Wall teichoic acid function, biosynthesis, and inhibition

Antiadhesion agents against Gram-positive pathogens

ABSTRACT 

A fundamental step of Gram-positive pathogenesis is the bacterial adhesion to the host tissue involving interaction between bacterial surface molecules and host ligands. This review is focused on antivirulence compounds that target Gram-positive adhesins and on their potential development as therapeutic agents alternative or complementary to conventional antibiotics in the contrast of pathogens. In particular, compounds that target the sortase A, wall theicoic acid inhibitors, carbohydrates able to bind bacterial proteins and proteins capable of influencing the bacterial adhesion, were described. We further discuss the advantages and disadvantages of this strategy in the development of novel antimicrobials and the future perspective of this research field still at its first steps.

Cell wall structure and function in lactic acid bacteria

The cell wall of Gram-positive bacteria is a complex assemblage of glycopolymers and proteins. It consists of a thick peptidoglycan sacculus that surrounds the cytoplasmic membrane and that is decorated with teichoic acids, polysaccharides, and proteins. It plays a major role in bacterial physiology since it maintains cell shape and integrity during growth and division; in addition, it acts as the interface between the bacterium and its environment. Lactic acid bacteria (LAB) are traditionally and widely used to ferment food, and they are also the subject of more and more research because of their potential health-related benefits. It is now recognized that understanding the composition, structure, and properties of LAB cell walls is a crucial part of developing technological and health applications using these bacteria. In this review, we examine the different components of the Gram-positive cell wall: peptidoglycan, teichoic acids, polysaccharides, and proteins. We present recent findings regarding the structure and function of these complex compounds, results that have emerged thanks to the tandem development of structural analysis and whole genome sequencing. Although general structures and biosynthesis pathways are conserved among Gram-positive bacteria, studies have revealed that LAB cell walls demonstrate unique properties; these studies have yielded some notable, fundamental, and novel findings. Given the potential of this research to contribute to future applied strategies, in our discussion of the role played by cell wall components in LAB physiology, we pay special attention to the mechanisms controlling bacterial autolysis, bacterial sensitivity to bacteriophages and the mechanisms underlying interactions between probiotic bacteria and their hosts.

Recognition of flavin mononucleotide, Haemophilus influenzae type b and its capsular polysaccharide vaccines by antibodies specific to D-ribitol-5-phosphate

D-Ribitol-5-phosphate (Rbt-5-P) is an important metabolite in the pentose phosphate pathway and an integral part of bacterial cell wall polysaccharides, specifically as polyribosyl ribitol phosphate (PRP) in Haemophilus influenzae type b (Hib). The major objective of this study was to investigate whether an antibody specific to Rbt-5-P can recognize the PRP of Hib. D-Ribose-5-phosphate was reacted with proteins in the presence of sodium cyanoborohydride to obtain Rbt-5-P epitopes; 120 h reaction resulted in conjugation of ~30 and ~17 moles of Rbt-5-P/mole of BSA and OVA, respectively, based on decrease in amino groups, MALDI-TOF analyses, an increase in apparent molecular weight (SDS-PAGE) and glycoprotein staining. Immunization of rabbits with Rbt-5-P-BSA conjugate generated antibodies to Rbt-5-P as demonstrated by dot immunoblot and non-competitive ELISA. Homogeneous Rbt-5-P-specific antibody was purified from Rbt-5-P-BSA antiserum subjected to caprylic acid precipitation followed by hapten-affinity chromatography; its affinity constant is 7.1 × 10(8) M(-1). Rbt-5-P antibody showed 100 % specificity to Rbt-5-P, ~230 %, 10 % and 3.4 % cross-reactivity to FMN, riboflavin and FAD, respectively; the antibody showed ~4 % cross-reactivity to D-ribitol and <3 % to other sugars/sugar alcohols. Rbt-5-P-specific antibody recognized Hib conjugate vaccines containing PRP which was inhibited specifically by Rbt-5-P, and also detected Hib cell-surface capsular polysaccharides by immunofluorescence. In conclusion, Rbt-5-P-protein conjugate used as an immunogen elicited antibodies binding to an epitope also present in PRP and Hib bacteria. Rbt-5-P-specific antibody has potential applications in the detection and quantification of free/bound Rbt-5-P and FMN as well as immunological recognition of Hib bacteria and its capsular polysaccharide.

D-amino acid mediated recruitment of endogenous antibodies to bacterial surfaces

The number of antibiotic resistant bacterial strains has been continuously increasing over the last few decades. Nontraditional routes to combat bacteria may offer an attractive alternative to the ongoing problem of drug discovery in this field. Herein, we describe the initial framework toward the development of bacterial d-amino acid antibody recruitment therapy (DART). DART represents a promising antibiotic strategy by exploiting the promiscuity of bacteria to incorporate unnatural d-amino acids and subsequently recruit antibodies to the bacterial surface. The conjugation of 2,4-dinitrophenyl (DNP) to various d-amino acids led to the discovery of a d-amino acid that specifically tags the surface of Bacillus subtilis and Staphylococcus aureus for the recruitment of anti-DNP antibodies (a highly abundant antibody in human serum). This system represents a novel strategy as an antibacterial therapy that targets planktonic Gram-positive bacteria.

Additivity and synergy between an antimicrobial peptide and inhibitory ions

Recently we described the pH dependence of activity for a family of cationic antimicrobial peptides (CAMPs) selected from a combinatorial library. In the current work we report on the effects of toxic ions (Cu(2+), Zn(2+), and F(-)) and the chelator EDTA on the activity profiles of one member of this family, the 12-residue cationic antimicrobial peptide *ARVA, against a panel of microorganisms. All four ions exhibited either synergy or additivity with *ARVA for all organisms tested with the exception of *ARVA combined with NaF against Candida albicans which exhibited indifference. CuCl2 and ZnCl2 exhibited synergy with *ARVA against both the Gram negative Pseudomonas aeruginosa and the Gram positive Staphylococcus aureus as well as strong additivity against Escherichia coli at submillimolar concentrations. The chelator EDTA was synergistic with *ARVA against the two Gram negative organisms but showed only simple additivity with S. aureus and C. albicans despite their much lower MICs with EDTA. This effect may be related to the known differences in the divalent ion binding properties of the Gram negative LPS layer as compared to the peptidoglycan layer of the Gram positive organism. Unlike the other ions, NaF showed only additivity or indifference when combined with *ARVA and required much higher concentrations for activity. The yeast C. albicans did not show synergy or strong additivity with any of the inhibitory compounds tested. The effects of toxic ions and chelators observed here have important implications for applications using CAMPs and for the design of novel formulations involving CAMPs. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Biosynthesis of the unique wall teichoic acid of Staphylococcus aureus lineage ST395

The major clonal lineages of the human pathogen Staphylococcus aureus produce cell wall-anchored anionic poly-ribitol-phosphate (RboP) wall teichoic acids (WTA) substituted with d-Alanine and N-acetyl-d-glucosamine. The phylogenetically isolated S. aureus ST395 lineage has recently been found to produce a unique poly-glycerol-phosphate (GroP) WTA glycosylated with N-acetyl-d-galactosamine (GalNAc). ST395 clones bear putative WTA biosynthesis genes on a novel genetic element probably acquired from coagulase-negative staphylococci (CoNS). We elucidated the ST395 WTA biosynthesis pathway and identified three novel WTA biosynthetic genes, including those encoding an α-O-GalNAc transferase TagN, a nucleotide sugar epimerase TagV probably required for generation of the activated sugar donor substrate for TagN, and an unusually short GroP WTA polymerase TagF. By using a panel of mutants derived from ST395, the GalNAc residues carried by GroP WTA were found to be required for infection by the ST395-specific bacteriophage Φ187 and to play a crucial role in horizontal gene transfer of S. aureus pathogenicity islands (SaPIs). Notably, ectopic expression of ST395 WTA biosynthesis genes rendered normal S. aureus susceptible to Φ187 and enabled Φ187-mediated SaPI transfer from ST395 to regular S. aureus. We provide evidence that exchange of WTA genes and their combination in variable, mosaic-like gene clusters have shaped the evolution of staphylococci and their capacities to undergo horizontal gene transfer events.

The structural highly diverse wall teichoic acids (WTA) are cell wall-anchored glycopolymers produced by most Gram-positive bacteria. While most of the dominant Staphylococcus aureus lineages produce poly-ribitol-phosphate WTA, the recently described ST395 lineage produces a distinct poly-glycerol-phosphate WTA type resembling the WTA backbone of coagulase-negative staphylococci (CoNS). Here, we analyzed the ST395 WTA biosynthesis pathway and found new types of WTA biosynthesis genes along with an evolutionary link between ST395 and CoNS, from which the ST395 WTA genes probably originate. The elucidation of ST395 WTA biosynthesis will help to understand how Gram-positive bacteria produce highly variable WTA types and elucidate functional consequences of WTA variation.

In silico analysis of AHJD-like viruses, Staphylococcus aureus phages S24-1 and S13', and study of phage S24-1 adsorption

Staphylococcus aureus is a clinically important bacterium that is commensal in both humans and animals. Bacteriophage (phage) attachment to the host bacterial surface is an important process during phage infection, which involves interactions between phage receptor-binding proteins and host receptor molecules. However, little information is available on the receptor-binding protein of S. aureus phages. S. aureus virulent phages S24-1 and S13' (family Podoviridae, genus AHJD-like viruses) were isolated from sewage. In the present study, we investigated the receptor-binding protein of AHJD-like viruses using phage S24-1. First, based on a comparative genomic analysis of phages S24-1 and S13', open reading frame 16 (ORF16) of phage S24-1 was speculated to be the receptor-binding protein, which possibly determines the host range. Second, we demonstrated that this was the receptor-binding protein of phage S24-1. Third, our study suggested that wall teichoic acids in the cell walls of S. aureus are the main receptor molecules for ORF16 and phage S24-1. Finally, the C-terminal region of ORF16 may be essential for binding to S. aureus. These results strongly suggest that ORF16 of phage S24-1 and its homologs may be the receptor-binding proteins of AHJD-like viruses.

Recognition of riboflavin and the capsular polysaccharide of Haemophilus influenzae type b by antibodies generated to the haptenic epitope D-ribitol

D-Ribitol, a five-carbon sugar alcohol, is an important metabolite in the pentose phosphate pathway; it is an integral part of riboflavin (vitamin B2) and cell wall polysaccharides in most Gram-positive and a few Gram-negative bacteria. Antibodies specific to D-ribitol were generated in New Zealand white rabbits by using reductively aminated D-ribose-BSA conjugate as the immunogen. MALDI-TOF and amino group analyses of ribitol-BSA conjugate following 120 h reaction showed ~27-30 mol of ribitol conjugated per mole BSA. The presence of sugar alcohol in the conjugates was also confirmed by an increase in molecular mass and a positive periodic acid-Schiff staining in SDS-PAGE. Caprylic acid precipitation of rabbit serum followed by hapten affinity chromatography on ribitol-KLH-Sepharose CL-6B resulted in pure ribitol-specific antibodies (~45-50 μg/mL). The affinity constant of ribitol antibodies was found to be 2.9 × 10(7) M(-1) by non-competitive ELISA. Ribitol antibodies showed 100% specificity towards ribitol, ~800% cross-reactivity towards riboflavin, 10-15% cross-reactivity with sorbitol, xylitol and mannitol, and 5-7% cross-reactivity with L-arabinitol and meso-erythritol. The specificity of antibody to ribitol was further confirmed by its low cross-reactivity (0.4%) with lumichrome. Antibodies to D-ribitol recognized the purified capsular polysaccharide of Haemophilus influenzae type b, which could be specifically inhibited by ribitol. In conclusion, antibodies specific to D-ribitol have been generated and characterized, which have potential applications in the detection of free riboflavin and ribitol in biological samples, as well as identification of cell-surface macromolecules containing ribitol.

Scalable one-pot bacteria-templating synthesis route toward hierarchical, porous-Co3O4 superstructures for supercapacitor electrodes

Template-driven strategy has been widely used to synthesize inorganic nano/micro materials. Here, we used a bottom-up controlled synthesis route to develop a powerful solution-based method of fabricating three-dimensional (3D), hierarchical, porous-Co₃O₄ superstructures that exhibit the morphology of flower-like microspheres (hereafter, RT-Co₃O₄). The gram-scale RT-Co₃O₄ was facilely prepared using one-pot synthesis with bacterial templating at room temperature. Large-surface-area RT-Co₃O₄ also has a noticeable pseudocapacitive performance because of its high mass loading per area (~10 mg cm⁻²), indicating a high capacitance of 214 F g⁻¹ (2.04 F cm⁻²) at 2 A g⁻¹ (19.02 mA cm⁻²), a Coulombic efficiency averaging over 95%, and an excellent cycling stability that shows a capacitance retention of about 95% after 4,000 cycles.

Insights into naturally minimised Streptomyces albus J1074 genome

Background

The Streptomyces albus J1074 strain is one of the most widely used chassis for the heterologous production of bioactive natural products. The fast growth and an efficient genetic system make this strain an attractive model for expressing cryptic biosynthetic pathways to aid drug discovery.

Results

To improve its capabilities for the heterologous expression of biosynthetic gene clusters, the complete genomic sequence of S. albus J1074 was obtained. With a size of 6,841,649 bp, coding for 5,832 genes, its genome is the smallest within the genus streptomycetes. Genome analysis revealed a strong tendency to reduce the number of genetic duplicates. The whole transcriptomes were sequenced at different time points to identify the early metabolic switch from the exponential to the stationary phase in S. albus J1074.

Conclusions

S. albus J1074 carries the smallest genome among the completely sequenced species of the genus Streptomyces. The detailed genome and transcriptome analysis discloses its capability to serve as a premium host for the heterologous production of natural products. Moreover, the genome revealed 22 additional putative secondary metabolite gene clusters that reinforce the strain’s potential for natural product synthesis.

Design and synthesis of membrane-targeting antibiotics: from peptides- to aminosugar-based antimicrobial cationic amphiphiles

Infections caused by drug resistant and/or slow-growing bacteria are increasingly becoming some of the greatest challenges of health organizations worldwide.

Murgocil is a highly bioactive staphylococcal-specific inhibitor of the peptidoglycan glycosyltransferase enzyme MurG

Modern medicine is founded on the discovery of penicillin and subsequent small molecules that inhibit bacterial peptidoglycan (PG) and cell wall synthesis. However, the discovery of new chemically and mechanistically distinct classes of PG inhibitors has become exceedingly rare, prompting speculation that intracellular enzymes involved in PG precursor synthesis are not 'druggable' targets. Here, we describe a β-lactam potentiation screen to identify small molecules that augment the activity of β-lactams against methicillin-resistant Staphylococcus aureus (MRSA) and mechanistically characterize a compound resulting from this screen, which we have named murgocil. We provide extensive genetic, biochemical, and structural modeling data demonstrating both in vitro and in whole cells that murgocil specifically inhibits the intracellular membrane-associated glycosyltransferase, MurG, which synthesizes the lipid II PG substrate that penicillin binding proteins (PBPs) polymerize and cross-link into the cell wall. Further, we demonstrate that the chemical synergy and cidality achieved between murgocil and the β-lactam imipenem is mediated through MurG dependent localization of PBP2 to the division septum. Collectively, these data validate our approach to rationally identify new target-specific bioactive β-lactam potentiation agents and demonstrate that murgocil now serves as a highly selective and potent chemical probe to assist our understanding of PG biosynthesis and cell wall biogenesis across Staphylococcal species.

Pathways and roles of wall teichoic acid glycosylation in Staphylococcus aureus

The thick peptidoglycan layers of Gram-positive bacteria are connected to polyanionic glycopolymers called wall teichoic acids (WTA). Pathogens such as Staphylococcus aureus, Listeria monocytogenes, or Enterococcus faecalis produce WTA with diverse, usually strain-specific structure. Extensive studies on S. aureus WTA mutants revealed important functions of WTA in cell division, growth, morphogenesis, resistance to antimicrobials, and interaction with host or phages. While most of the S. aureus WTA-biosynthetic genes have been identified it remained unclear for long how and why S. aureus glycosylates WTA with α- or β-linked N-acetylglucosamine (GlcNAc). Only recently the discovery of two WTA glycosyltransferases, TarM and TarS, yielded fundamental insights into the roles of S. aureus WTA glycosylation. Mutants lacking WTA GlcNAc are resistant towards most of the S. aureus phages and, surprisingly, TarS-mediated WTA β-O-GlcNAc modification is essential for β-lactam resistance in methicillin-resistant S. aureus. Notably, S. aureus WTA GlcNAc residues are major antigens and activate the complement system contributing to opsonophagocytosis. WTA glycosylation with a variety of sugars and corresponding glycosyltransferases were also identified in other Gram-positive bacteria, which paves the way for detailed investigations on the diverse roles of WTA modification with sugar residues.

Contributions of the σ(W) , σ(M) and σ(X) regulons to the lantibiotic resistome of Bacillus subtilis

In Bacillus subtilis, the extracytoplasmic function (ECF) σ factors σ(M) , σ(W) and σ(X) all contribute to resistance against lantibiotics. Nisin, a model lantibiotic, has a dual mode of action: it inhibits cell wall synthesis by binding lipid II, and this complex also forms pores in the cytoplasmic membrane. These activities can be separated in a nisin hinge-region variant (N20P M21P) that binds lipid II, but no longer permeabilizes membranes. The major contribution of σ(M) to nisin resistance is expression of ltaSa, encoding a stress-activated lipoteichoic acid synthase, and σ(X) functions primarily by activation of the dlt operon controlling d-alanylation of teichoic acids. Together, σ(M) and σ(X) regulate cell envelope structure to decrease access of nisin to its lipid II target. In contrast, σ(W) is principally involved in protection against membrane permeabilization as it provides little protection against the nisin hinge region variant. σ(W) contributes to nisin resistance by regulation of a signal peptide peptidase (SppA), phage shock proteins (PspA and YvlC, a PspC homologue) and tellurite resistance related proteins (YceGHI). These defensive mechanisms are also effective against other lantibiotics such as mersacidin, gallidermin and subtilin and comprise an important subset of the intrinsic antibiotic resistome of B. subtilis.

Two unique phosphorylation-driven signaling pathways crosstalk in Staphylococcus aureus to modulate the cell-wall charge: Stk1/Stp1 meets GraSR

The Stk1/Stp1 and GraSR signal-transduction pathways are two distinct pathways in Staphylococcus aureus that rely on a reversible phosphorylation process in transducing external stimuli intracellularly. Stk1/Stp1 is an eukaryote-like Ser/Thr kinase phosphatase pair involved in purine biosynthesis, cell-wall metabolism, and autolysis. GraSR is a two-component system involved in resistance to cationic antimicrobial peptides. Both systems are implicated in S. aureus virulence and resistance to cell-wall inhibitors. Our study shows that the response regulator protein GraR undergoes phosphorylation by Stk1 at three threonine residues in the DNA-binding domain. Phosphorylation by Stk1 depends on the structural integrity of GraR as well as the amino acid sequences flanking the phosphorylation sites. Its homologue in Bacillus subtilis , BceR, which harbors two of the three phosphorylation sites in GraR, does not undergo Stk1-dependent phosphorylation. GraR is involved in regulation of the dltABCD operon, the gene products of which add the d-Ala on wall teichoic acid (WTA). Investigation of WTA isolated from the S. aureus RN6390 ΔgraR strain by NMR spectroscopy showed a clear negative effect that graR deletion has on the d-Ala content of WTA. Moreover, complementation of ΔgraR mutant with graR lacking the Stk1 phosphorylation sites mirrors this effect. These findings provide evidence that GraR is a target of Stk1 in vivo and suggest that modification of WTA by d-Ala is modulated by Stk1. The crosstalk between these two otherwise independent signaling pathways may facilitate S. aureus interaction with its environment to modulate processes such as cell growth and division and virulence.

Conservation of the PTEN catalytic motif in the bacterial undecaprenyl pyrophosphate phosphatase, BacA/UppP

Isoprenoid lipid carriers are essential in protein glycosylation and bacterial cell envelope biosynthesis. The enzymes involved in their metabolism (synthases, kinases and phosphatases) are therefore critical to cell viability. In this review, we focus on two broad groups of isoprenoid pyrophosphate phosphatases. One group, containing phosphatidic acid phosphatase motifs, includes the eukaryotic dolichyl pyrophosphate phosphatases and proposed recycling bacterial undecaprenol pyrophosphate phosphatases, PgpB, YbjB and YeiU/LpxT. The second group comprises the bacterial undecaprenol pyrophosphate phosphatase, BacA/UppP, responsible for initial formation of undecaprenyl phosphate, which we predict contains a tyrosine phosphate phosphatase motif resembling that of the tumour suppressor, phosphatase and tensin homologue (PTEN). Based on protein sequence alignments across species and 2D structure predictions, we propose catalytic and lipid recognition motifs unique to BacA/UppP enzymes. The verification of our proposed active-site residues would provide new strategies for the development of substrate-specific inhibitors which mimic both the lipid and pyrophosphate moieties, leading to the development of novel antimicrobial agents.

Glycoepitopes of staphylococcal wall teichoic acid govern complement-mediated opsonophagocytosis via human serum antibody and mannose-binding lectin

Serum antibodies and mannose-binding lectin (MBL) are important host defense factors for host adaptive and innate immunity, respectively. Antibodies and MBL also initiate the classical and lectin complement pathways, respectively, leading to opsonophagocytosis. We have shown previously that Staphylococcus aureus wall teichoic acid (WTA), a cell wall glycopolymer consisting of ribitol phosphate substituted with α- or β-O-N-acetyl-d-glucosamine (GlcNAc) and d-alanine, is recognized by MBL and serum anti-WTA IgG. However, the exact antigenic determinants to which anti-WTA antibodies or MBL bind have not been determined. To answer this question, several S. aureus mutants, such as α-GlcNAc glycosyltransferase-deficient S. aureus ΔtarM, β-GlcNAc glycosyltransferase-deficient ΔtarS, and ΔtarMS double mutant cells, were prepared from a laboratory and a community-associated methicillin-resistant S. aureus strain. Here, we describe the unexpected finding that β-GlcNAc WTA-deficient ΔtarS mutant cells (which have intact α-GlcNAc) escape from anti-WTA antibody-mediated opsonophagocytosis, whereas α-GlcNAc WTA-deficient ΔtarM mutant cells (which have intact β-GlcNAc) are efficiently engulfed by human leukocytes via anti-WTA IgG. Likewise, MBL binding in S. aureus cells was lost in the ΔtarMS double mutant but not in either single mutant. When we determined the serum concentrations of the anti-α- or anti-β-GlcNAc-specific WTA IgGs, anti-β-GlcNAc WTA-IgG was dominant in pooled human IgG fractions and in the intact sera of healthy adults and infants. These data demonstrate the importance of the WTA sugar conformation for human innate and adaptive immunity against S. aureus infection.

Fluorescent probes for investigation of isoprenoid configuration and size discrimination by bactoprenol-utilizing enzymes

Undecaprenyl Pyrophosphate Synthase (UPPS) is an enzyme critical to the production of complex polysaccharides in bacteria, as it produces the crucial bactoprenol scaffold on which these materials are assembled. Methods to characterize the systems associated with polysaccharide production are non-trivial, in part due to the lack of chemical tools to investigate their assembly. In this report, we develop a new fluorescent tool using UPPS to incorporate a powerful fluorescent anthranilamide moiety into bactoprenol. The activity of this analogue in polysaccharide biosynthesis is then tested with the initiating hexose-1-phosphate transferases involved in Capsular Polysaccharide A biosynthesis in the symbiont Bacteroides fragilis and the asparagine-linked glycosylation system of the pathogenic Campylobacter jejuni. In addition, it is shown that the UPPS used to make this probe is not specific for E-configured isoprenoid substrates and that elongation by UPPS is required for activity with the downstream enzymes.

Discovery of wall teichoic acid inhibitors as potential anti-MRSA β-lactam combination agents

Innovative strategies are needed to combat drug resistance associated with methicillin-resistant Staphylococcus aureus (MRSA). Here, we investigate the potential of wall teichoic acid (WTA) biosynthesis inhibitors as combination agents to restore β-lactam efficacy against MRSA. Performing a whole-cell pathway-based screen, we identified a series of WTA inhibitors (WTAIs) targeting the WTA transporter protein, TarG. Whole-genome sequencing of WTAI-resistant isolates across two methicillin-resistant Staphylococci spp. revealed TarG as their common target, as well as a broad assortment of drug-resistant bypass mutants mapping to earlier steps of WTA biosynthesis. Extensive in vitro microbiological analysis and animal infection studies provide strong genetic and pharmacological evidence of the potential effectiveness of WTAIs as anti-MRSA β-lactam combination agents. This work also highlights the emerging role of whole-genome sequencing in antibiotic mode-of-action and resistance studies.

Inhibition of the first step in synthesis of the mycobacterial cell wall core, catalyzed by the GlcNAc-1-phosphate transferase WecA, by the novel caprazamycin derivative CPZEN-45

Because tuberculosis is one of the most prevalent and serious infections, countermeasures against it are urgently required. We isolated the antitubercular agents caprazamycins from the culture of an actinomycete strain and created CPZEN-45 as the most promising derivative of the caprazamycins. Herein, we describe the mode of action of CPZEN-45 first against Bacillus subtilis. Unlike the caprazamycins, CPZEN-45 strongly inhibited incorporation of radiolabeled glycerol into growing cultures and showed antibacterial activity against caprazamycin-resistant strains, including a strain overexpressing translocase-I (MraY, involved in the biosynthesis of peptidoglycan), the target of the caprazamycins. By contrast, CPZEN-45 was not effective against a strain overexpressing undecaprenyl-phosphate-GlcNAc-1-phosphate transferase (TagO, involved in the biosynthesis of teichoic acid), and a mutation was found in the tagO gene of the spontaneous CPZEN-45-resistant strain. This suggested that the primary target of CPZEN-45 in B. subtilis is TagO, which is a different target from that of the parent caprazamycins. This suggestion was confirmed by evaluation of the activities of these enzymes. Finally, we showed that CPZEN-45 was effective against WecA (Rv1302, also called Rfe) of Mycobacterium tuberculosis, the ortholog of TagO and involved in the biosynthesis of the mycolylarabinogalactan of the cell wall of M. tuberculosis. The outlook for WecA as a promising target for the development of antituberculous drugs as a countermeasure of drug resistant tuberculosis is discussed.

Efforts toward broadening the spectrum of arylomycin antibiotic activity

New antibiotics are needed, and one source may be 'latent' antibiotics, natural products whose once broad-spectrum activity is currently limited by the evolution of resistance in nature. We have identified a potential class of latent antibiotics, the arylomycins, which are lipopeptides with a C-terminal macrocycle that target signal peptidase and whose spectrum is limited by a resistance-conferring mutation in many bacteria. Herein, we report the synthesis and evaluation of several arylomycin derivatives, and demonstrate that both C-terminal homologation with a glycyl aldehyde and addition of a positive charge to the macrocycle increase the activity and spectrum of the arylomycin scaffold.

Intradermal immunization with wall teichoic acid (WTA) elicits and augments an anti-WTA IgG response that protects mice from methicillin-resistant Staphylococcus aureus infection independent of mannose-binding lectin status

The objectives of this study were to investigate the immune response to intradermal immunization with wall teichoic acid (WTA) and the effect of MBL deficiency in a murine model of infection with methicillin-resistant Staphylococcus aureus (MRSA). WTA is a bacterial cell wall component that is implicated in invasive infection. We tested susceptibility to MRSA infection in wild type (WT) and MBL deficient mice using two strains of MRSA: MW2, a community-associated MRSA (CA-MRSA); and COL, a healthcare-associated MRSA (HA-MRSA). We also performed in vitro assays to investigate the effects of anti-WTA IgG containing murine serum on complement activation and bacterial growth in whole blood. We found that MBL knockout (KO) mice are relatively resistant to a specific MRSA strain, MW2 CA-MRSA, compared to WT mice, while both strains of mice had similar susceptibility to a different strain, COL HA-MRSA. Intradermal immunization with WTA elicited and augmented an anti-WTA IgG response in both WT and MBL KO mice. WTA immunization significantly reduced susceptibility to both MW2 CA-MRSA and COL HA-MRSA, independent of the presence of MBL. The protective mechanisms of anti-WTA IgG are mediated at least in part by complement activation and clearance of bacteria from blood. The significance of these findings is that 1) Intradermal immunization with WTA induces production of anti-WTA IgG; and 2) This anti-WTA IgG response protects from infection with both MW2 CA-MRSA and COL HA-MRSA even in the absence of MBL, the deficiency of which is common in humans.

Teichoic acid biosynthesis as an antibiotic target

The relentless spread of antibiotic-resistant pathogens makes it imperative to develop new chemotherapeutic strategies to overcome infection. The bacterial cell wall has served as a rich source for both validated and unexploited pathways that are essential for virulence and survival. Lipoteichoic acids (LTAs) and wall teichoic acids (WTAs) are cell wall polymers that play fundamental roles in Gram-positive bacterial physiology and pathogenesis, and both have been proposed as novel antibacterial targets. Here we describe recent progress toward the discovery of teichoic acid biosynthesis inhibitors and their potential as antibiotics to combat Staphylococcus aureus infections.

Auxiliary factors: a chink in the armor of MRSA resistance to β-lactam antibiotics

Combination agents provide an important orthogonal approach to treat infectious diseases, particularly those caused by drug resistant pathogens. Indeed, applying a biologically 'rational' and systems-level paradigm to discover potent, selective, and synergistic agents would augment current (and arguably overly relied upon) empirical and serendipitous approaches to such discovery efforts. Here, we review the cellular mechanisms of β-lactam drug resistance and tolerance achieved amongst methicillin-resistant Staphylococcus aureus (MRSA) as well as their molecular targets and strategies to identify cognate inhibitors as potential combination agents to restore β-lactam efficacy against MRSA.

pH Dependence of microbe sterilization by cationic antimicrobial peptides

We recently described a family of cationic antimicrobial peptides (CAMPs) selected from a combinatorial library that exhibited potent, broad-spectrum activity at neutral pH and low ionic strength. To further delimit the utility and activity profiles of these peptides, we investigated the effects of solution conditions, such as pH and ionic strength, on the efficacy of the peptide antimicrobials against a panel of microorganisms. Peptide minimum sterilizing concentrations (MSCs) varied linearly with pH for each subtype within our family of CAMPs for all organisms tested. The peptides were much less effective against Gram-negative bacteria at high pH, consistent with a decrease in net positive charge on the peptides. A similar trend was observed for the fungus Candida albicans. Surprisingly, the opposite pH trend was observed with the Gram-positive Staphylococcus aureus. In addition, an additive ionic strength effect was observed with increasing buffer strengths at identical pH values. The extreme difference in the observed pH behavior between Gram-negative and Gram-positive organisms is attributed to the presence of native charged molecules in the much thicker peptidoglycan layer of the Gram-positive organism. The novel species-specific effects of pH observed here have important implications for applications using CAMPs and for the design of novel CAMPs.

Preconditioning with cations increases the attachment of Anoxybacillus flavithermus and Geobacillus species to stainless steel

Preconditioning of Anoxybacillus flavithermus E16 and Geobacillus sp. strain F75 with cations prior to attachment often significantly increased (P ≤ 0.05) the number of viable cells that attached to stainless steel (by up to 1.5 log CFU/cm(2)) compared with unconditioned bacteria. It is proposed that the transition of A. flavithermus and Geobacillus spp. from milk formulations to stainless steel product contact surfaces in milk powder manufacturing plants is mediated predominantly by bacterial physiological factors (e.g., surface-exposed adhesins) rather than the concentrations of cations in milk formulations surrounding bacteria.

Wall teichoic acids of gram-positive bacteria

The peptidoglycan layers of many gram-positive bacteria are densely functionalized with anionic glycopolymers known as wall teichoic acids (WTAs). These polymers play crucial roles in cell shape determination, regulation of cell division, and other fundamental aspects of gram-positive bacterial physiology. Additionally, WTAs are important in pathogenesis and play key roles in antibiotic resistance. We provide an overview of WTA structure and biosynthesis, review recent studies on the biological roles of these polymers, and highlight remaining questions. We also discuss prospects for exploiting WTA biosynthesis as a target for new therapies to overcome resistant infections.

Design of membrane targeting tobramycin-based cationic amphiphiles with reduced hemolytic activity

Hemolysis caused by amphiphilic tobramycin antimicrobial agents is significantly reduced by altering the bond linking their hydrophobic and hydrophilic parts.

A bird's eye view of the bacterial landscape

Bacteria interact with the environment through their cell surface. Activities as diverse as attaching to a catheter, crawling on a surface, swimming through a pond, or being preyed on by a bacteriophage depend on the composition and structure of the cell surface. The cell surface must also protect bacteria from harmful chemicals present in the environment while allowing the intake of nutrients and excretion of toxic molecules. Bacteria have evolved four main types of bacterial cell surfaces to accomplish these functions: those of the typical gram-negative and gram-positive bacteria, and those of the Actinobacteria and Mollicutes. So few types seems remarkable since bacteria are very diverse and abundant, and they can live in many different environments. However, each species has tweaked these stereotypical bacterial surfaces to best fit its needs. The result is an amazing diversity of the bacterial landscape, most of which remains unexplored. Here I give an overview of the main features of the bacterial cell surface and highlight how advances in methodology have moved forward this field of study.

Viable screening targets related to the bacterial cell wall

The synthesis of the bacterial peptidoglycan has been recognized for over 50 years as fertile ground for antibacterial discovery. Initially, empirical screening of natural products for inhibition of bacterial growth detected many chemical classes of antibiotics whose specific mechanisms of action were eventually dissected and defined. Of the nontoxic antibiotics discovered, most were found to be inhibitors of either protein synthesis or cell wall synthesis, which led to more directed screening for inhibitors of these pathways. Directed screening and design programs for cell wall inhibitors have been undertaken since the 1960s. In that time it has become clear that, while certain steps and intermediates have yielded selective inhibitors and are established targets, other potential targets have not yielded inhibitors whose antibacterial activity is proven to be solely due to that inhibition. Why has this search been so problematic? Are the established targets still worth pursuing? This review will attempt to answer these and other questions and evaluate the viability of targets related to peptidoglycan synthesis.

Anti-infectious agents against MRSA

Clinically useful antibiotics, β-lactams and vancomycin, are known to inhibit bacterial cell wall peptidoglycan synthesis. Methicillin-resistant Staphylococcus aureus (MRSA) has a unique cell wall structure consisting of peptidoglycan and wall teichoic acid. In recent years, new anti-infectious agents (spirohexaline, tripropeptin C, DMPI, CDFI, cyslabdan, 1835F03, and BPH-652) targeting MRSA cell wall biosynthesis have been discovered using unique screening methods. These agents were found to inhibit important enzymes involved in cell wall biosynthesis such as undecaprenyl pyrophosphate (UPP) synthase, FemA, flippase, or UPP phosphatase. In this review, the discovery, the mechanism of action, and the future of these anti-infectious agents are described.

Dual role for the O-acetyltransferase OatA in peptidoglycan modification and control of cell septation in Lactobacillus plantarum

Until now, peptidoglycan O-acetyl transferases (Oat) were only described for their peptidoglycan O-acetylating activity and for their implication in the control of peptidoglycan hydrolases. In this study, we show that a Lactobacillus plantarum mutant lacking OatA is unable to uncouple cell elongation and septation. Wild-type cells showed an elongation arrest during septation while oatA mutant cells continued to elongate at a constant rate without any observable pause during the cell division process. Remarkably, this defect does not result from a default in peptidoglycan O-acetylation, since it can be rescued by wild-type OatA as well as by a catalytic mutant or a truncated variant containing only the transmembrane domain of the protein. Consistent with a potential involvement in division, OatA preferentially localizes at mid-cell before membrane invagination and remains at this position until the end of septation. Overexpression of oatA or its inactive variants induces septation-specific aberrations, including asymmetrical and dual septum formation. Overproduction of the division inhibitors, MinC or MinD, leads to cell filamentation in the wild type while curved and branched cells are observed in the oatA mutant, suggesting that the Min system acts differently on the division process in the absence of OatA. Altogether, the results suggest that OatA plays a key role in the spatio-temporal control of septation, irrespective of its catalytic activity.

Methicillin resistance in Staphylococcus aureus requires glycosylated wall teichoic acids

Staphylococcus aureus peptidoglycan (PG) is densely functionalized with anionic polymers called wall teichoic acids (WTAs). These polymers contain three tailoring modifications: d-alanylation, α-O-GlcNAcylation, and β-O-GlcNAcylation. Here we describe the discovery and biochemical characterization of a unique glycosyltransferase, TarS, that attaches β-O-GlcNAc (β-O-N-acetyl-D-glucosamine) residues to S. aureus WTAs. We report that methicillin resistant S. aureus (MRSA) is sensitized to β-lactams upon tarS deletion. Unlike strains completely lacking WTAs, which are also sensitive to β-lactams, ΔtarS strains have no growth or cell division defects. Because neither α-O-GlcNAc nor β-O-Glucose modifications can confer resistance, the resistance phenotype requires a highly specific chemical modification of the WTA backbone, β-O-GlcNAc residues. These data suggest β-O-GlcNAcylated WTAs scaffold factors required for MRSA resistance. The β-O-GlcNAc transferase identified here, TarS, is a unique target for antimicrobials that sensitize MRSA to β-lactams.

High-throughput assessment of bacterial growth inhibition by optical density measurements

The increasing incidence of antibiotic-resistant bacterial infections both in hospitals and in the community intensifies the need for new antibacterial strategies and targets. Although high-throughput screening against live bacteria allows rapid discovery of compounds with growth-inhibitory activities, these efforts have failed to fill the pipeline with the anticipated antibacterial compounds because target identification is often onerous. Recently, a strategy was reported that employs a bacteria growth inhibition assay readout using optical density measurements on paired strains - both a wildtype strain and a pathway-null mutant - to find inhibitors of wildtype bacterial growth that specifically target conditionally essential enzymes in the pathway of interest. Protocols are provided here for determining the robustness of an assay, screening in a high-throughput format and setting up dose-response curves in paired Staphylococcus aureus strains. However, the protocols can be used to screen for growth-inhibitory compounds in any bacterial strain of interest.

Wall teichoic acids restrict access of bacteriophage endolysin Ply118, Ply511, and PlyP40 cell wall binding domains to the Listeria monocytogenes peptidoglycan

The C-terminal cell wall binding domains (CBDs) of phage endolysins direct the enzymes to their binding ligands on the bacterial cell wall with high affinity and specificity. The Listeria monocytogenes Ply118, Ply511, and PlyP40 endolysins feature related CBDs which recognize the directly cross-linked peptidoglycan backbone structure of Listeria. However, decoration with fluorescently labeled CBDs primarily occurs at the poles and septal regions of the rod-shaped cells. To elucidate the potential role of secondary cell wall-associated carbohydrates such as the abundant wall teichoic acid (WTA) on this phenomenon, we investigated CBD binding using L. monocytogenes serovar 1/2 and 4 cells deficient in WTA. Mutants were obtained by deletion of two redundant tagO homologues, whose products catalyze synthesis of the WTA linkage unit. While inactivation of either tagO1 (EGDe lmo0959) or tagO2 (EGDe lmo2519) alone did not affect WTA content, removal of both alleles following conditional complementation yielded WTA-deficient Listeria cells. Substitution of tagO from an isopropyl-β-d-thiogalactopyranoside-inducible single-copy integration vector restored the original phenotype. Although WTA-deficient cells are viable, they featured severe growth inhibition and an unusual coccoid morphology. In contrast to CBDs from other Listeria phage endolysins which directly utilize WTA as binding ligand, the data presented here show that WTAs are not required for attachment of CBD118, CBD511, and CBDP40. Instead, lack of the cell wall polymers enables unrestricted spatial access of CBDs to the cell wall surface, indicating that the abundant WTA can negatively regulate sidewall localization of the cell wall binding domains.

Secondary cell wall polymers of Enterococcus faecalis are critical for resistance to complement activation via mannose-binding lectin

The complement system is part of our first line of defense against invading pathogens. The strategies used by Enterococcus faecalis to evade recognition by human complement are incompletely understood. In this study, we identified an insertional mutant of the wall teichoic acid (WTA) synthesis gene tagB in E. faecalis V583 that exhibited an increased susceptibility to complement-mediated killing by neutrophils. Further analysis revealed that increased killing of the mutant was due to a higher rate of phagocytosis by neutrophils, which correlated with higher C3b deposition on the bacterial surface. Our studies indicated that complement activation via the lectin pathway was much stronger on the tagB mutant compared with wild type. In concordance, we found an increased binding of the key lectin pathway components mannose-binding lectin and mannose-binding lectin-associated serine protease-2 (MASP-2) on the mutant. To understand the mechanism of lectin pathway inhibition by E. faecalis, we purified and characterized cell wall carbohydrates of E. faecalis wild type and V583ΔtagB. NMR analysis revealed that the mutant strain lacked two WTAs with a repeating unit of →6)[α-l-Rhap-(1→3)]β-D-GalpNAc-(1→5)-Rbo-1-P and →6) β-D-Glcp-(1→3) [α-D-Glcp-(1→4)]-β-D-GalpNAc-(1→5)-Rbo-1-P→, respectively (Rbo, ribitol). In addition, compositional changes in the enterococcal rhamnopolysaccharide were noticed. Our study indicates that in E. faecalis, modification of peptidoglycan by secondary cell wall polymers is critical to evade recognition by the complement system.

Proton-binding capacity of Staphylococcus aureus wall teichoic acid and its role in controlling autolysin activity

Wall teichoic acid (WTA) or related polyanionic cell wall glycopolymers are produced by most gram-positive bacterial species and have been implicated in various cellular functions. WTA and the proton gradient across bacterial membranes are known to control the activity of autolysins but the molecular details of these interactions are poorly understood. We demonstrate that WTA contributes substantially to the proton-binding capacity of Staphylococcus aureus cell walls and controls autolysis largely via the major autolysin AtlA whose activity is known to decline at acidic pH values. Compounds that increase or decrease the activity of the respiratory chain, a main source of protons in the cell wall, modulated autolysis rates in WTA-producing cells but did not affect the augmented autolytic activity observed in a WTA-deficient mutant. We propose that WTA represents a cation-exchanger like mesh in the gram-positive cell envelopes that is required for creating a locally acidified milieu to govern the pH-dependent activity of autolysins.

Global regulatory functions of the Staphylococcus aureus endoribonuclease III in gene expression

RNA turnover plays an important role in both virulence and adaptation to stress in the Gram-positive human pathogen Staphylococcus aureus. However, the molecular players and mechanisms involved in these processes are poorly understood. Here, we explored the functions of S. aureus endoribonuclease III (RNase III), a member of the ubiquitous family of double-strand-specific endoribonucleases. To define genomic transcripts that are bound and processed by RNase III, we performed deep sequencing on cDNA libraries generated from RNAs that were co-immunoprecipitated with wild-type RNase III or two different cleavage-defective mutant variants in vivo. Several newly identified RNase III targets were validated by independent experimental methods. We identified various classes of structured RNAs as RNase III substrates and demonstrated that this enzyme is involved in the maturation of rRNAs and tRNAs, regulates the turnover of mRNAs and non-coding RNAs, and autoregulates its synthesis by cleaving within the coding region of its own mRNA. Moreover, we identified a positive effect of RNase III on protein synthesis based on novel mechanisms. RNase III–mediated cleavage in the 5′ untranslated region (5′UTR) enhanced the stability and translation of cspA mRNA, which encodes the major cold-shock protein. Furthermore, RNase III cleaved overlapping 5′UTRs of divergently transcribed genes to generate leaderless mRNAs, which constitutes a novel way to co-regulate neighboring genes. In agreement with recent findings, low abundance antisense RNAs covering 44% of the annotated genes were captured by co-immunoprecipitation with RNase III mutant proteins. Thus, in addition to gene regulation, RNase III is associated with RNA quality control of pervasive transcription. Overall, this study illustrates the complexity of post-transcriptional regulation mediated by RNase III.

Control of mRNA stability is crucial for bacteria to survive and rapidly adapt to environmental changes and stress conditions. The molecular players and the degradation pathways involved in these adaptive processes are poorly understood in Staphylococcus aureus. The universally conserved double-strand-specific endoribonuclease III (RNase III) in S. aureus is known to repress the synthesis of several virulence factors and was recently implicated in genome-wide mRNA processing mediated by antisense transcripts. We present here the first global map of direct RNase III targets in S. aureus. Deep sequencing was used to identify RNAs associated with epitope-tagged wild-type RNase III and two catalytically impaired but binding-competent mutant proteins in vivo. Experimental validation revealed an unexpected variety of structured RNA transcripts as novel RNase III substrates. In addition to rRNA operon maturation, autoregulation, degradation of structured RNAs, and antisense regulation, we propose novel mechanisms by which RNase III increases mRNA translation. Overall, this study shows that RNase III has a broad function in gene regulation of S. aureus. We can now address more specifically the roles of this universally conserved enzyme in gene regulation in response to stress and during host infection.

Deletion of hypothetical wall teichoic acid ligases in Staphylococcus aureus activates the cell wall stress response

The Staphylococcus aureus cell wall stress stimulon (CWSS) is activated by cell envelope-targeting antibiotics or depletion of essential cell wall biosynthesis enzymes. The functionally uncharacterized S. aureus LytR-CpsA-Psr (LCP) proteins, MsrR, SA0908 and SA2103, all belong to the CWSS. Although not essential, deletion of all three LCP proteins severely impairs cell division. We show here that VraSR-dependent CWSS expression was up to 250-fold higher in single, double and triple LCP mutants than in wild type S. aureus in the absence of external stress. The LCP triple mutant was virtually depleted of wall teichoic acids (WTA), which could be restored to different degrees by any of the single LCP proteins. Subinhibitory concentrations of tunicamycin, which inhibits the first WTA synthesis enzyme TarO (TagO), could partially complement the severe growth defect of the LCP triple mutant. Both of the latter findings support a role for S. aureus LCP proteins in late WTA synthesis, as in Bacillus subtilis where LCP proteins were recently proposed to transfer WTA from lipid carriers to the cell wall peptidoglycan. Intrinsic activation of the CWSS upon LCP deletion and the fact that LCP proteins were essential for WTA-loading of the cell wall, highlight their important role(s) in S. aureus cell envelope biogenesis.

Role of the Group B antigen of Streptococcus agalactiae: a peptidoglycan-anchored polysaccharide involved in cell wall biogenesis

Streptococcus agalactiae (Group B streptococcus, GBS) is a leading cause of infections in neonates and an emerging pathogen in adults. The Lancefield Group B carbohydrate (GBC) is a peptidoglycan-anchored antigen that defines this species as a Group B Streptococcus. Despite earlier immunological and biochemical characterizations, the function of this abundant glycopolymer has never been addressed experimentally. Here, we inactivated the gene gbcO encoding a putative UDP-N-acetylglucosamine-1-phosphate:lipid phosphate transferase thought to catalyze the first step of GBC synthesis. Indeed, the gbcO mutant was unable to synthesize the GBC polymer, and displayed an important growth defect in vitro. Electron microscopy study of the GBC-depleted strain of S. agalactiae revealed a series of growth-related abnormalities: random placement of septa, defective cell division and separation processes, and aberrant cell morphology. Furthermore, vancomycin labeling and peptidoglycan structure analysis demonstrated that, in the absence of GBC, cells failed to initiate normal PG synthesis and cannot complete polymerization of the murein sacculus. Finally, the subcellular localization of the PG hydrolase PcsB, which has a critical role in cell division of streptococci, was altered in the gbcO mutant. Collectively, these findings show that GBC is an essential component of the cell wall of S. agalactiae whose function is reminiscent of that of conventional wall teichoic acids found in Staphylococcus aureus or Bacillus subtilis. Furthermore, our findings raise the possibility that GBC-like molecules play a major role in the growth of most if not all beta-hemolytic streptococci.

Wall teichoic acid protects Staphylococcus aureus from inhibition by Congo red and other dyes

OBJECTIVES

Polyanionic polymers, including lipoteichoic acid and wall teichoic acid, are important determinants of the charged character of the staphylococcal cell wall. This study was designed to investigate the extent to which teichoic acid contributes to protection from anionic azo dyes and to identify barriers to drug penetration for development of new antibiotics for multidrug-resistant Staphylococcus aureus infection.

METHODS

We studied antimicrobial activity of azo dyes against S. aureus strains with or without inhibition of teichoic acid in vitro and in vivo.

RESULTS

We observed that inhibition of wall teichoic acid expression resulted in an ∼1000-fold increase in susceptibility to azo dyes such as Congo red, reducing its MIC from >1024 to <4 mg/L. Sensitization occurred when the first step in the wall teichoic acid pathway, catalysed by TarO, was inhibited either by mutation or by chemical inhibition. In contrast, genetic blockade of lipoteichoic acid biosynthesis did not confer Congo red susceptibility. Based on this finding, combination therapy was tested using the highly synergistic combination of Congo red plus tunicamycin at sub-MIC concentrations (to inhibit wall teichoic acid biosynthesis). The combination rescued Caenorhabditis elegans from a lethal challenge of S. aureus.

CONCLUSIONS

Our studies show that wall teichoic acid confers protection to S. aureus from anionic azo dyes and related compounds, and its inhibition raises the prospect of development of new combination therapies based on this inhibition.

Localization and structural analysis of a conserved pyruvylated epitope in Bacillus anthracis secondary cell wall polysaccharides and characterization of the galactose-deficient wall polysaccharide from avirulent B. anthracis CDC 684

Bacillus anthracis CDC 684 is a naturally occurring, avirulent variant and close relative of the highly pathogenic B. anthracis Vollum. Bacillus anthracis CDC 684 contains both virulence plasmids, pXO1 and pXO2, yet is non-pathogenic in animal models, prompting closer scrutiny of the molecular basis of attenuation. We structurally characterized the secondary cell wall polysaccharide (SCWP) of B. anthracis CDC 684 (Ba684) using chemical and NMR spectroscopy analysis. The SCWP consists of a HexNAc trisaccharide backbone having identical structure as that of B. anthracis Pasteur, Sterne and Ames, →4)-β-d-ManpNAc-(1 → 4)-β-d-GlcpNAc-(1 → 6)-α-d-GlcpNAc-(1→. Remarkably, although the backbone is fully polymerized, the SCWP is the devoid of all galactosyl side residues, a feature which normally comprises 50% of the glycosyl residues on the highly galactosylated SCWPs from pathogenic strains. This observation highlights the role of defective wall assembly in virulence and indicates that polymerization occurs independently of galactose side residue attachment. Of particular interest, the polymerized Ba684 backbone retains the substoichiometric pyruvate acetal, O-acetate and amino group modifications found on SCWPs from normal B. anthracis strains, and immunofluorescence analysis confirms that SCWP expression coincides with the ability to bind the surface layer homology (SLH) domain containing S-layer protein extractable antigen-1. Pyruvate was previously demonstrated as part of a conserved epitope, mediating SLH-domain protein attachment to the underlying peptidoglycan layer. We find that a single repeating unit, located at the distal (non-reducing) end of the Ba684 SCWP, is structurally modified and that this modification is present in identical manner in the SCWPs of normal B. anthracis strains. These polysaccharides terminate in the sequence: (S)-4,6-O-(1-carboxyethylidene)-β-d-ManpNAc-(1 → 4)-[3-O-acetyl]-β-d-GlcpNAc-(1 → 6)-α-d-GlcpNH(2)-(1→.

Biosynthesis of teichoic acids in Streptococcus pneumoniae and closely related species: lessons from genomes

The cell wall of Streptococcus pneumoniae contains an unusually complex wall teichoic acid (WTA), which has identical repeating units as the membrane-anchored lipoteichoic acid (LTA). Both polymers share a common cytoplasmic pathway of precursor synthesis, but several TA enzymes have remained elusive. Bioinformatic analysis of the genome of various pneumococcal strains, including choline-independent mutant strains, has allowed us to identify the missing TA genes. We present here the deduced complete pathways of WTA and LTA synthesis in S. pneumoniae and point to the variations occurring in different pneumococcal strains and in closely related species such as Streptococcus oralis and Streptococcus mitis.

Comparative genomics of enterococci: variation in Enterococcus faecalis, clade structure in E. faecium, and defining characteristics of E. gallinarum and E. casseliflavus

The enterococci are Gram-positive lactic acid bacteria that inhabit the gastrointestinal tracts of diverse hosts. However, Enterococcus faecium and E. faecalis have emerged as leading causes of multidrug-resistant hospital-acquired infections. The mechanism by which a well-adapted commensal evolved into a hospital pathogen is poorly understood. In this study, we examined high-quality draft genome data for evidence of key events in the evolution of the leading causes of enterococcal infections, including E. faecalis, E. faecium, E. casseliflavus, and E. gallinarum. We characterized two clades within what is currently classified as E. faecium and identified traits characteristic of each, including variation in operons for cell wall carbohydrate and putative capsule biosynthesis. We examined the extent of recombination between the two E. faecium clades and identified two strains with mosaic genomes. We determined the underlying genetics for the defining characteristics of the motile enterococci E. casseliflavus and E. gallinarum. Further, we identified species-specific traits that could be used to advance the detection of medically relevant enterococci and their identification to the species level.

The enterococci, in particular, vancomycin-resistant enterococci, have emerged as leading causes of multidrug-resistant hospital-acquired infections. In this study, we examined genome sequence data to define traits with the potential to influence host-microbe interactions and to identify sequences and biochemical functions that could form the basis for the rapid identification of enterococcal species or lineages of importance in clinical and environmental samples.

Phosphate-containing cell wall polymers of bacilli

Anionic phosphate-containing cell wall polymers of bacilli are represented by teichoic acids and poly(glycosyl 1-phosphates). Different locations of phosphodiester bonds in the main chain of teichoic acids as well as the nature and combination of the constituent structural elements underlie their structural diversity. Currently, the structures of teichoic acids of bacilli can be classified into three types, viz. poly(polyol phosphates) with glycerol or ribitol as the polyol; poly(glycosylpolyol phosphates), mainly glycerol-containing polymers; and poly(acylglycosylglycerol phosphate), in which the components are covalently linked through glycosidic, phosphodiester, and amide bonds. In addition to teichoic acids, poly(glycosyl 1-phosphates) with mono- and disaccharide residues in the repeating units have been detected in cell walls of several Bacillus subtilis and Bacillus pumilus strains. The known structures of teichoic acids and poly(glycosyl 1-phosphates) of B. subtilis, B. atrophaeus, B. licheniformis, B. pumilus, B. stearothermophilus, B. coagulans, B. cereus as well as oligomers that link the polymers to peptidoglycan are surveyed. The reported data on the structures of phosphate-containing polymers of different strains of B. subtilis suggest heterogeneity of the species and may be of interest for the taxonomy of bacilli to allow differentiation of closely related organisms according to the "structures and composition of cell wall polymers" criterion.