Structure and mechanism of Staphylococcus aureus TarM, the wall teichoic acid α-glycosyltransferase

Identification and Characterization of the Two Glycosyltransferases Required for the Polymerization of the HS:1 Serotype Capsular Polysaccharide of Campylobacter jejuni G1

Campylobacter jejuni is a Gram-negative pathogenic bacterium commonly found in poultry and is the leading cause of gastrointestinal infections in the United States. Similar to other Gram-negative bacteria, C. jejuni possesses an extracellular carbohydrate-based capsular polysaccharide (CPS) composed of repeating units of monosaccharides bound via glycosidic linkages. The gene cluster for serotype 1 (HS:1) of C. jejuni contains 13 different genes required for the production and presentation of the CPS. Each repeating unit within the HS:1 CPS structure contains a backbone of glycerol phosphate and d-galactose. Here, the enzyme HS1.11 was shown to catalyze the formation of CDP-(2R)-glycerol from MgCTP and l-glycerol-3-phosphate. HS1.09 was found to be a multidomain protein that catalyzes the polymerization of l-glycerol-3-phosphate and d-galactose using UDP-d-galactose and CDP-(2R)-glycerol as substrates. The domain of HS1.09 that extends from residues 286 to 703 was shown to catalyze the transfer of l-glycerol-P from CDP-glycerol to the hydroxyl group at C4 of the d-galactose moiety at the nonreducing end of the growing oligosaccharide. The transfer of d-galactose to the C2 hydroxyl group of the glycerol-phosphate moiety was shown to be catalyzed with retention of configuration by the domain of HS1.09 that extends from residues 704 to 1095. Primers as short as a single d-galactoside were accepted as initial substrates. Oligosaccharide products were isolated by ion exchange chromatography and identified by high-resolution ESI-mass spectrometry and NMR spectroscopy.

The two-component system ArlRS is essential for wall teichoic acid glycoswitching in Staphylococcus aureus

Staphylococcus aureus is among the leading causes of hospital-acquired infections. Critical to S. aureus biology and pathogenesis are the cell wall-anchored glycopolymers wall teichoic acids (WTA). Approximately one-third of S. aureus isolates decorates WTA with a mixture of α1,4- and β1,4-N-acetylglucosamine (GlcNAc), which requires the dedicated glycosyltransferases TarM and TarS, respectively. Environmental conditions, such as high salt concentrations, affect the abundance and ratio of α1,4- and β1,4-GlcNAc WTA decorations, thereby impacting biological properties such as antibody binding and phage infection. To identify regulatory mechanisms underlying WTA glycoswitching, we screened 1,920 S. aureus mutants (Nebraska Transposon Mutant Library) by immunoblotting for differential expression of WTA-linked α1,4- or β1,4-GlcNAc using specific monoclonal antibody Fab fragments. Three two-component systems (TCS), GraRS, ArlRS, and AgrCA, were among the 230 potential hits. Using isogenic TCS mutants, we demonstrated that ArlRS is essential for WTA β1,4-GlcNAc decoration. ArlRS repressed tarM expression through the transcriptional regulator MgrA. In bacteria lacking arlRS, the increased expression of tarM correlated with the absence of WTA β1,4-GlcNAc, likely by outcompeting TarS enzymatic activity. ArlRS was responsive to Mg2+, but not Na+, revealing its role in the previously reported salt-induced WTA glycoswitch from α1,4-GlcNAc to β1,4-GlcNAc. Importantly, ArlRS-mediated regulation of WTA glycosylation affected S. aureus interaction with the innate receptor langerin and lysis by β1,4-GlcNAc-dependent phages. Since WTA represents a promising target for future immune-based treatments and vaccines, our findings provide important insight to align strategies targeting S. aureus WTA glycosylation patterns during infection.IMPORTANCEStaphylococcus aureus is a common colonizer but can also cause severe infections in humans. The development of antibiotic resistance complicates the treatment of S. aureus infections, increasing the need for antibiotic alternatives such as vaccines and therapies with bacterial viruses also known as phages. Wall teichoic acids (WTA) are abundant glycosylated structures of the S. aureus cell wall that have gained attention as a promising target for new treatments. Importantly, WTA glycosylation patterns show variation depending on environmental conditions, thereby impacting phage binding and interaction with host factors, such as antibodies and innate pattern-recognition receptors. Here, we show that the two-component system ArlRS is involved in the regulation of WTA glycosylation by responding to environmental changes in Mg2+ concentration. These findings may support the design of new treatment strategies that target WTA glycosylation patterns of S. aureus during infection.

Molecular properties of the RmlT wall teichoic acid rhamnosyltransferase that modulates virulence in Listeria monocytogenes

Wall teichoic acids (WTAs) from the major Gram-positive foodborne pathogen Listeria monocytogenes are peptidoglycan-associated glycopolymers decorated by monosaccharides that, while not essential for bacterial growth, are required for bacterial virulence and resistance to antimicrobials. Here we report the structure and function of a bacterial WTAs rhamnosyltransferase, RmlT, strictly required for L. monocytogenes WTAs rhamnosylation. In particular, we demonstrated that RmlT transfers rhamnose from dTDP-L-rhamnose to naked WTAs, and that specificity towards TDP-rhamnose is not determined by its binding affinity. Structures of RmlT with and without its substrates showed that this enzyme is a dimer, revealed the residues responsible for interaction with the substrates and that the catalytic residue pre-orients the acceptor substrate towards the nucleophilic attack to the sugar. Additionally, the structures provided indications for two potential interaction pathways for the long WTAs on the surface of RmlT. Finally, we confirmed that WTAs glycosyltransferases are promising targets for next-generation strategies against Gram-positive pathogens by showing that inactivation of the RmlT catalytic activity results in a decreased infection in vivo.

Transition transferases prime bacterial capsule polymerization

Capsules are long-chain carbohydrate polymers that envelop the surfaces of many bacteria, protecting them from host immune responses. Capsule biosynthesis enzymes are potential drug targets and valuable biotechnological tools for generating vaccine antigens. Despite their importance, it remains unknown how structurally variable capsule polymers of Gram-negative pathogens are linked to the conserved glycolipid anchoring these virulence factors to the bacterial membrane. Using Actinobacillus pleuropneumoniae as an example, we demonstrate that CpsA and CpsC generate a poly(glycerol-3-phosphate) linker to connect the glycolipid with capsules containing poly(galactosylglycerol-phosphate) backbones. We reconstruct the entire capsule biosynthesis pathway in A. pleuropneumoniae serotypes 3 and 7, solve the X-ray crystal structure of the capsule polymerase CpsD, identify its tetratricopeptide repeat domain as essential for elongating poly(glycerol-3-phosphate) and show that CpsA and CpsC stimulate CpsD to produce longer polymers. We identify the CpsA and CpsC product as a wall teichoic acid homolog, demonstrating similarity between the biosynthesis of Gram-positive wall teichoic acid and Gram-negative capsules.

Chitin Translocation Is Functionally Coupled with Synthesis in Chitin Synthase

Chitin, an extracellular polysaccharide, is synthesized by membrane-embedded chitin synthase (CHS) utilizing intracellular substrates. The mechanism of the translocation of synthesized chitin across the membrane to extracellular locations remains unresolved. We prove that the chitin synthase from Phytophthora sojae (PsCHS) is a processive glycosyltransferase, which can rapidly produce and tightly bind with the highly polymerized chitin. We further demonstrate that PsCHS is a bifunctional enzyme, which is necessary and sufficient to translocate the synthesized chitin. PsCHS was purified and then reconstituted into proteoliposomes (PLs). The nascent chitin is generated and protected from chitinase degradation unless detergent solubilizes the PLs, showing that PsCHS translocates the newly produced chitin into the lumen of the PLs. We also attempted to resolve the PsCHS structure of the synthesized chitin-bound state, although it was not successful; the obtained high-resolution structure of the UDP/Mn2+-bound state could still assist in describing the characterization of the PsCHS's transmembrane channel. Consistently, we demonstrate that PsCHS is indispensable and capable of translocating chitin in a process that is tightly coupled to chitin synthesis.

Enzymatic Synthesis of Novel Terpenoid Glycoside Derivatives Decorated with N-Acetylglucosamine Catalyzed by UGT74AC1

The addition of the O-linked N-acetylglucosamine (O-GlcNAc) is a significant modification for active molecules, such as proteins, carbohydrates, and natural products. However, the synthesis of terpenoid glycoside derivatives decorated with GlcNAc remains a challenging task due to the absence of glycosyltransferases, key enzymes for catalyzing the transfer of GlcNAc to terpenoids. In this study, we demonstrated that the enzyme mutant UGT74AC1T79Y/L48M/R28H/L109I/S15A/M76L/H47R efficiently transferred GlcNAc from uridine diphosphate (UDP)-GlcNAc to a variety of terpenoids. This powerful enzyme was employed to synthesize GlcNAc-decorated derivatives of terpenoids, including mogrol, steviol, andrographolide, protopanaxadiol, glycyrrhetinic acid, ursolic acid, and betulinic acid for the first time. To unravel the mechanism of UDP-GlcNAc recognition, we determined the X-ray crystal structure of the inactivated mutant UGT74AC1His18A/Asp111A in complex with UDP-GlcNAc at a resolution of 1.66 Å. Through molecular dynamic simulation and activity analysis, we revealed the molecular mechanism and catalytically important amino acids directly involved in the recognition of UDP-GlcNAc. Overall, this study not only provided a potent biocatalyst capable of glycodiversifying natural products but also elucidated the structural basis for UDP-GlcNAc recognition by glycosyltransferases.

Evaluation of Aging Effect on the Durability of Antibacterial Treatments Applied on Textile Materials for the Automotive Industry

The automotive industry is always seeking novel solutions to improve the durability and the performance of textile materials used in vehicles. Indeed, especially after the coronavirus pandemic, antibacterial treatments have gained interest for their potential of ensuring cleanliness and safety toward microbial contamination within vehicles. This study gives a panoramic view of the durability of antibacterial treatments applied on textile materials in the automotive industry, focusing on their performance after experiencing accelerated aging processes. Two different textile materials, a fabric and a synthetic leather, both treated with antibacterial agents, were tested according to ISO 22196 and ISO 20743 standards, respectively, using two model microorganisms, Escherichia coli and Staphylococcus aureus. The impact of mechanical, thermal, and solar aging on the antibacterial properties has been evaluated. In addition, scanning electron microscope (SEM) analysis was performed to investigate the surface morphology of the materials before and after aging. Furthermore, contact angle measurements were conducted. The results suggest that neither mechanical nor thermal aging processes determined diminished antibacterial action. It was determined, instead, that the most damaging stressor for both textile materials was UV aging, causing severe surface alterations and a reduction in antibacterial activity.

Conyza canadensis from Jordan: Phytochemical Profiling, Antioxidant, and Antimicrobial Activity Evaluation

In this investigation, the chemical composition of the hydro-distilled essential oil (HD-EO), obtained from the fresh aerial parts (inflorescence heads (Inf), leaves (L), and stems (St)) of Conyza canadensis growing wild in Jordan was determined by GC/MS. Additionally, the methanolic extract obtained from the whole aerial parts of C. canadensis (CCM) was examined for its total phenolic content (TPC), total flavonoids content (TFC), DPPH radical scavenging activity, iron chelating activity and was then analyzed with LC-MS/MS for the presence of certain selected phenolic compounds and flavonoids. The GC/MS analysis of CCHD-EOs obtained from the different aerial parts revealed the presence of (2E, 8Z)-matricaria ester as the main component, amounting to 15.4% (Inf), 60.7% (L), and 31.6% (St) of the total content. Oxygenated monoterpenes were the main class of volatile compounds detected in the Inf-CCHD-EO. However, oils obtained from the leaves and stems were rich in polyacetylene derivatives. The evaluation of the CCM extract showed a richness in phenolic content (95.59 ± 0.40 mg GAE/g extract), flavonoids contents (467.0 ± 10.5 mg QE/ g extract), moderate DPPH radical scavenging power (IC50 of 23.75 ± 0.86 µg/mL) and low iron chelating activity (IC50 = 5396.07 ± 15.05 µg/mL). The LC-MS/MS profiling of the CCM extract allowed for the detection of twenty-five phenolic compounds and flavonoids. Results revealed that the CCM extract contained high concentration levels of rosmarinic acid (1441.1 mg/kg plant), in addition to caffeic acid phenethyl ester (231.8 mg/kg plant). An antimicrobial activity assessment of the CCM extract against a set of Gram-positive and Gram-negative bacteria, in addition to two other fungal species including Candida and Cryptococcus, showed significant antibacterial activity of the extract against S. aureus with MIC value of 3.125 µg/mL. The current study is the first phytochemical screening for the essential oil and methanolic extract composition of C. canadensis growing in Jordan, its antioxidant and antimicrobial activity.

Unveiling the Multifaceted Capabilities of Endophytic Aspergillus flavus Isolated from Annona squamosa Fruit Peels against Staphylococcus Isolates and HCoV 229E-In Vitro and In Silico Investigations

Recently, there has been a surge towards searching for primitive treatment strategies to discover novel therapeutic approaches against multi-drug-resistant pathogens. Endophytes are considered unexplored yet perpetual sources of several secondary metabolites with therapeutic significance. This study aims to isolate and identify the endophytic fungi from Annona squamosa L. fruit peels using morphological, microscopical, and transcribed spacer (ITS-rDNA) sequence analysis; extract the fungus's secondary metabolites by ethyl acetate; investigate the chemical profile using UPLC/MS; and evaluate the potential antibacterial, antibiofilm, and antiviral activities. An endophytic fungus was isolated and identified as Aspergillus flavus L. from the fruit peels. The UPLC/MS revealed seven compounds with various chemical classes. The antimicrobial activity of the fungal ethyl acetate extract (FEA) was investigated against different Gram-positive and Gram-negative standard strains, in addition to resistant clinical isolates using the agar diffusion method. The CPE-inhibition assay was used to identify the potential antiviral activity of the crude fungal extract against low pathogenic human coronavirus (HCoV 229E). Selective Gram-positive antibacterial and antibiofilm activities were evident, demonstrating pronounced efficacy against both methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA). However, the extract exhibited very weak activity against Gram-negative bacterial strains. The ethyl acetate extract of Aspergillus flavus L exhibited an interesting antiviral activity with a half maximal inhibitory concentration (IC50) value of 27.2 µg/mL against HCoV 229E. Furthermore, in silico virtual molecular docking-coupled dynamics simulation highlighted the promising affinity of the identified metabolite, orienting towards three MRSA biotargets and HCoV 229E main protease as compared to reported reference inhibitors/substrates. Finally, ADME analysis was conducted to evaluate the potential oral bioavailability of the identified metabolites.

Discovering New Substrates of a UDP-Glycosyltransferase with a High-Throughput Method

UDP-glycosyltransferases (UGTs) form a large enzyme family that is found in a wide range of organisms. These enzymes are known for accepting a wide variety of substrates, and they derivatize xenobiotics and metabolites for detoxification. However, most UGT homologs have not been well characterized, and their potential for biomedical and environmental applications is underexplored. In this work, we have used a fluorescent assay for screening substrates of a plant UGT homolog by monitoring the formation of UDP. We optimized the assay such that it could be used for high-throughput screening of substrates of the Medicago truncatula UGT enzyme, UGT71G1, and our results show that 34 of the 159 screened compound samples are potential substrates. With an LC-MS/MS method, we confirmed that three of these candidates indeed were glycosylated by UGT71G1, which includes bisphenol A (BPA) and 7-Ethyl-10-hydroxycamptothecin (SN-38); derivatization of these toxic compounds can lead to new environmental and medical applications. This work suggests that UGT homologs may recognize a substrate profile that is much broader than previously anticipated. Additionally, it demonstrates that this screening method provides a new means to study UDP-glycosyltransferases, facilitating the use of these enzymes to tackle a wide range of problems.

Invasive Staphylococcus epidermidis uses a unique processive wall teichoic acid glycosyltransferase to evade immune recognition

Staphylococcus epidermidis expresses glycerol phosphate wall teichoic acid (WTA), but some health care-associated methicillin-resistant S. epidermidis (HA-MRSE) clones produce a second, ribitol phosphate (RboP) WTA, resembling that of the aggressive pathogen Staphylococcus aureus. RboP-WTA promotes HA-MRSE persistence and virulence in bloodstream infections. We report here that the TarM enzyme of HA-MRSE [TarM(Se)] glycosylates RboP-WTA with glucose, instead of N-acetylglucosamine (GlcNAc) by TarM(Sa) in S. aureus. Replacement of GlcNAc with glucose in RboP-WTA impairs HA-MRSE detection by human immunoglobulin G, which may contribute to the immune-evasion capacities of many invasive S. epidermidis. Crystal structures of complexes with uridine diphosphate glucose (UDP-glucose), and with UDP and glycosylated poly(RboP), reveal the binding mode and glycosylation mechanism of this enzyme and explain why TarM(Se) and TarM(Sa) link different sugars to poly(RboP). These structural data provide evidence that TarM(Se) is a processive WTA glycosyltransferase. Our study will support the targeted inhibition of TarM enzymes, and the development of RboP-WTA targeting vaccines and phage therapies.

Controlled processivity in glycosyltransferases: A way to expand the enzymatic toolbox

Glycosyltransferases (GT) catalyse the biosynthesis of complex carbohydrates which are the most abundant group of molecules in nature. They are involved in several key mechanisms such as cell signalling, biofilm formation, host immune system invasion or cell structure and this in both prokaryotic and eukaryotic cells. As a result, research towards complete enzyme mechanisms is valuable to understand and elucidate specific structure-function relationships in this group of molecules. In a next step this knowledge could be used in GT protein engineering, not only for rational drug design but also for multiple biotechnological production processes, such as the biosynthesis of hyaluronan, cellooligosaccharides or chitooligosaccharides. Generation of these poly- and/or oligosaccharides is possible due to a common feature of several of these GTs: processivity. Enzymatic processivity has the ability to hold on to the growing polymer chain and some of these GTs can even control the number of glycosyl transfers. In a first part, recent advances in understanding the mechanism of various processive enzymes are discussed. To this end, an overview is given of possible engineering strategies for the purpose of new industrial and fundamental applications. In the second part of this review, we focused on specific chain length-controlling mechanisms, i.e., key residues or conserved regions, and this for both eukaryotic and prokaryotic enzymes.

Genetic diversity of Staphylococcus aureus wall teichoic acid glycosyltransferases affects immune recognition

Staphylococcus aureus is a leading cause of skin and soft tissue infections and systemic infections. Wall teichoic acids (WTAs) are cell wall-anchored glycopolymers that are important for S. aureus nasal colonization, phage-mediated horizontal gene transfer, and antibiotic resistance. WTAs consist of a polymerized ribitol phosphate (RboP) chain that can be glycosylated with N-acetylglucosamine (GlcNAc) by three glycosyltransferases: TarS, TarM, and TarP. TarS and TarP modify WTA with β-linked GlcNAc at the C-4 (β1,4-GlcNAc) and the C-3 position (β1,3-GlcNAc) of the RboP subunit, respectively, whereas TarM modifies WTA with α-linked GlcNAc at the C-4 position (α1,4-GlcNAc). Importantly, these WTA glycosylation patterns impact immune recognition and clearance of S. aureus. Previous studies suggest that tarS is near-universally present within the S. aureus population, whereas a smaller proportion co-contain either tarM or tarP. To gain more insight into the presence and genetic variation of tarS, tarM and tarP in the S. aureus population, we analysed a collection of 25 652 S. aureus genomes within the PubMLST database. Over 99 % of isolates contained tarS. Co-presence of tarS/tarM or tarS/tarP occurred in 37 and 7 % of isolates, respectively, and was associated with specific S. aureus clonal complexes. We also identified 26 isolates (0.1 %) that contained all three glycosyltransferase genes. At sequence level, we identified tar alleles with amino acid substitutions in critical enzymatic residues or with premature stop codons. Several tar variants were expressed in a S. aureus tar-negative strain. Analysis using specific monoclonal antibodies and human langerin showed that WTA glycosylation was severely attenuated or absent. Overall, our data provide a broad overview of the genetic diversity of the three WTA glycosyltransferases in the S. aureus population and the functional consequences for immune recognition.

3-O-Sulfation induces sequence-specific compact topologies in heparan sulfate that encode a dynamic sulfation code

Heparan sulfate (HS) is arguably the most diverse linear biopolymer that is known to modulate hundreds of proteins. Whereas the configurational and conformational diversity of HS is well established in terms of varying sulfation patterns and iduronic acid (IdoA) puckers, a linear helical topology resembling a cylindrical rod is the only topology thought to be occupied by the biopolymer. We reasoned that 3-O-sulfation, a rare modification in natural HS, may induce novel topologies that contribute to selective recognition of proteins. In this work, we studied a library of 24 distinct HS hexasaccharides using molecular dynamics (MD). We discovered novel compact (C) topologies that are populated significantly by a unique group of 3-O-sulfated sequences containing IdoA residues. 3-O-sulfated sequences containing glucuronic acid (GlcA) residue and sequences devoid of 3-O-sulfate groups did not exhibit high levels of the C topology and primarily exhibited only the canonical linear (L) form. The C topology arises under dynamical conditions due to rotation around an IdoA → GlcN glycosidic linkage, especially in psi (Ψ) torsion. At an atomistic level, the L → C transformation is a multi-factorial phenomenon engineered to reduce like-charge repulsion, release one or more HS-bound water molecules, and organize a bi-dentate "IdoA-cation-IdoA" interaction. These forces also drive an L → C transformation in a 3-O-sulfated octasaccharide, which has shown evidence of the unique C topology in the co-crystallized state. The 3-O-sulfate-based generation of unique, sequence-specific, compact topologies indicate that natural HS encodes a dynamic sulfation code that could be exploited for selective recognition of target proteins.

Towards Computationally Guided Design and Engineering of a Neisseria meningitidis Serogroup W Capsule Polymerase with Altered Substrate Specificity

Heavy metal contamination of drinking water is a public health concern that requires the development of more efficient bioremediation techniques. Absorption technologies, including biosorption, provide opportunities for improvements to increase the diversity of target metal ions and overall binding capacity. Microorganisms are a key component in wastewater treatment plants, and they naturally bind metal ions through surface macromolecules but with limited capacity. The long-term goal of this work is to engineer capsule polymerases to synthesize molecules with novel functionalities. In previously published work, we showed that the Neisseria meningitidis serogroup W (NmW) galactose-sialic acid (Gal-NeuNAc) heteropolysaccharide binds lead ions effectively, thereby demonstrating the potential for its use in environmental decontamination applications. In this study, computational analysis of the NmW capsule polymerase galactosyltransferase (GT) domain was used to gain insight into how the enzyme could be modified to enable the synthesis of N-acetylgalactosamine-sialic acid (GalNAc-NeuNAc) heteropolysaccharide. Various computational approaches, including molecular modeling with I-TASSER and molecular dynamics (MD) simulations with NAMD, were utilized to identify key amino acid residues in the substrate binding pocket of the GT domain that may be key to conferring UDP-GalNAc specificity. Through these combined strategies and using BshA, a UDP-GlcNAc transferase, as a structural template, several NmW active site residues were identified as mutational targets to accommodate the proposed N-acetyl group in UDP-GalNAc. Thus, a rational approach for potentially conferring new properties to bacterial capsular polysaccharides is demonstrated.

Structural basis for SdgB- and SdgA-mediated glycosylation of staphylococcal adhesive proteins

The initiation of infection of host tissues by Staphylococcus aureus requires a family of staphylococcal adhesive proteins containing serine-aspartate repeat (SDR) domains, such as ClfA. The O-linked glycosylation of the long-chain SDR domain mediated by SdgB and SdgA is a key virulence factor that protects the adhesive SDR proteins against host proteolytic attack in order to promote successful tissue colonization, and has also been implicated in staphylococcal agglutination, which leads to sepsis and an immunodominant epitope for a strong antibody response. Despite the biological significance of these two glycosyltransferases involved in pathogenicity and avoidance of the host innate immune response, their structures and the molecular basis of their activity have not been investigated. This study reports the crystal structures of SdgB and SdgA from S. aureus as well as multiple structures of SdgB in complex with its substrates (for example UDP, N-acetylglucosamine or SDR peptides), products (glycosylated SDR peptides) or phosphate ions. Together with biophysical and biochemical analyses, this structural work uncovered the novel mechanism by which SdgB and SdgA carry out the glycosyl-transfer process to the long SDR region in SDR proteins. SdgB undergoes dynamic changes in its structure such as a transition from an open to a closed conformation upon ligand binding and takes diverse forms, both as a homodimer and as a heterodimer with SdgA. Overall, these findings not only elucidate the putative role of the three domains of SdgB in recognizing donor and acceptor substrates, but also provide new mechanistic insights into glycosylation of the SDR domain, which can serve as a starting point for the development of antibacterial drugs against staphylococcal infections.

Structural and mechanistic investigations of protein S-glycosyltransferases

Attachment of sugars to nitrogen and oxygen in peptides is ubiquitous in biology, but glycosylation of sulfur atoms has only been recently described. Here, we characterize two S-glycosyltransferases SunS and ThuS that selectively glycosylate one of five Cys residues in their substrate peptides; substitution of this Cys with Ser results in a strong decrease in glycosylation activity. Crystal structures of SunS and ThuS in complex with UDP-glucose or a derivative reveal an unusual architecture in which a glycosyltransferase type A (GTA) fold is decorated with additional domains to support homodimerization. Dimer formation creates an extended cavity for the substrate peptide, drawing functional analogy with O-glycosyltransferases involved in cell wall biosynthesis. This extended cavity contains a sharp bend that may explain the site selectivity of the glycosylation because the target Cys is in a Gly-rich stretch that can accommodate the bend. These studies establish a molecular framework for understanding the unusual S-glycosyltransferases.

Antibacterial Potential of Biosynthesized Zinc Oxide Nanoparticles against Poultry-Associated Foodborne Pathogens: An In Vitro Study

Simple Summary

The overuse of antibiotics in the poultry industry has led to the emergence of multidrug-resistant microorganisms. Thus, there is a need to find an alternative to conventional antibiotics. Recently, zinc oxide nanoparticles (ZnO NPs) have gained much attention due to their excellent antibacterial activity. In addition, ZnO NPs is an essential trace mineral in poultry diets. In this sense, incorporating ZnO NPs into poultry can promote growth and performance while serving as an alternative antibacterial agent to control diseases. Therefore, this study aimed to assess the in vitro antibacterial activity and antibacterial mechanisms of ZnO NPs against poultry-associated foodborne pathogens (Salmonella spp., Escherichia coli, and Staphylococcus aureus). The obtained findings demonstrated effective antibacterial actions against the tested microorganisms. The nanotechnology approach could represent a new tool for combating pathogens in the poultry industry.

Abstract

Since the emergence of multidrug-resistant bacteria in the poultry industry is currently a serious threat, there is an urgent need to develop a more efficient and alternative antibacterial substance. Zinc oxide nanoparticles (ZnO NPs) have exhibited antibacterial efficacy against a wide range of microorganisms. Although the in vitro antibacterial activity of ZnO NPs has been studied, little is known about the antibacterial mechanisms of ZnO NPs against poultry-associated foodborne pathogens. In the present study, ZnO NPs were successfully synthesized using Lactobacillus plantarum TA4, characterized, and their antibacterial potential against common avian pathogens (Salmonella spp., Escherichia coli, and Staphylococcus aureus) was investigated. Confirmation of ZnO NPs by UV-Visual spectroscopy showed an absorption band center at 360 nm. Morphologically, the synthesized ZnO NPs were oval with an average particle size of 29.7 nm. Based on the dissolution study of Zn²⁺, ZnO NPs released more ions than their bulk counterparts. Results from the agar well diffusion assay indicated that ZnO NPs effectively inhibited the growth of the three poultry-associated foodborne pathogens. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were assessed using various concentrations of ZnO NPs, which resulted in excellent antibacterial activity as compared to their bulkier counterparts. S. aureus was more susceptible to ZnO NPs compared to the other tested bacteria. Furthermore, the ZnO NPs demonstrated substantial biofilm inhibition and eradication. The formation of reactive oxygen species (ROS) and cellular material leakage was quantified to determine the underlying antibacterial mechanisms, whereas a scanning electron microscope (SEM) was used to examine the morphological changes of tested bacteria treated with ZnO NPs. The findings suggested that ROS-induced oxidative stress caused membrane damage and bacterial cell death. Overall, the results demonstrated that ZnO NPs could be developed as an alternative antibiotic in poultry production and revealed new possibilities in combating pathogenic microorganisms.

A Glycolipid Glycosyltransferase with Broad Substrate Specificity from the Marine Bacterium "Candidatus Pelagibacter sp." Strain HTCC7211

In the marine environment, phosphorus availability significantly affects the lipid composition in many cosmopolitan marine heterotrophic bacteria, including members of the SAR11 clade and the Roseobacter clade. Under phosphorus stress conditions, nonphosphorus sugar-containing glycoglycerolipids are substitutes for phospholipids in these bacteria. Although these glycoglycerolipids play an important role as surrogates for phospholipids under phosphate deprivation, glycoglycerolipid synthases in marine microbes are poorly studied. In the present study, we biochemically characterized a glycolipid glycosyltransferase (GT_cp) from the marine bacterium “Candidatus Pelagibacter sp.” strain HTCC7211, a member of the SAR11 clade. Our results showed that GT_cp is able to act as a multifunctional enzyme by synthesizing different glycoglycerolipids with UDP-glucose, UDP-galactose, or UDP-glucuronic acid as sugar donors and diacylglycerol (DAG) as the acceptor. Analyses of enzyme kinetic parameters demonstrated that Mg²⁺ notably changes the enzyme’s affinity for UDP-glucose, which improves its catalytic efficiency. Homology modeling and mutational analyses revealed binding sites for the sugar donor and the diacylglycerol lipid acceptor, which provided insights into the retaining mechanism of GT_cp with its GT-B fold. A phylogenetic analysis showed that GT_cp and its homologs form a group in the GT4 glycosyltransferase family. These results not only provide new insights into the glycoglycerolipid synthesis mechanism in lipid remodeling but also describe an efficient enzymatic tool for the future synthesis of bioactive molecules.

IMPORTANCE The bilayer formed by membrane lipids serves as the containment unit for living microbial cells. In the marine environment, it has been firmly established that phytoplankton and heterotrophic bacteria can replace phospholipids with nonphosphorus sugar-containing glycoglycerolipids in response to phosphorus limitation. However, little is known about how these glycoglycerolipids are synthesized. Here, we determined the biochemical characteristics of a glycolipid glycosyltransferase (GT_cp) from the marine bacterium “Candidatus Pelagibacter sp.” strain HTCC7211. GT_cp and its homologs form a group in the GT4 glycosyltransferase family and can synthesize neutral glycolipids (monoglucosyl-1,2-diacyl-sn-glycerol [MGlc-DAG] and monogalactosyl [MGal]-DAG) and monoglucuronic acid diacylglycerol (MGlcA-DAG). We also uncovered the key residues for DAG binding through molecular docking, site-direct mutagenesis, and subsequent enzyme activity assays. Our data provide new insights into the glycoglycerolipid synthesis mechanism in lipid remodeling.

(Automated) Synthesis of Well-defined Staphylococcus Aureus Wall Teichoic Acid Fragments

Wall teichoic acids (WTAs) are important components of the cell wall of the opportunistic Gram‐positive bacterium Staphylococcus aureus. WTAs are composed of repeating ribitol phosphate (RboP) residues that are decorated with d‐alanine and N‐acetyl‐d‐glucosamine (GlcNAc) modifications, in a seemingly random manner. These WTA‐modifications play an important role in shaping the interactions of WTA with the host immune system. Due to the structural heterogeneity of WTAs, it is impossible to isolate pure and well‐defined WTA molecules from bacterial sources. Therefore, here synthetic chemistry to assemble a broad library of WTA‐fragments, incorporating all possible glycosylation modifications (α‐GlcNAc at the RboP C4; β‐GlcNAc at the RboP C4; β‐GlcNAc at the RboP C3) described for S. aureus WTAs, is reported. DNA‐type chemistry, employing ribitol phosphoramidite building blocks, protected with a dimethoxy trityl group, was used to efficiently generate a library of WTA‐hexamers. Automated solid phase syntheses were used to assemble a WTA‐dodecamer and glycosylated WTA‐hexamer. The synthetic fragments have been fully characterized and diagnostic signals were identified to discriminate the different glycosylation patterns. The different glycosylated WTA‐fragments were used to probe binding of monoclonal antibodies using WTA‐functionalized magnetic beads, revealing the binding specificity of these WTA‐specific antibodies and the importance of the specific location of the GlcNAc modifications on the WTA‐chains.

Resource sharing between central metabolism and cell envelope synthesis

Synthesis of the bacterial cell envelope requires a regulated partitioning of resources from central metabolism. Here, we consider the key metabolic junctions that provide the precursors needed to assemble the cell envelope. Peptidoglycan synthesis requires redirection of a glycolytic intermediate, fructose-6-phosphate, into aminosugar biosynthesis by the highly regulated branchpoint enzyme GlmS. MurA directs the downstream product, UDP-GlcNAc, specifically into peptidoglycan synthesis. Other shared resources required for cell envelope synthesis include the isoprenoid carrier lipid undecaprenyl phosphate and amino acids required for peptidoglycan cross-bridges. Assembly of the envelope requires a sharing of limited resources between competing cellular pathways and may additionally benefit from scavenging of metabolites released from neighboring cells or the formation of symbiotic relationships with a host.

β-Lactams against the Fortress of the Gram-Positive Staphylococcus aureus Bacterium

The biological diversity of the unicellular bacteria-whether assessed by shape, food, metabolism, or ecological niche-surely rivals (if not exceeds) that of the multicellular eukaryotes. The relationship between bacteria whose ecological niche is the eukaryote, and the eukaryote, is often symbiosis or stasis. Some bacteria, however, seek advantage in this relationship. One of the most successful-to the disadvantage of the eukaryote-is the small (less than 1 μm diameter) and nearly spherical Staphylococcus aureus bacterium. For decades, successful clinical control of its infection has been accomplished using β-lactam antibiotics such as the penicillins and the cephalosporins. Over these same decades S. aureus has perfected resistance mechanisms against these antibiotics, which are then countered by new generations of β-lactam structure. This review addresses the current breadth of biochemical and microbiological efforts to preserve the future of the β-lactam antibiotics through a better understanding of how S. aureus protects the enzyme targets of the β-lactams, the penicillin-binding proteins. The penicillin-binding proteins are essential enzyme catalysts for the biosynthesis of the cell wall, and understanding how this cell wall is integrated into the protective cell envelope of the bacterium may identify new antibacterials and new adjuvants that preserve the efficacy of the β-lactams.

Identification of the Key Residues of the Uridine Diphosphate Glycosyltransferase 91D2 and its Effect on the Accumulation of Steviol Glycosides in Stevia rebaudiana

Stevia (Stevia rebaudiana Bertoni) possesses substantial value for its unique sweet compounds-steviol glycosides (SGs). In the metabolic glycosylation grid of SGs, SrUGT91D2 has been shown to catalyze formation of 1,2-β-d-glucoside linkages at the C₁₃- and C₁₉-positions and play a crucial role in the synthesis of SGs, including the formation of stevioside (ST), rebaudioside E (RE), and rebaudioside D (RD). However, the key residues of the SrUGT91D2 enzyme and how SrUGT91D2 affects the accumulation of SGs in S. rebaudiana remain unclear. In the present study, cloning and functional analysis of full-length SrUGT91D2 gene sequences were performed in 10 different S. rebaudiana genotypes with divergent SG compositions. After sequence analysis, it was found that most of the sequences of this gene (more than 50%) in each genotype were consistent with the UGT91D2e_No.5 allele, which has been reported to exert catalytic activity on 1,2-β-d-glucoside. Moreover, six variants (UGT91D2e_No.5, SrUGT91D2-11-14, SrUGT91D2-110, SrUGT91D2-023, SrUGT91D2-N01, and SrUGT91D2-N04) of this gene were obtained, and their activities were identified. Although there were some differences among these variants, the only type of mutation was partial base substitution at a very low level. In addition, the expression analysis of SrUGT91D2 in each genotype showed that the expression level of the gene was significantly different among genotypes, and a significant positive correlation was found between the content of RD (which was closely influenced by SrUGT91D2) and the expression level of SrUGT91D2 in each genotype (correlation coefficient = 0.91). Thus, it was indicated that SrUGT91D2 was relatively conserved in S. rebaudiana, and the differential effect of SrUGT91D2 on the accumulation of related SGs mainly derived from its expression level. Furthermore, based on homologous modeling and molecular docking analysis, T84, T144, A194, S284, E285, V286, G365, E369, R404, and G409 were predicted to be key residues in the glucosylation of SGs by SrUGT91D2. After site-mutation and enzyme assays, it was confirmed that T84, T144, R404, A194, and G409 are the key residues in the SrUGT91D2 protein, especially T144 and G409. This work provided valuable information for understanding the structure-activity relationship of the SrUGT91D2 protein and the molecular mechanism of SG accumulation in stevia.

Cell wall glycosylation in Staphylococcus aureus: targeting the tar glycosyltransferases

Peptidoglycan (PG) is the major structural polymer of the bacterial cell wall. The PG layer of gram-positive bacterial pathogens such as Staphylococcus aureus (S. aureus) is permeated with anionic glycopolymers known as wall teichoic acids (WTAs) and lipoteichoic acids (LTAs). In S. aureus, the WTA backbone typically consists of repeating ribitol-5-phosphate units, which are modified by enzymes that introduce glycosylation as well as amino acids at different locations. These modifications are key determinants of phage adhesion, bacterial biofilm formation and virulence of S. aureus. In this review, we examine differences in WTA structures in gram-positive bacteria, focusing in particular on three enzymes, TarM, TarS, and TarP that glycosylate the WTA of S. aureus at different locations. Infections with S. aureus pose an increasing threat to human health, particularly through the emergence of multidrug-resistant strains. Recently obtained structural information on TarM, TarS and TarP has helped to better understand the strategies used by S. aureus to establish resistance and to evade host defense mechanisms. Moreover, structures of complexes with poly-RboP and its analogs can serve as a platform for the development of new inhibitors that could form a basis for the development of antibiotic agents.

New Textile for Personal Protective Equipment—Plasma Chitosan/Silver Nanoparticles Nylon Fabric

Fabric structures are prone to contamination with microorganisms, as their morphology and ability to retain moisture creates a proper environment for their growth. In this work, a novel, easily processed and cheap coating for a nylon fabric with antimicrobial characteristics was developed. After plasma treatment, made to render the fabric surface more reactive sites, the fabric was impregnated with chitosan and silver nanoparticles by simply dipping it into a mixture of different concentrations of both components. Silver nanoparticles were previously synthesized using the Lee–Meisel method, and their successful obtention was proven by UV–Vis, showing the presence of the surface plasmon resonance band at 410 nm. Nanoparticles with 25 nm average diameter observed by STEM were stable, mainly in the presence of chitosan, which acted as a surfactant for silver nanoparticles, avoiding their aggregation. The impregnated fabric possessed bactericidal activity higher for Gram-positive Staphylococcus aureus than for Gram-negative Pseudomonas aeruginosa bacteria for all combinations. The percentage of live S. aureus and P. aeruginosa CFU was reduced to less than 20% and 60%, respectively, when exposed to each of the coating combinations. The effect was more pronounced when both chitosan and silver were present in the coating, suggesting an effective synergy between these components. After a washing process, the antimicrobial effect was highly reduced, suggesting that the coating is unstable after washing, being almost completely removed from the fabric. Nevertheless, the new-coated fabric can be successfully used in single-use face masks. To our knowledge, the coating of nylon fabrics intended for face-mask material with both agents has never been reported.

Wall teichoic acids: physiology and applications

Wall teichoic acids (WTAs) are charged glycopolymers containing phosphodiester-linked polyol units and represent one of the major components of Gram-positive cell envelope. WTAs have important physiological functions in cell division, gene transfer, surface adhesion, drug resistance and biofilm formation, and are critical virulence factors and vital determinants in mediating cell interaction with and tolerance to environmental factors. Here, we first briefly introduce WTA structure, biosynthesis and its regulation, and then summarize in detail four major physiological roles played by WTAs, i.e. WTA-mediated resistance to antimicrobials, virulence to mammalian cells, interaction with bacteriolytic enzymes and regulation of cell metabolism. We also review the applications of WTAs in these fields that are closely related to the human society, including antibacterial drug discovery targeting WTA biosynthesis, development of vaccines and antibodies regarding WTA-mediated pathogenicity, specific and sensitive detection of pathogens in food using WTAs as a surface epitope and regulation of WTA-related pathways for efficient microbial production of useful compounds. We also point out major problems remaining in these fields, and discuss some possible directions in the future exploration of WTA physiology and applications.

Biosynthesis and Export of Bacterial Glycolipids

Complex carbohydrates are essential for many biological processes, from protein quality control to cell recognition, energy storage, and cell wall formation. Many of these processes are performed in topologically extracellular compartments or on the cell surface; hence, diverse secretion systems evolved to transport the hydrophilic molecules to their sites of action. Polyprenyl lipids serve as ubiquitous anchors and facilitators of these transport processes. Here, we summarize and compare bacterial biosynthesis pathways relying on the recognition and transport of lipid-linked complex carbohydrates. In particular, we compare transporters implicated in O antigen and capsular polysaccharide biosyntheses with those facilitating teichoic acid and N-linked glycan transport. Further, we discuss recent insights into the generation, recognition, and recycling of polyprenyl lipids.

Immunization with a Bacterial Lipoprotein Establishes an Immuno-Protective Response with Upregulation of Effector CD4+ T Cells and Neutrophils Against Methicillin-Resistant Staphylococcus aureus Infection

Staphylococcus aureus (S. aureus) is a commensal bacterium in the human body; however, the bacterium frequently generates serious inflammation and infectious diseases. Some strains of S. aureus, such as methicillin-resistant Staphylococcus aureus (MRSA), are still a serious problem in public health facilities. Thus, an effective protection strategy is eagerly expected for the prevention and cure of MRSA infection. Here, we report that a specific fraction of an S. aureus lipoprotein (SA-LP) established a protective response against MRSA infection. The fractionated S. aureus lipoprotein SA-LP-F2, which is contained in 30–50 kDa of crude S. aureus lipoprotein (SA-LP-C), effectively activated dendritic cells (DCs) and the SA-LP-F2-pulsed DCs generated IFN-γ+CD4+ T (Th1) and IL-17A+CD4+ T (Th17) cells by in vitro antigen presentation. The SA-LP-F2 immunization upregulated the Th1 and Th17 populations so that MRSA colonization on the skin was suppressed during the challenge phase with MRSA. By following the effector T cell upregulation, the neutrophil function, which was a substantial effector cell against MRSA, was also enhanced in the SA-LP-F2-immunized mice. Finally, we found that the protective effect of SA-LP-F2 immunization was maintained for at least 90 days because the immunized mice continued to show a protective response during the MRSA challenge period. In the MRSA challenge, reactivated Th1 and Th17 populations were maintained in the SA-LP-F2-immunized mice as compared to naive mice. In addition, the neutrophil population was also upregulated in the mice. The memory CD4+ T cell (central memory T; T_CM and effector memory T; T_EM) population was established by SA-LP-F2 immunization and was maintained at higher levels than usual. Taken together, our findings may provide a breakthrough in the establishment of an immunization strategy against MRSA infection.

Processivity in Bacterial Glycosyltransferases

Extracellular polysaccharides and glycoproteins of pathogenic bacteria assist in adherence, autoaggregation, biofilm formation, and host immune system evasion. As a result, considerable research in the field of glycobiology is dedicated to study the composition and function of glycans associated with virulence, as well as the enzymes involved in their biosynthesis with the aim to identify novel antibiotic targets. Especially, insights into the enzyme mechanism, substrate binding, and transition-state structures are valuable as a starting point for rational inhibitor design. An intriguing aspect of enzymes that generate or process polysaccharides and glycoproteins is the level of processivity. The existence of enzymatic processivity reflects the need for regulation of the final glycan/glycoprotein length and structure, depending on the role they perform. In this Review, we describe the currently reported examples of various processive enzymes involved in polymerization and transfer of sugar moieties, predominantly in bacterial pathogens, with a focus on the biochemical methods, to showcase the importance of studying processivity for understanding the mechanism.

Determinants of Phage Host Range in Staphylococcus Species

Bacteria in the genus Staphylococcus are important targets for phage therapy due to their prevalence as pathogens and increasing antibiotic resistance. Here we review Staphylococcus outer surface features and specific phage resistance mechanisms that define the host range, the set of strains that an individual phage can potentially infect. Phage infection goes through five distinct phases: attachment, uptake, biosynthesis, assembly, and lysis. Adsorption inhibition, encompassing outer surface teichoic acid receptor alteration, elimination, or occlusion, limits successful phage attachment and entry. Restriction-modification systems (in particular, type I and IV systems), which target phage DNA inside the cell, serve as the major barriers to biosynthesis as well as transduction and horizontal gene transfer between clonal complexes and species. Resistance to late stages of infection occurs through mechanisms such as assembly interference, in which staphylococcal pathogenicity islands siphon away superinfecting phage proteins to package their own DNA. While genes responsible for teichoic acid biosynthesis, capsule, and restriction-modification are found in most Staphylococcus strains, a variety of other host range determinants (e.g., clustered regularly interspaced short palindromic repeats, abortive infection, and superinfection immunity) are sporadic. The fitness costs of phage resistance through teichoic acid structure alteration could make staphylococcal phage therapies promising, but host range prediction is complex because of the large number of genes involved, and the roles of many of these are unknown. In addition, little is known about the genetic determinants that contribute to host range expansion in the phages themselves. Future research must identify host range determinants, characterize resistance development during infection and treatment, and examine population-wide genetic background effects on resistance selection.

Structure and mechanism of TagA, a novel membrane-associated glycosyltransferase that produces wall teichoic acids in pathogenic bacteria

Staphylococcus aureus and other bacterial pathogens affix wall teichoic acids (WTAs) to their surface. These highly abundant anionic glycopolymers have critical functions in bacterial physiology and their susceptibility to β-lactam antibiotics. The membrane-associated TagA glycosyltransferase (GT) catalyzes the first-committed step in WTA biosynthesis and is a founding member of the WecB/TagA/CpsF GT family, more than 6,000 enzymes that synthesize a range of extracellular polysaccharides through a poorly understood mechanism. Crystal structures of TagA from T. italicus in its apo- and UDP-bound states reveal a novel GT fold, and coupled with biochemical and cellular data define the mechanism of catalysis. We propose that enzyme activity is regulated by interactions with the bilayer, which trigger a structural change that facilitates proper active site formation and recognition of the enzyme's lipid-linked substrate. These findings inform upon the molecular basis of WecB/TagA/CpsF activity and could guide the development of new anti-microbial drugs.

Glycosylation of Staphylococcus aureus cell wall teichoic acid is influenced by environmental conditions

Wall teichoic acid (WTA) are major constituents of Staphylococcus aureus (S. aureus) cell envelopes with important roles in the bacteria's physiology, resistance to antimicrobial molecules, host interaction, virulence and biofilm formation. They consist of ribitol phosphate repeat units in which the ribitol residue is substituted with D-alanine (D-Ala) and N-acetyl-D-glucosamine (GlcNAc). The complete S. aureus WTA biosynthesis pathways was recently revealed with the identification of the two glycosyltransferases, TarM and TarS, respectively responsible for the α- and β-GlcNAc anomeric substitutions. We performed structural analyses to characterize WTAs from a panel of 24 S. aureus strains responsible for invasive infections. A majority of the S. aureus strains produced the β-GlcNAc WTA form in accordance with the presence of the tarS gene in all strains assessed. The β-GlcNAc anomer was preferentially expressed at the expense of the α-GlcNAc anomer when grown on stress-inducing culture medium containing high NaCl concentration. Furthermore, WTA glycosylation of the prototype S. aureus Newman strain was characterized in vivo in two different animal models, namely peritonitis and deep wound infection. While the inoculum used to infect animals produced almost exclusively α-GlcNAc WTA, a complete switch to β-glycosylation was observed in infected kidneys, livers and muscles. Overall, our data demonstrate that S. aureus WTA glycosylation is strongly influenced by environmental conditions and suggest that β-GlcNAc WTA may bring competitive advantage in vivo.

Staphylococcus aureus Lipoprotein Induces Skin Inflammation, Accompanied with IFN-γ-Producing T Cell Accumulation through Dermal Dendritic Cells

Staphylococcus aureus (S. aureus) is a commensal bacteria on the human skin, which causes serious skin inflammation. Several immune cells, especially effector T cells (Teff), have been identified as key players in S. aureus-derived skin inflammation. However, the bacterial component that induces dramatic host immune responses on the skin has not been well characterized. Here, we report that S. aureus lipoprotein (SA-LP) was recognized by the host immune system as a strong antigen, so this response induced severe skin inflammation. SA-LP activated dendritic cells (DCs), and this activation led to Teff accumulation on the inflamed skin in the murine intradermal (ID) injection model. The skin-accumulated Teff pool was established by IFN-ɤ-producing CD4⁺ and CD8⁺T (Th1 and Tc1). SA-LP activated dermal DC (DDC) in a dominant manner, so that these DCs were presumed to possess the strong responsibility of SA-LP-specific Teff generation in the skin-draining lymph nodes (dLN). SA-LP activated DC transfer into the mice ear, which showed similar inflammation, accompanied with Th1 and Tc1 accumulation on the skin. Thus, we revealed that SA-LP has a strong potential ability to establish skin inflammation through the DC-Teff axis. This finding provides novel insights not only for therapy, but also for the prevention of S. aureus-derived skin inflammation.

Unprotonated Short-Chain Alkylamines Inhibit Staphylolytic Activity of Lysostaphin in a Wall Teichoic Acid-Dependent Manner

Lysostaphin (Lst) is a potent bacteriolytic enzyme that kills Staphylococcus aureus, a common bacterial pathogen of humans and animals. With high activity against both planktonic cells and biofilms, Lst has the potential to be used in industrial products, such as commercial cleansers, for decontamination. However, Lst is inhibited in the presence of monoethanolamine (MEA), a chemical widely used in cleaning solutions and pharmaceuticals, and the underlying mechanism of inhibition remains unknown. In this study, we examined the cell binding and killing capabilities of Lst against S. aureus ATCC 6538 in buffered salt solution with MEA at different pH values (7.5 to 10.5) and discovered that only the unprotonated form of MEA inhibited Lst binding to the cell surface, leading to low Lst activity, despite retention of its secondary structure. This reduced enzyme activity could be largely recovered via a reduction in wall teichoic acid (WTA) biosynthesis through tunicamycin treatment, indicating that the suppression of Lst activity was dependent on the presence and amount of WTA. We propose that the decreased cell binding and killing capabilities of Lst are associated with the influence of uncharged MEA on the conformation of WTA. A similar effect was confirmed with other short-chain alkylamines. This study offers new insight into the impact of short-chain alkylamines on both Lst and WTA structure and function and provides guidance for the application of Lst in harsh environments.IMPORTANCE Lysostaphin (Lst) effectively and selectively kills Staphylococcus aureus, the bacterial culprit of many hospital- and community-acquired skin and respiratory infections and food poisoning. Lst has been investigated in animal models and clinical trials, industrial formulations, and environmental settings. Here, we studied the mechanistic basis of the inhibitory effect of alkylamines, such as monoethanolamine (MEA), a widely used chemical in commercial detergents, on Lst activity, for the potential incorporation of Lst in disinfectant solutions. We have found that protonated MEA has little influence on Lst activity, while unprotonated MEA prevents Lst from binding to S. aureus cells and hence dramatically decreases the enzyme's bacteriolytic efficacy. Following partial removal of the wall teichoic acid, an important component of the bacterial cell envelope, the inhibitory effect of unprotonated MEA on Lst is reduced. This phenomenon can be extended to other short-chain alkylamines. This mechanistic report of the impact of alkylamines on Lst functionality will help guide future applications of Lst in disinfection and decontamination of health-related commercial products.

Enzyme structures of the bacterial peptidoglycan and wall teichoic acid biogenesis pathways

The bacterial cell wall is a complex polymeric structure with essential roles in defence, survival and pathogenesis. Common to both Gram-positive and Gram-negative bacteria is the mesh-like peptidoglycan sacculus that surrounds the outer leaflet of the cytoplasmic membrane. Recent crystallographic studies of enzymes that comprise the peptidoglycan biosynthetic pathway have led to significant new understanding of all stages. These include initial multi-step cytosolic formation of sugar-pentapeptide precursors, transfer of the precursors to activated polyprenyl lipids at the membrane inner leaflet and flippase mediated relocalization of the resulting lipid II precursors to the outer leaflet where glycopolymerization and subsequent peptide crosslinking are finalized. Additional, species-specific enzymes allow customized peptidoglycan modifications and biosynthetic regulation that are important to bacterial virulence and survival. These studies have reinforced the unique and specific catalytic mechanisms at play in cell wall biogenesis and expanded the atomic foundation to develop novel, structure guided, antibacterial agents.

A New Family of Capsule Polymerases Generates Teichoic Acid-Like Capsule Polymers in Gram-Negative Pathogens

Group 2 capsule polymers represent crucial virulence factors of Gram-negative pathogenic bacteria. They are synthesized by enzymes called capsule polymerases. In this report, we describe a new family of polymerases that combine glycosyltransferase and hexose- and polyol-phosphate transferase activity to generate complex poly(oligosaccharide phosphate) and poly(glycosylpolyol phosphate) polymers, the latter of which display similarity to wall teichoic acid (WTA), a cell wall component of Gram-positive bacteria. Using modeling and multiple-sequence alignment, we showed homology between the predicted polymerase domains and WTA type I biosynthesis enzymes, creating a link between Gram-negative and Gram-positive cell wall biosynthesis processes. The polymerases of the new family are highly abundant and found in a variety of capsule-expressing pathogens such as Neisseria meningitidis, Actinobacillus pleuropneumoniae, Haemophilus influenzae, Bibersteinia trehalosi, and Escherichia coli with both human and animal hosts. Five representative candidates were purified, their activities were confirmed using nuclear magnetic resonance (NMR) spectroscopy, and their predicted folds were validated by site-directed mutagenesis.IMPORTANCE Bacterial capsules play an important role in the interaction between a pathogen and the immune system of its host. During the last decade, capsule polymerases have become attractive tools for the production of capsule polymers applied as antigens in glycoconjugate vaccine formulations. Conventional production of glycoconjugate vaccines requires the cultivation of the pathogen and thus the highest biosafety standards, leading to tremendous costs. With regard to animal husbandry, where vaccines could avoid the extensive use of antibiotics, conventional production is not sufficiently cost-effective. In contrast, enzymatic synthesis of capsule polymers is pathogen-free and fast, offers high stereo- and regioselectivity, and works with high efficacy. The new capsule polymerase family described here vastly increases the toolbox of enzymes available for biotechnology purposes. Representatives are abundantly found in human pathogens but also in animal pathogens, paving the way for the exploitation of polymerases for the development of a new generation of vaccines for animal husbandry.

Salt-Induced Stress Stimulates a Lipoteichoic Acid-Specific Three-Component Glycosylation System in Staphylococcus aureus

Lipoteichoic acid (LTA) in Staphylococcus aureus is a poly-glycerophosphate polymer anchored to the outer surface of the cell membrane. LTA has numerous roles in cell envelope physiology, including regulating cell autolysis, coordinating cell division, and adapting to environmental growth conditions. LTA is often further modified with substituents, including d-alanine and glycosyl groups, to alter cellular function. While the genetic determinants of d-alanylation have been largely defined, the route of LTA glycosylation and its role in cell envelope physiology have remained unknown, in part due to the low levels of basal LTA glycosylation in S. aureus We demonstrate here that S. aureus utilizes a membrane-associated three-component glycosylation system composed of an undecaprenol (Und) N-acetylglucosamine (GlcNAc) charging enzyme (CsbB; SAOUHSC_00713), a putative flippase to transport loaded substrate to the outside surface of the cell (GtcA; SAOUHSC_02722), and finally an LTA-specific glycosyltransferase that adds α-GlcNAc moieties to LTA (YfhO; SAOUHSC_01213). We demonstrate that this system is specific for LTA with no cross recognition of the structurally similar polyribitol phosphate containing wall teichoic acids. We show that while wild-type S. aureus LTA has only a trace of GlcNAcylated LTA under normal growth conditions, amounts are raised upon either overexpressing CsbB, reducing endogenous d-alanylation activity, expressing the cell envelope stress responsive alternative sigma factor SigB, or by exposure to environmental stress-inducing culture conditions, including growth media containing high levels of sodium chloride.IMPORTANCE The role of glycosylation in the structure and function of Staphylococcus aureus lipoteichoic acid (LTA) is largely unknown. By defining key components of the LTA three-component glycosylation pathway and uncovering stress-induced regulation by the alternative sigma factor SigB, the role of N-acetylglucosamine tailoring during adaptation to environmental stresses can now be elucidated. As the dlt and glycosylation pathways compete for the same sites on LTA and induction of glycosylation results in decreased d-alanylation, the interplay between the two modification systems holds implications for resistance to antibiotics and antimicrobial peptides.

Discovery of genes required for lipoteichoic acid glycosylation predicts two distinct mechanisms for wall teichoic acid glycosylation

The bacterial cell wall is an important and highly complex structure that is essential for bacterial growth because it protects bacteria from cell lysis and environmental insults. A typical Gram-positive bacterial cell wall is composed of peptidoglycan and the secondary cell wall polymers, wall teichoic acid (WTA) and lipoteichoic acid (LTA). In many Gram-positive bacteria, LTA is a polyglycerol-phosphate chain that is decorated with d-alanine and sugar residues. However, the function of and proteins responsible for the glycosylation of LTA are either unknown or not well-characterized. Here, using bioinformatics, genetic, and NMR spectroscopy approaches, we found that the Bacillus subtilis csbB and yfhO genes are essential for LTA glycosylation. Interestingly, the Listeria monocytogenes gene lmo1079, which encodes a YfhO homolog, was not required for LTA glycosylation, but instead was essential for WTA glycosylation. LTA is polymerized on the outside of the cell and hence can only be glycosylated extracellularly. Based on the similarity of the genes coding for YfhO homologs that are required in B. subtilis for LTA glycosylation or in L. monocytogenes for WTA glycosylation, we hypothesize that WTA glycosylation might also occur extracellularly in Listeria species. Finally, we discovered that in L. monocytogenes, lmo0626 (gtlB) was required for LTA glycosylation, indicating that the encoded protein has a function similar to that of YfhO, although the proteins are not homologous. Together, our results enable us to propose an updated model for LTA glycosylation and also indicate that glycosylation of WTA might occur through two different mechanisms in Gram-positive bacteria.

Crystal structure of lipid A disaccharide synthase LpxB from Escherichia coli

Most Gram-negative bacteria are surrounded by a glycolipid called lipopolysaccharide (LPS), which forms a barrier to hydrophobic toxins and, in pathogenic bacteria, is a virulence factor. During LPS biosynthesis, a membrane-associated glycosyltransferase (LpxB) forms a tetra-acylated disaccharide that is further acylated to form the membrane anchor moiety of LPS. Here we solve the structure of a soluble and catalytically competent LpxB by X-ray crystallography. The structure reveals that LpxB has a glycosyltransferase-B family fold but with a highly intertwined, C-terminally swapped dimer comprising four domains. We identify key catalytic residues with a product, UDP, bound in the active site, as well as clusters of hydrophobic residues that likely mediate productive membrane association or capture of lipidic substrates. These studies provide the basis for rational design of antibiotics targeting a crucial step in LPS biosynthesis.

Identification and biochemical characterization of WbwB, a novel UDP-Gal: Neu5Ac-R α1,4-galactosyltransferase from the intestinal pathogen Escherichia coli serotype O104

The intestinal pathogen Escherichia coli serotype O104:H4 (ECO104) can cause bloody diarrhea and haemolytic uremic syndrome. The ECO104 O antigen has the unique repeating unit structure [4Galα1-4Neu5,7,9Ac₃α2-3Galβ1-3GalNAcβ1-], which includes the mammalian sialyl-T antigen as an internal structure. Previously, we identified WbwC from ECO104 as the β3Gal-transferase that synthesizes the T antigen, and showed that α3-sialyl-transferase WbwA transfers sialic acid to the T antigen. Here we identify the wbwB gene product as a unique α1,4-Gal-transferase WbwB that transfers Gal from UDP-Gal to the terminal sialic acid residue of Neu5Acα2-3Galβ1-3GalNAcα-diphosphate-lipid acceptor. NMR analysis of the WbwB enzyme reaction product indicated that Galα1-4Neu5Acα2-3Galβ1-3GalNAcα-diphosphate-lipid was synthesized. WbwB from ECO104 has a unique acceptor specificity for terminal sialic acid as well as the diphosphate group in the acceptor. The characterization studies showed that WbwB does not require divalent metal ion as a cofactor. Mutagenesis identified Lys243 within an RKR motif and both Glu315 and Glu323 of the fourth EX₇E motif as essential for the activity. WbwB is the final glycosyltransferase in the biosynthesis pathway of the ECO104 antigen repeating unit. This work contributes to knowledge of the biosynthesis of bacterial virulence factors.

Structural and functional diversity in Listeria cell wall teichoic acids

Wall teichoic acids (WTAs) are the most abundant glycopolymers found on the cell wall of many Gram-positive bacteria, whose diverse surface structures play key roles in multiple biological processes. Despite recent technological advances in glycan analysis, structural elucidation of WTAs remains challenging due to their complex nature. Here, we employed a combination of ultra-performance liquid chromatography-coupled electrospray ionization tandem-MS/MS and NMR to determine the structural complexity of WTAs from Listeria species. We unveiled more than 10 different types of WTA polymers that vary in their linkage and repeating units. Disparity in GlcNAc to ribitol connectivity, as well as variable O-acetylation and glycosylation of GlcNAc contribute to the structural diversity of WTAs. Notably, SPR analysis indicated that constitution of WTA determines the recognition by bacteriophage endolysins. Collectively, these findings provide detailed insight into Listeria cell wall-associated carbohydrates, and will guide further studies on the structure-function relationship of WTAs.

Extended Physicochemical Characterization of the Synthetic Anticoagulant Pentasaccharide Fondaparinux Sodium by Quantitative NMR and Single Crystal X-ray Analysis

Fondaparinux sodium is a synthetic pentasaccharide representing the high affinity antithrombin III binding site in heparin. It is the active pharmaceutical ingredient of the anticoagulant drug Arixtra^®. The single crystal X-ray structure of Fondaparinux sodium is reported, unequivocally confirming both structure and absolute configuration. The iduronic acid adopts a somewhat distorted chair conformation. Due to the presence of many sulfur atoms in the highly sulfated pentasaccharide, anomalous dispersion could be applied to determine the absolute configuration. A comparison with the conformation of Fondaparinux in solution, as well as complexed with proteins is presented. The content of the solution reference standard was determined by quantitative NMR using an internal standard both in 1999 and in 2016. A comparison of the results allows the conclusion that this method shows remarkable precision over time, instrumentation and analysts.

The 2.8 Å Electron Microscopy Structure of Adeno-Associated Virus-DJ Bound by a Heparinoid Pentasaccharide

Atomic structures of adeno-associated virus (AAV)-DJ, alone and in complex with fondaparinux, have been determined by cryoelectron microscopy at 3 Å resolution. The gene therapy vector, AAV-DJ, is a hybrid of natural serotypes that was previously derived by directed evolution, selecting for hepatocyte entry and resistance to neutralization by human serum. The structure of AAV-DJ differs from that of parental serotypes in two regions where neutralizing antibodies bind, so immune escape appears to have been the primary driver of AAV-DJ's directed evolution. Fondaparinux is an analog of cell surface heparan sulfate to which several AAVs bind during entry. Fondaparinux interacts with viral arginines at a known heparin binding site, without the large conformational changes whose presence was controversial in low-resolution imaging of AAV2-heparin complexes. The glycan density suggests multi-modal binding that could accommodate sequence variation and multivalent binding along a glycan polymer, consistent with a role in attachment, prior to more specific interactions with a receptor protein mediating entry.

Identification and biochemical characterization of a novel α-1,3-mannosyltransferase WfcD from Escherichia coli O141

Glycosyltransferases (GTs) catalyze the formation of regio- and stereospecific glycosidic linkages between specific sugar donors and recipients. In this study, the function of the wfcD gene from the Escherichia coli O141 O-antigen gene cluster encoding an α-1,3-mannosyltransferase that catalyzed the formation of the linkage Man(α1-3)-GlcNAc was biochemically characterized. WfcD was expressed in E. coli BL21 (DE3), and the enzymatic product was identified by liquid chromatography-mass spectrometry (LC-MS), collision-induced dissociation electrospray ionization ion trap multiple tandem MS (CID-ESI-IT-MSⁿ) and glycosidase digestion using the donor substrate GDP-Man and the synthetic acceptor substrate decyl diphosphate 2-acetamido-2-deoxy-α-D-glucopyranose (GlcNAc-PP-De). The kinetic and physiochemical properties and the substrate specificity of WfcD were investigated. WfcD is the first characterized bacterial mannosyltransferase that acts on the Man(α1-3)-GlcNAc linkage. This study enhances our knowledge of the diverse functions of GTs.

Structure and Mechanism of Staphylococcus aureus TarS, the Wall Teichoic Acid β-glycosyltransferase Involved in Methicillin Resistance

In recent years, there has been a growing interest in teichoic acids as targets for antibiotic drug design against major clinical pathogens such as Staphylococcus aureus, reflecting the disquieting increase in antibiotic resistance and the historical success of bacterial cell wall components as drug targets. It is now becoming clear that β-O-GlcNAcylation of S. aureus wall teichoic acids plays a major role in both pathogenicity and antibiotic resistance. Here we present the first structure of S. aureus TarS, the enzyme responsible for polyribitol phosphate β-O-GlcNAcylation. Using a divide and conquer strategy, we obtained crystal structures of various TarS constructs, mapping high resolution overlapping N-terminal and C-terminal structures onto a lower resolution full-length structure that resulted in a high resolution view of the entire enzyme. Using the N-terminal structure that encapsulates the catalytic domain, we furthermore captured several snapshots of TarS, including the native structure, the UDP-GlcNAc donor complex, and the UDP product complex. These structures along with structure-guided mutants allowed us to elucidate various catalytic features and identify key active site residues and catalytic loop rearrangements that provide a valuable platform for anti-MRSA drug design. We furthermore observed for the first time the presence of a trimerization domain composed of stacked carbohydrate binding modules, commonly observed in starch active enzymes, but adapted here for a poly sugar-phosphate glycosyltransferase.

Historically, β-lactam class antibiotics such as methicillin have been very successful in the treatment of bacterial infections, effectively destroying bacteria by rupturing their cell walls while posing little harm to the human organism. In recent years, however, the alarming emergence of Methicillin Resistant S. aureus or MRSA has resulted in a world-wide health crisis, calling on new strategies to combat pathogenesis and antibiotic resistance. As such, understanding the pathways and players that orchestrate resistance is important for overcoming these mechanisms and restoring our powerful β-lactam antibiotic arsenal. In this article we describe the crystal structure of TarS, an enzyme responsible for the glycosylation of wall teichoic acid polymers of the S. aureus cell wall, a process that has been shown to be specifically responsible for methicillin resistance in MRSA. TarS is therefore a promising drug target whose inhibition in combinational therapies would result in MRSA re-sensitization to β-lactam antibiotics. Here we present the first structure of TarS together with several snap-shots of its substrate/product complexes, and elucidate important catalytic features that are valuable for rational drug design efforts to combat resistance in MRSA.

Mechanism of a cytosolic O-glycosyltransferase essential for the synthesis of a bacterial adhesion protein

O-glycosylation of Ser and Thr residues is an important process in all organisms, which is only poorly understood. Such modification is required for the export and function of adhesin proteins that mediate the attachment of pathogenic Gram-positive bacteria to host cells. Here, we have analyzed the mechanism by which the cytosolic O-glycosyltransferase GtfA/B of Streptococcus gordonii modifies the Ser/Thr-rich repeats of adhesin. The enzyme is a tetramer containing two molecules each of GtfA and GtfB. The two subunits have the same fold, but only GtfA contains an active site, whereas GtfB provides the primary binding site for adhesin. During a first phase of glycosylation, the conformation of GtfB is restrained by GtfA to bind substrate with unmodified Ser/Thr residues. In a slow second phase, GtfB recognizes residues that are already modified with N-acetylglucosamine, likely by converting into a relaxed conformation in which one interface with GtfA is broken. These results explain how the glycosyltransferase modifies a progressively changing substrate molecule.

Potentiating activity of luteolin on membrane permeabilizing agent and ATPase inhibitor against methicillin-resistant Staphylococcus aureus

OBJECTIVE

To investigate the mechanism of antibacterial activity of luteolin (LUT) against methicillin-resistant Staphylococcus aureus (MRSA).

METHODS

The mechanism of anti-MRSA activity of LUT was analyzed by the viability assay in membrane permeabilizing agent, ATPase inhibitors, and peptidoglycan (PGN) derived from Staphylococcus aureus (S. aureus). Also, transmission electron microscopy was used to monitor survival characteristics and changes in S. aureus morphology.

RESULTS

Compared to the LUT alone, the optical density of suspensions treated with the combination of 125 μg/mL Tris and 250 μg/mL N,N'-dicyclohexylcarbodiimide were reduced to 60% and 46% of the control, respectively. PGN (15.6 μg/mL) gradually impeded the activity of LUT, and PGN (62.5 μg/mL) completely blocked the activity of LUT on S. aureus.

CONCLUSIONS

Increased susceptibility to LUT with the Tris-dicyclohexylcarbodiimide combinations is evident in all tested MRSA isolates. The results indicate LUT synergy in increasing cytoplasmic membrane permeability and inhibiting ATPase. S. aureus PGN directly blocks the antibacterial activity of LUT, suggesting the direct binding of LUT with PGN. These findings may be validated for the development of antibacterial agent for low MRSA resistance.