O-Acetylation of peptidoglycan is required for proper cell separation and S-layer anchoring in Bacillus anthracis

VimA-dependent modulation of the secretome in Porphyromonas gingivalis

The VimA protein of Porphyromonas gingivalis is a multifunctional protein involved in cell surface biogenesis. To further determine if its acetyl coenzyme A (acetyl-CoA) transfer and putative sorting functions can affect the secretome, its role in peptidoglycan biogenesis and effects on the extracellular proteins of P. gingivalis FLL92, a vimA-defective mutant, were evaluated. There were structural and compositional differences in the peptidoglycan of P. gingivalis FLL92 compared with the wild-type strain. Sixty-eight proteins were present only in the extracellular fraction of FLL92. Fifteen proteins present in the extracellular fraction of the parent strain were missing in the vimA-defective mutant. These proteins had protein sorting characteristics that included a C-terminal motif with a common consensus Gly-Gly-CTERM pattern and a polar tail consisting of aromatic amino acid residues. These observations suggest that the VimA protein is likely involved in peptidoglycan synthesis, and corroborates our previous report, which suggests a role in protein sorting.

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→.

A conformational switch controls cell wall-remodelling enzymes required for bacterial cell division

Remodelling of the peptidoglycan (PG) exoskeleton is intimately tied to the growth and division of bacteria. Enzymes that hydrolyse PG are critical for these processes, but their activities must be tightly regulated to prevent the generation of lethal breaches in the PG matrix. Despite their importance, the mechanisms regulating PG hydrolase activity have remained elusive. Here we investigate the control of cell division hydrolases called amidases (AmiA, AmiB and AmiC) required for Escherichia coli cell division. Poorly regulated amiB mutants were isolated encoding lytic AmiB variants with elevated basal PG hydrolase activities in vitro. The structure of an AmiB orthologue was also solved, revealing that the active site of AmiB is occluded by a conserved alpha helix. Strikingly, most of the amino acid substitutions in the lytic AmiB variants mapped to this domain and are predicted to disrupt its interaction with the active site. Our results therefore support a model in which cell separation is stimulated by the reversible relief of amidase autoinhibition governed by conserved subcomplexes within the cytokinetic ring. Analogous conformational control mechanisms are likely to be part of a general strategy used to control PG hydrolases present within multienzyme PG-remodelling machines.

Lysozyme resistance in Streptococcus suis is highly variable and multifactorial

Background

Streptococcus suis is an important infectious agent for pigs and occasionally for humans. The host innate immune system plays a key role in preventing and eliminating S. suis infections. One important constituent of the innate immune system is the protein lysozyme, which is present in a variety of body fluids and immune cells. Lysozyme acts as a peptidoglycan degrading enzyme causing bacterial lysis. Several pathogens have developed mechanisms to evade lysozyme-mediated killing. In the present study we compared the lysozyme sensitivity of various S. suis isolates and investigated the molecular basis of lysozyme resistance for this pathogen.

Results

The lysozyme minimal inhibitory concentrations of a wide panel of S. suis isolates varied between 0.3 to 10 mg/ml. By inactivating the oatA gene in a serotype 2 and a serotype 9 strain, we showed that OatA-mediated peptidoglycan modification partly contributes to lysozyme resistance. Furthermore, inactivation of the murMN operon provided evidence that additional peptidoglycan crosslinking is not involved in lysozyme resistance in S. suis. Besides a targeted approach, we also used an unbiased approach for identifying factors involved in lysozyme resistance. Based on whole genome comparisons of a lysozyme sensitive strain and selected lysozyme resistant derivatives, we detected several single nucleotide polymorphisms (SNPs) that were correlated with the lysozyme resistance trait. Two SNPs caused defects in protein expression of an autolysin and a capsule sugar transferase. Analysis of specific isogenic mutants, confirmed the involvement of autolysin activity and capsule structures in lysozyme resistance of S. suis.

Conclusions

This study shows that lysozyme resistance levels are highly variable among S. suis isolates and serotypes. Furthermore, the results show that lysozyme resistance in S. suis can involve different mechanisms including OatA-mediated peptidolycan modification, autolysin activity and capsule production.

CpsY influences Streptococcus iniae cell wall adaptations important for neutrophil intracellular survival

The ability of a pathogen to evade neutrophil phagocytic killing mechanisms is critically important for dissemination and establishment of a systemic infection. Understanding how pathogens overcome these innate defenses is essential for the development of optimal therapeutic strategies for invasive infections. CpsY is a conserved transcriptional regulator previously identified as an important virulence determinant for systemic infection of Streptococcus iniae. While orthologs of CpsY have been associated with the regulation of methionine metabolism and uptake pathways, CpsY additionally functions in protection from neutrophil-mediated killing. S. iniae does not alter neutrophil phagosomal maturation but instead is able to adapt to the extreme bactericidal environment of a mature neutrophil phagosome, a property dependent upon CpsY. This CpsY-dependent adaptation appears to involve stabilization of the cell wall through peptidoglycan O-acetylation and repression of cellular autolysins. Furthermore, S. iniae continues to be a powerful model for investigation of bacterial adaptations during systemic streptococcal infection.

Identification of the first known inhibitors of O-acetylpeptidoglycan esterase: a potential new antibacterial target

The O-acetylation of peptidoglycan (PG) is now known to occur in 53 species, including numerous human pathogens such as, Staphylococcus aureus, Bacillus anthracis, species of Enterococcus, Campylobacter jejuni, Helicobacter pylori, Neisseria gonorrhoeae and N. meningitidis. This modification, which occurs at the C-6 hydroxyl of N-acetylmuramoyl residues within PG, serves to regulate autolytic activity during PG metabolism and contributes to pathogenesis and persistence within a host. O-Acetylpeptidoglycan esterase (Ape) was recently discovered as an enzyme responsible for the removal of O-acetyl groups from PG, thus permitting the continued maintenance and metabolism of the sacculus. Recombinant Ape1 from N. gonorrhoeae was purified to apparent homogeneity and optimal storage conditions for the enzyme were determined. Using 4-methylumbelliferyl acetate as substrate, a fluorogenic assay amenable for the high-throughput screening for potential inhibitors was developed and Ape1 was screened against a subset of compounds of the Canadian Compound Collection. The overall Z' score for the screen was 0.62, indicative of a well-suited assay with a sufficient signal window, and the threshold was set at 65 %. After eliminating a number of false-positives, seven compounds were identified as true inhibitors of Ape1, the first to be identified for this class of enzyme. Dose-response curves were generated leading to the identification of five of these compounds with IC(50) values ranging between 0.3 and 23 μM. Of these, purpurin was selected for further analysis and it was found to inhibit the growth of both Gram-positive and Gram-negative bacteria that produce both O-acetylated PG and Ape.

Identification and in vitro analysis of the GatD/MurT enzyme-complex catalyzing lipid II amidation in Staphylococcus aureus

The peptidoglycan of Staphylococcus aureus is characterized by a high degree of crosslinking and almost completely lacks free carboxyl groups, due to amidation of the D-glutamic acid in the stem peptide. Amidation of peptidoglycan has been proposed to play a decisive role in polymerization of cell wall building blocks, correlating with the crosslinking of neighboring peptidoglycan stem peptides. Mutants with a reduced degree of amidation are less viable and show increased susceptibility to methicillin. We identified the enzymes catalyzing the formation of D-glutamine in position 2 of the stem peptide. We provide biochemical evidence that the reaction is catalyzed by a glutamine amidotransferase-like protein and a Mur ligase homologue, encoded by SA1707 and SA1708, respectively. Both proteins, for which we propose the designation GatD and MurT, are required for amidation and appear to form a physically stable bi-enzyme complex. To investigate the reaction in vitro we purified recombinant GatD and MurT His-tag fusion proteins and their potential substrates, i.e. UDP-MurNAc-pentapeptide, as well as the membrane-bound cell wall precursors lipid I, lipid II and lipid II-Gly₅. In vitro amidation occurred with all bactoprenol-bound intermediates, suggesting that in vivo lipid II and/or lipid II-Gly₅ may be substrates for GatD/MurT. Inactivation of the GatD active site abolished lipid II amidation. Both, murT and gatD are organized in an operon and are essential genes of S. aureus. BLAST analysis revealed the presence of homologous transcriptional units in a number of gram-positive pathogens, e.g. Mycobacterium tuberculosis, Streptococcus pneumonia and Clostridium perfringens, all known to have a D-iso-glutamine containing PG. A less negatively charged PG reduces susceptibility towards defensins and may play a general role in innate immune signaling.

O-acetylation of plant cell wall polysaccharides

Plant cell walls are composed of structurally diverse polymers, many of which are O-acetylated. How plants O-acetylate wall polymers and what its function is remained elusive until recently, when two protein families were identified in the model plant Arabidopsis that are involved in the O-acetylation of wall polysaccharides - the reduced wall acetylation (RWA) and the trichome birefringence-like (TBL) proteins. This review discusses the role of these two protein families in polysaccharide O-acetylation and outlines the differences and similarities of polymer acetylation mechanisms in plants, fungi, bacteria, and mammals. Members of the TBL protein family had been shown to impact pathogen resistance, freezing tolerance, and cellulose biosynthesis. The connection of TBLs to polysaccharide O-acetylation thus gives crucial leads into the biological function of wall polymer O-acetylation. From a biotechnological point understanding the O-acetylation mechanism is important as acetyl-substituents inhibit the enzymatic degradation of wall polymers and released acetate can be a potent inhibitor in microbial fermentations, thus impacting the economic viability of, e.g., lignocellulosic based biofuel production.

More than just lysins: peptidoglycan hydrolases tailor the cell wall

Enzymes that degrade the peptidoglycan (PG) cell wall layer called PG hydrolases or autolysins are often thought of as destructive forces. Phages employ them to lyse their host for the release of virion particles and some bacteria secrete them to eliminate (lyse) their competition. However, bacteria also harness the activity of PG hydrolases for important aspects of growth, division, and development. Of course, using PG hydrolases in this capacity requires that they be tightly regulated. While this has been appreciated for some time, we are only just beginning to understand the mechanisms governing the activities of these 'tailoring' enzymes. This review will focus on recent advances in this area with an emphasis on the regulation of PG hydrolases involved in cell division.

Recent progress in Bacillus subtilis sporulation

The Gram-positive bacterium Bacillus subtilis can initiate the process of sporulation under conditions of nutrient limitation. Here, we review some of the last 5 years of work in this area, with a particular focus on the decision to initiate sporulation, DNA translocation, cell-cell communication, protein localization and spore morphogenesis. The progress we describe has implications not only just for the study of sporulation but also for other biological systems where homologs of sporulation-specific proteins are involved in vegetative growth.

Bacillus subtilis σ(V) confers lysozyme resistance by activation of two cell wall modification pathways, peptidoglycan O-acetylation and D-alanylation of teichoic acids

The seven extracytoplasmic function (ECF) sigma (σ) factors of Bacillus subtilis are broadly implicated in resistance to antibiotics and other cell envelope stressors mediated, in part, by regulation of cell envelope synthesis and modification enzymes. We here define the regulon of σ(V) as including at least 20 operons, many of which are also regulated by σ(M), σ(X), or σ(W). The σ(V) regulon is strongly and specifically induced by lysozyme, and this induction is key to the intrinsic resistance of B. subtilis to lysozyme. Strains with null mutations in either sigV or all seven ECF σ factor genes (Δ7ECF) have essentially equal increases in sensitivity to lysozyme. Induction of σ(V) in the Δ7ECF background restores lysozyme resistance, whereas induction of σ(M), σ(X), or σ(W) does not. Lysozyme resistance results from the ability of σ(V) to activate the transcription of two operons: the autoregulated sigV-rsiV-oatA-yrhK operon and dltABCDE. Genetic analyses reveal that oatA and dlt are largely redundant with respect to lysozyme sensitivity: single mutants are not affected in lysozyme sensitivity, whereas an oatA dltA double mutant is as sensitive as a sigV null strain. Moreover, the sigV oatA dltA triple mutant is no more sensitive than the oatA dltA double mutant, indicating that there are no other σ(V)-dependent genes necessary for lysozyme resistance. Thus, we suggest that σ(V) confers lysozyme resistance by the activation of two cell wall modification pathways: O-acetylation of peptidoglycan catalyzed by OatA and D-alanylation of teichoic acids by DltABCDE.

Characterization of O-acetylation of N-acetylglucosamine: a novel structural variation of bacterial peptidoglycan

Peptidoglycan (PG) N-acetyl muramic acid (MurNAc) O-acetylation is widely spread in gram-positive bacteria and is generally associated with resistance against lysozyme and endogenous autolysins. We report here the presence of O-acetylation on N-acetylglucosamine (GlcNAc) in Lactobacillus plantarum PG. This modification of glycan strands was never described in bacteria. Fine structural characterization of acetylated muropeptides released from L. plantarum PG demonstrated that both MurNAc and GlcNAc are O-acetylated in this species. These two PG post-modifications rely on two dedicated O-acetyltransferase encoding genes, named oatA and oatB, respectively. By analyzing the resistance to cell wall hydrolysis of mutant strains, we showed that GlcNAc O-acetylation inhibits N-acetylglucosaminidase Acm2, the major L. plantarum autolysin. In this bacterial species, inactivation of oatA, encoding MurNAc O-acetyltransferase, resulted in marked sensitivity to lysozyme. Moreover, MurNAc over-O-acetylation was shown to activate autolysis through the putative N-acetylmuramoyl-L-alanine amidase LytH enzyme. Our data indicate that in L. plantarum, two different O-acetyltransferases play original and antagonistic roles in the modulation of the activity of endogenous autolysins.