capsular polysaccharide pathways instead require an α-d-Glc-β-1,4-l-rhamnosyltransferase. The insights into the RhaPS priming step obtained here open the door to targeting the early steps of the group carbohydrate biosynthesis pathways in species of the Streptococcus genus of high clinical and veterinary importance.

Antimalarial Activity of Human Group IIA Secreted Phospholipase A2 in Relation to Enzymatic Hydrolysis of Oxidized Lipoproteins

The level of human group IIA secreted phospholipase A₂ (hGIIA sPLA₂) is increased in the plasma of malaria patients, but its role is unknown. In parasite culture with normal plasma, hGIIA is inactive against Plasmodium falciparum, contrasting with hGIIF, hGV, and hGX sPLA₂s, which readily hydrolyze plasma lipoproteins, release nonesterified fatty acids (NEFAs), and inhibit parasite growth. Here, we revisited the anti-Plasmodium activity of hGIIA under conditions closer to those of malaria physiopathology where lipoproteins are oxidized. In parasite culture containing oxidized lipoproteins, hGIIA sPLA₂ was inhibitory, with a 50% inhibitory concentration value of 150.0 ± 40.8 nM, in accordance with its capacity to release NEFAs from oxidized particles. With oxidized lipoproteins, hGIIF, hGV, and hGX sPLA₂s were also more potent, by 4.6-, 2.1-, and 1.9-fold, respectively. Using specific immunoassays, we found that hGIIA sPLA₂ is increased in plasma from 41 patients with malaria over levels for healthy donors (median [interquartile range], 1.6 [0.7 to 3.4] nM versus 0.0 [0.0 to 0.1] nM, respectively; P < 0.0001). Other sPLA₂s were not detected. Malaria plasma, but not normal plasma, contains oxidized lipoproteins and was inhibitory to P. falciparum when spiked with hGIIA sPLA₂ Injection of recombinant hGIIA into mice infected with P. chabaudi reduced the peak of parasitemia, and this was effective only when the level of plasma peroxidation was increased during infection. In conclusion, we propose that malaria-induced oxidation of lipoproteins converts these into a preferential substrate for hGIIA sPLA₂, promoting its parasite-killing effect. This mechanism may contribute to host defense against P. falciparum in malaria where high levels of hGIIA are observed.

Discovery of glycerol phosphate modification on streptococcal rhamnose polysaccharides

Cell wall glycopolymers on the surface of Gram-positive bacteria are fundamental to bacterial physiology and infection biology. Here we identify gacH, a gene in the Streptococcus pyogenes group A carbohydrate (GAC) biosynthetic cluster, in two independent transposon library screens for its ability to confer resistance to zinc and susceptibility to the bactericidal enzyme human group IIA-secreted phospholipase A₂. Subsequent structural and phylogenetic analysis of the GacH extracellular domain revealed that GacH represents an alternative class of glycerol phosphate transferase. We detected the presence of glycerol phosphate in the GAC, as well as the serotype c carbohydrate from Streptococcus mutans, which depended on the presence of the respective gacH homologs. Finally, nuclear magnetic resonance analysis of GAC confirmed that glycerol phosphate is attached to approximately 25% of the GAC N-acetylglucosamine side-chains at the C6 hydroxyl group. This previously unrecognized structural modification impacts host-pathogen interaction and has implications for vaccine design.