Bacterial cell-wall recycling

Interplay between Peptidoglycan Biology and Virulence in Gram-Negative Pathogens

The clinical and epidemiological threat of the growing antimicrobial resistance in Gram-negative pathogens, particularly for β-lactams, the most frequently used and relevant antibiotics, urges research to find new therapeutic weapons to combat the infections caused by these microorganisms. An essential previous step in the development of these therapeutic solutions is to identify their potential targets in the biology of the pathogen. This is precisely what we sought to do in this review specifically regarding the barely exploited field analyzing the interplay among the biology of the peptidoglycan and related processes, such as β-lactamase regulation and virulence. Hence, here we gather, analyze, and integrate the knowledge derived from published works that provide information on the topic, starting with those dealing with the historically neglected essential role of the Gram-negative peptidoglycan in virulence, including structural, biogenesis, remodeling, and recycling aspects, in addition to proinflammatory and other interactions with the host. We also review the complex link between intrinsic β-lactamase production and peptidoglycan metabolism, as well as the biological costs potentially associated with the expression of horizontally acquired β-lactamases. Finally, we analyze the existing evidence from multiple perspectives to provide useful clues for identifying targets enabling the future development of therapeutic options attacking the peptidoglycan-virulence interconnection as a key weak point of the Gram-negative pathogens to be used, if not to kill the bacteria, to mitigate their capacity to produce severe infections.

Proteomic analysis of Lactobacillus casei GCRL163 cell-free extracts reveals a SecB homolog and other biomarkers of prolonged heat stress

Prolonged heat stress is one of the harsh conditions Lactobacillus casei strains encounter as non-starter lactic acid bacteria in dairy product manufacture. To understand the physiological and molecular mechanisms through which Lb. casei GCRL163 adapts to persistent elevated temperature, label-free quantitative proteomics of cell-free extracts was used to characterize the global responses of the strain cultured anaerobically in bioreactors at 30 to 45°C, pH 6.5, together with GC-MS for fatty acid methyl ester analysis at different growth phases. At higher growth temperatures, repression of energy-consuming metabolic pathways, such as fatty acid, nucleotide and amino acid biosynthesis, was observed, while PTS- and ABC-type transporter systems associated with uptake of nitrogen and carbon sources were up-regulated. Alkaline shock protein Asp23_2 was only detected at 45°C, expressed at high abundance, and presumptive α-L-fucosidase only at 40 and 45°C, with highly increased abundance (log2-fold change of 7) at 45°C. We identified a novel SecB homolog as a protein export chaperone putatively involved in posttranslational translocation systems, which was down-regulated as growth temperature increased and where the modelled 3D-structure shared architectural similarities with the Escherichia coli SecB protein. Membrane lipid analyses revealed temporal changes in fatty acid composition, cyclization of oleic acid to cyclopropane and novel cyclopentenyl moieties, and reduced synthesis of vaccenic acid, at higher temperatures. An 18kDa α-crystallin domain, Hsp20 family heat shock protein was more highly up-regulated in response to heat stress compared to other molecular chaperones, suggesting this protein could be a useful biomarker of prolonged heat stress in Lb. casei GCRL163.

A Structural Dissection of the Active Site of the Lytic Transglycosylase MltE from Escherichia coli

Lytic transglycosylases (LTs) are bacterial enzymes that catalyze the cleavage of the glycan strands of the bacterial cell wall. The mechanism of this cleavage is a remarkable intramolecular transacetalization reaction, accomplished by an ensemble of active-site residues. Because the LT reaction occurs in parallel with the cell wall bond-forming reactions catalyzed by the penicillin-binding proteins, simultaneous inhibition of both enzymes can be particularly bactericidal to Gram-negative bacteria. The MltE lytic transglycosylase is the smallest of the eight LTs encoded by the Escherichia coli genome. Prior crystallographic and computational studies identified four active-site residues-E64, S73, S75, and Y192-as playing roles in catalysis. Each of these four residues was individually altered by mutation to give four variant enzymes (E64Q, S73A, S75A, and Y192F). All four variants showed reduced catalytic activity [soluble wild type (100%) > soluble Y192F and S75A (both 40%) > S73A (4%) > E64Q (≤1%)]. The crystal structure of each variant protein was determined at the resolution of 2.12 Å for E64Q, 2.33 Å for Y192F, 1.38 Å for S73A, and 1.35 Å for S75A. These variants show alteration of the hydrogen-bond interactions of the active site. Within the framework of a prior computational study of the LT mechanism, we suggest the mechanistic role of these four active-site residues in MltE catalysis.

Cell-Wall Recycling of the Gram-Negative Bacteria and the Nexus to Antibiotic Resistance

The importance of the cell wall to the viability of the bacterium is underscored by the breadth of antibiotic structures that act by blocking key enzymes that are tasked with cell-wall creation, preservation, and regulation. The interplay between cell-wall integrity, and the summoning forth of resistance mechanisms to deactivate cell-wall-targeting antibiotics, involves exquisite orchestration among cell-wall synthesis and remodeling and the detection of and response to the antibiotics through modulation of gene regulation by specific effectors. Given the profound importance of antibiotics to the practice of medicine, the assertion that understanding this interplay is among the most fundamentally important questions in bacterial physiology is credible. The enigmatic regulation of the expression of the AmpC β-lactamase, a clinically significant and highly regulated resistance response of certain Gram-negative bacteria to the β-lactam antibiotics, is the exemplar of this challenge. This review gives a current perspective to this compelling, and still not fully solved, 35-year enigma.

Structural and functional determination of homologs of the Mycobacterium tuberculosis N-acetylglucosamine-6-phosphate deacetylase (NagA)

The Mycobacterium tuberculosis (Mtb) pathogen encodes a GlcNAc-6-phosphate deacetylase enzyme, NagA (Rv3332), that belongs to the amidohydrolase superfamily. NagA enzymes catalyze the deacetylation of GlcNAc-6-phosphate (GlcNAc6P) to glucosamine-6-phosphate (GlcN6P). NagA is a potential antitubercular drug target because it represents the key enzymatic step in the generation of essential amino-sugar precursors required for Mtb cell wall biosynthesis and also influences recycling of cell wall peptidoglycan fragments. Here, we report the structural and functional characterization of NagA from Mycobacterium smegmatis (MSNagA) and Mycobacterium marinum (MMNagA), close relatives of Mtb. Using a combination of X-ray crystallography, site-directed mutagenesis, and biochemical and biophysical assays, we show that these mycobacterial NagA enzymes are selective for GlcNAc6P. Site-directed mutagenesis studies revealed crucial roles of conserved residues in the active site that underpin stereoselective recognition, binding, and catalysis of substrates. Moreover, we report the crystal structure of MSNagA in both ligand-free form and in complex with the GlcNAc6P substrate at 2.6 and 2.0 Å resolutions, respectively. The GlcNAc6P complex structure disclosed the precise mode of GlcNAc6P binding and the structural framework of the active site, including two divalent metals located in the α/β binuclear site. Furthermore, we observed a cysteine residue located on a flexible loop region that occludes the active site. This cysteine is unique to mycobacteria and may represent a unique subsite for targeting mycobacterial NagA enzymes. Our results provide critical insights into the structural and mechanistic properties of mycobacterial NagA enzymes having an essential role in amino-sugar and nucleotide metabolism in mycobacteria.

Exolytic and endolytic turnover of peptidoglycan by lytic transglycosylase Slt of Pseudomonas aeruginosa

β-Lactam antibiotics inhibit cell-wall transpeptidases, preventing the peptidoglycan, the major constituent of the bacterial cell wall, from cross-linking. This causes accumulation of long non-cross-linked strands of peptidoglycan, which leads to bacterial death. Pseudomonas aeruginosa, a nefarious bacterial pathogen, attempts to repair this aberrantly formed peptidoglycan by the function of the lytic transglycosylase Slt. We document in this report that Slt turns over the peptidoglycan by both exolytic and endolytic reactions, which cause glycosidic bond scission from a terminus or in the middle of the peptidoglycan, respectively. These reactions were characterized with complex synthetic peptidoglycan fragments that ranged in size from tetrasaccharides to octasaccharides. The X-ray structure of the wild-type apo Slt revealed it to be a doughnut-shaped protein. In a series of six additional X-ray crystal structures, we provide insights with authentic substrates into how Slt is enabled for catalysis for both the endolytic and exolytic reactions. The substrate for the exolytic reaction binds Slt in a canonical arrangement and reveals how both the glycan chain and the peptide stems are recognized by the Slt. We document that the apo enzyme does not have a fully formed active site for the endolytic reaction. However, binding of the peptidoglycan at the existing subsites within the catalytic domain causes a conformational change in the protein that assembles the surface for binding of a more expansive peptidoglycan between the catalytic domain and an adjacent domain. The complexes of Slt with synthetic peptidoglycan substrates provide an unprecedented snapshot of the endolytic reaction.

Mechanisms of antimicrobial resistance among hospital-associated pathogens

INTRODUCTION

The introduction of antibiotics revolutionized medicine in the 20th-century permitting the treatment of once incurable infections. Widespread use of antibiotics, however, has led to the development of resistant organisms, particularly in the healthcare setting. Today, the clinician is often faced with pathogens carrying a cadre of resistance determinants that severely limit therapeutic options. The genetic plasticity of microbes allows them to adapt to stressors via genetic mutations, acquisition or sharing of genetic material and modulation of genetic expression leading to resistance to virtually any antimicrobial used in clinical practice. Areas covered: This is a comprehensive review that outlines major mechanisms of resistance in the most common hospital-associated pathogens including bacteria and fungi. Expert commentary: Understanding the genetic and biochemical mechanisms of such antimicrobial adaptation is crucial to tackling the rapid spread of resistance, can expose unconventional therapeutic targets to combat multidrug resistant pathogens and lead to more accurate prediction of antimicrobial susceptibility using rapid molecular diagnostics. Clinicians making treatment decisions based on the molecular basis of resistance may design therapeutic strategies that include de-escalation of broad spectrum antimicrobial usage, more focused therapies or combination therapies. These strategies are likely to improve patient outcomes and decrease the risk of resistance in hospital settings.

Imaging Bacterial Cell Wall Biosynthesis

Peptidoglycan is an essential component of the cell wall that protects bacteria from environmental stress. A carefully coordinated biosynthesis of peptidoglycan during cell elongation and division is required for cell viability. This biosynthesis involves sophisticated enzyme machineries that dynamically synthesize, remodel, and degrade peptidoglycan. However, when and where bacteria build peptidoglycan, and how this is coordinated with cell growth, have been long-standing questions in the field. The improvement of microscopy techniques has provided powerful approaches to study peptidoglycan biosynthesis with high spatiotemporal resolution. Recent development of molecular probes further accelerated the growth of the field, which has advanced our knowledge of peptidoglycan biosynthesis dynamics and mechanisms. Here, we review the technologies for imaging the bacterial cell wall and its biosynthesis activity. We focus on the applications of fluorescent d-amino acids, a newly developed type of probe, to visualize and study peptidoglycan synthesis and dynamics, and we provide direction for prospective research.

Peptidoglycan synthesis in Tannerella forsythia: Scavenging is the modus operandi

Tannerella forsythia is a Gram-negative oral pathogen strongly associated with periodontitis. This bacterium has an absolute requirement for exogenous N-acetylmuramic acid (MurNAc), an amino sugar that forms the repeating disaccharide unit with amino sugar N-acetylglucosamine (GlcNAc) of the peptidoglycan backbone. In silico genome analysis indicates that T. forsythia lacks the key biosynthetic enzymes needed for the de novo synthesis of MurNAc, and so relies on alternative ways to meet its requirement for peptidoglycan biosynthesis. In the subgingival niche, the bacterium can acquire MurNAc and peptidoglycan fragments (muropeptides) released by the cohabiting bacteria during their cell wall breakdown associated with cell division. Tannerella forsythia is able to also use host sialic acid (Neu5Ac) in lieu of MurNAc or muropeptides for its survival during the biofilm growth. Evidence suggests that the bacterium might be able to shunt sialic acid into a metabolic pathway leading to peptidoglycan synthesis. In this review, we explore the mechanisms by which T. forsythia is able to scavenge MurNAc, muropeptide and sialic acid for its peptidoglycan synthesis, and the impact of these scavenging activities on pathogenesis.

Cell Wall Biogenesis During Elongation and Division in the Plant Pathogen Agrobacterium tumefaciens

A great diversity of bacterial cell shapes can be found in nature, suggesting that cell wall biogenesis is regulated both spatially and temporally. Although Agrobacterium tumefaciens has a rod-shaped morphology, the mechanisms underlying cell growth are strikingly different than other well-studied rod-shaped bacteria including Escherichia coli. Technological advances, such as the ability to deplete essential genes and the development of fluorescent D-amino acids, have enabled recent advances in our understanding of cell wall biogenesis during cell elongation and division of A. tumefaciens. In this review, we address how the field has evolved over the years by providing a historical overview of cell elongation and division in rod-shaped bacteria. Next, we summarize the current understanding of cell growth and cell division processes in A. tumefaciens. Finally, we highlight the need for further research to answer key questions related to the regulation of cell wall biogenesis in A. tumefaciens.

Comparative proteomic analysis of Xanthomonas citri ssp. citri periplasmic proteins reveals changes in cellular envelope metabolism during in vitro pathogenicity induction

Citrus canker is a plant disease caused by Gram-negative bacteria from the genus Xanthomonas. The most virulent species is Xanthomonas citri ssp. citri (XAC), which attacks a wide range of citrus hosts. Differential proteomic analysis of the periplasm-enriched fraction was performed for XAC cells grown in pathogenicity-inducing (XAM-M) and pathogenicity-non-inducing (nutrient broth) media using two-dimensional electrophoresis combined with liquid chromatography-tandem mass spectrometry. Amongst the 40 proteins identified, transglycosylase was detected in a highly abundant spot in XAC cells grown under inducing condition. Additional up-regulated proteins related to cellular envelope metabolism included glucose-1-phosphate thymidylyltransferase, dTDP-4-dehydrorhamnose-3,5-epimerase and peptidyl-prolyl cis-trans-isomerase. Phosphoglucomutase and superoxide dismutase proteins, known to be involved in pathogenicity in other Xanthomonas species or organisms, were also detected. Western blot and quantitative real-time polymerase chain reaction analyses for transglycosylase and superoxide dismutase confirmed that these proteins were up-regulated under inducing condition, consistent with the proteomic results. Multiple spots for the 60-kDa chaperonin and glyceraldehyde-3-phosphate dehydrogenase were identified, suggesting the presence of post-translational modifications. We propose that substantial alterations in cellular envelope metabolism occur during the XAC infectious process, which are related to several aspects, from defence against reactive oxygen species to exopolysaccharide synthesis. Our results provide new candidates for virulence-related proteins, whose abundance correlates with the induction of pathogenicity and virulence genes, such as hrpD6, hrpG, hrpB7, hpa1 and hrpX. The results present new potential targets against XAC to be investigated in further functional studies.

B. subtilis LytR-CpsA-Psr Enzymes Transfer Wall Teichoic Acids from Authentic Lipid-Linked Substrates to Mature Peptidoglycan In Vitro

Gram-positive bacteria endow their peptidoglycan with glycopolymers that are crucial for viability and pathogenesis. However, the cellular machinery that executes this function is not well understood. While decades of genetic and phenotypic work have highlighted the LytR-CpsA-Psr (LCP) family of enzymes as cell-wall glycopolymer transferases, their in vitro characterization has been elusive, largely due to a paucity of tools for functional assays. In this report, we synthesized authentic undecaprenyl diphosphate-linked wall teichoic acid (WTA) intermediates and built an assay system capable of monitoring LCP-mediated glycopolymer transfer. We report that all Bacillus subtilis LCP enzymes anchor WTAs to peptidoglycan in vitro. Furthermore, we probed the catalytic requirements and substrate preferences for these LCP enzymes and elaborated in vitro conditions for facile tests of enzyme function. This work sheds light on the molecular features of glycopolymer transfer and aims to aid drug discovery and development programs exploiting this promising antibacterial target.

A Single Dual-Function Enzyme Controls the Production of Inflammatory NOD Agonist Peptidoglycan Fragments by Neisseria gonorrhoeae

Neisseria gonorrhoeae gonococcus (GC) is a Gram-negative betaproteobacterium and causative agent of the sexually transmitted infection gonorrhea. During growth, GC releases lipooligosaccharide (LOS) and peptidoglycan (PG) fragments, which contribute significantly to the inflammatory damage observed during human infection. In ascending infection of human Fallopian tubes, inflammation leads to increased risk of ectopic pregnancy, pelvic inflammatory disease, and sterility. Of the PG fragments released by GC, most are disaccharide peptide monomers, and of those, 80% have tripeptide stems despite the observation that tetrapeptide stems make up 80% of the assembled cell wall. We identified a serine-protease l,d-carboxypeptidase, NGO1274 (LdcA), as the enzyme responsible for converting cell wall tetrapeptide-stem PG to released tripeptide-stem PG. Unlike characterized cytoplasmic LdcA homologs in gammaproteobacteria, LdcA in GC is exported to the periplasm, and its localization is critical for its activity in modifying PG fragments for release. Distinct among other characterized l,d-carboxypeptidases, LdcA from GC is also capable of catalyzing the cleavage of specific peptide cross-bridges (endopeptidase activity). To define the role of ldcA in pathogenesis, we demonstrate that ldcA disruption results in both loss of NOD1-dependent NF-κB activation and decreased NOD2-dependent NF-κB activation while not affecting Toll-like receptor (TLR) agonist release. Since the human intracellular peptidoglycan receptor NOD1 (hNOD1) specifically recognizes PG fragments with a terminal meso-DAP rather than d-alanine, we conclude that LdcA is required for GC to provoke NOD1-dependent responses in cells of the human host.IMPORTANCE The macromolecular meshwork of peptidoglycan serves essential functions in determining bacterial cell shape, protecting against osmotic lysis, and defending cells from external assaults. The conserved peptidoglycan structure, however, is also recognized by eukaryotic pattern recognition receptors, which can trigger immune responses against bacteria. Many bacteria can induce an inflammatory response through the intracellular peptidoglycan receptor NOD1, but Neisseria gonorrhoeae serves as an extreme example, releasing fragments of peptidoglycan into the environment during growth that specifically antagonize human NOD1. Understanding the peptidoglycan breakdown mechanisms that allow Neisseria to promote NOD1 activation, rather than avoiding or suppressing immune detection, is critical to understanding the pathogenesis of this increasingly drug-resistant organism. We identify a peptidoglycan l,d-carboxypeptidase responsible for converting liberated peptidoglycan fragments into the human NOD1 agonist and find that the same enzyme has endopeptidase activity on certain peptidoglycan cross-links, the first described combination of those two activities in a single enzyme.

Bacterial secretion of D-arginine controls environmental microbial biodiversity

Bacteria face tough competition in polymicrobial communities. To persist in a specific niche, many species produce toxic extracellular effectors to interfere with the growth of nearby microbes. These effectors include the recently reported non-canonical D-amino acids (NCDAAs). In Vibrio cholerae, the causative agent of cholera, NCDAAs control cell wall integrity in stationary phase. Here, an analysis of the composition of the extracellular medium of V. cholerae revealed the unprecedented presence of D-Arg. Compared with other D-amino acids, D-Arg displayed higher potency and broader toxicity in terms of the number of bacterial species affected. Tolerance to D-Arg was associated with mutations in the phosphate transport and chaperone systems, whereas D-Met lethality was suppressed by mutations in cell wall determinants. These observations suggest that NCDAAs target different cellular processes. Finally, even though virtually all Vibrio species are tolerant to D-Arg, only a few can produce this D-amino acid. Indeed, we demonstrate that D-Arg may function as part of a cooperative strategy in vibrio communities to protect non-producing members from competing bacteria. Because NCDAA production is widespread in bacteria, we anticipate that D-Arg is a relevant modulator of microbial subpopulations in diverse ecosystems.

AmiD Is a Novel Peptidoglycan Amidase in Wolbachia Endosymbionts of Drosophila melanogaster

Wolbachia endobacteria are obligate intracellular bacteria with a highly reduced genome infecting many arthropod and filarial species, in which they manipulate arthropod reproduction to increase their transmission and are essential for nematode development and survival. The Wolbachia genome encodes all enzymes required for the synthesis of the cell wall building block lipid II, although a peptidoglycan-like structure has not been detected. Despite the ability to synthesize lipid II, Wolbachia from arthropods and nematodes have only a subset of genes encoding enzymes involved in the periplasmic processing of lipid II and peptidoglycan recycling, with arthropods having two more than nematodes. We functionally analyzed the activity of the putative cell wall hydrolase AmiD from the Wolbachia endosymbiont of Drosophila melanogaster, an enzyme not encoded by the nematode endobacteria. Wolbachia AmiD has Zn²⁺-dependent amidase activity and cleaves intact peptidoglycan, monomeric lipid II and anhydromuropeptides, substrates that are generated during bacterial growth. AmiD may have been maintained in arthropod Wolbachia to avoid host immune recognition by degrading cell wall fragments in the periplasm. This is the first description of a wolbachial lipid II processing enzyme putatively expressed in the periplasm.

Deciphering the Nature of Enzymatic Modifications of Bacterial Cell Walls

The major constituent of bacterial cell walls is peptidoglycan, which, in its crosslinked form, is a polymer of considerable complexity that encases the entire bacterium. A functional cell wall is indispensable for survival of the organism. There are several dozen enzymes that assemble and disassemble the peptidoglycan dynamically within each bacterial generation. Understanding of the nature of these transformations is critical knowledge for these events. Octasaccharide peptidoglycans were prepared and studied with seven recombinant cell-wall-active enzymes (SltB1, MltB, RlpA, mutanolysin, AmpDh2, AmpDh3, and PBP5). With the use of highly sensitive mass spectrometry methods, we described the breadth of reactions that these enzymes catalyzed with peptidoglycan and shed light on the nature of the cell wall alteration performed by these enzymes. The enzymes exhibit broadly distinct preferences for their substrate peptidoglycans in the reactions that they catalyze.

Mechanisms of Antibiotic Resistance

Emergence of resistance among the most important bacterial pathogens is recognized as a major public health threat affecting humans worldwide. Multidrug-resistant organisms have not only emerged in the hospital environment but are now often identified in community settings, suggesting that reservoirs of antibiotic-resistant bacteria are present outside the hospital. The bacterial response to the antibiotic "attack" is the prime example of bacterial adaptation and the pinnacle of evolution. "Survival of the fittest" is a consequence of an immense genetic plasticity of bacterial pathogens that trigger specific responses that result in mutational adaptations, acquisition of genetic material, or alteration of gene expression producing resistance to virtually all antibiotics currently available in clinical practice. Therefore, understanding the biochemical and genetic basis of resistance is of paramount importance to design strategies to curtail the emergence and spread of resistance and to devise innovative therapeutic approaches against multidrug-resistant organisms. In this chapter, we will describe in detail the major mechanisms of antibiotic resistance encountered in clinical practice, providing specific examples in relevant bacterial pathogens.

Lytic transglycosylases: concinnity in concision of the bacterial cell wall

The lytic transglycosylases (LTs) are bacterial enzymes that catalyze the non-hydrolytic cleavage of the peptidoglycan structures of the bacterial cell wall. They are not catalysts of glycan synthesis as might be surmised from their name. Notwithstanding the seemingly mundane reaction catalyzed by the LTs, their lytic reactions serve bacteria for a series of astonishingly diverse purposes. These purposes include cell-wall synthesis, remodeling, and degradation; for the detection of cell-wall-acting antibiotics; for the expression of the mechanism of cell-wall-acting antibiotics; for the insertion of secretion systems and flagellar assemblies into the cell wall; as a virulence mechanism during infection by certain Gram-negative bacteria; and in the sporulation and germination of Gram-positive spores. Significant advances in the mechanistic understanding of each of these processes have coincided with the successive discovery of new LTs structures. In this review, we provide a systematic perspective on what is known on the structure-function correlations for the LTs, while simultaneously identifying numerous opportunities for the future study of these enigmatic enzymes.

Promoting acid resistance and nisin yield of Lactococcus lactis F44 by genetically increasing D-Asp amidation level inside cell wall

Nisin fermentation by Lactococcus lactis requires a low pH to maintain a relatively higher nisin activity. However, the acidic environment will result in cell arrest, and eventually decrease the relative nisin production. Hence, constructing an acid-resistant L. lactis is crucial for nisin harvest in acidic nisin fermentation. In this paper, the first discovery of the relationship between D-Asp amidation-associated gene (asnH) and acid resistance was reported. Overexpression of asnH in L. lactis F44 (F44A) resulted in a sevenfold increase in survival capacity during acid shift (pH 3) and enhanced nisin desorption capacity compared to F44 (wild type), which subsequently contributed to higher nisin production, reaching 5346 IU/mL, 57.0% more than that of F44 in the fed-batch fermentation. Furthermore, the engineered F44A showed a moderate increase in D-Asp amidation level (from 82 to 92%) compared to F44. The concomitant decrease of the negative charge inside the cell wall was detected by a newly developed method based on the nisin adsorption amount onto cell surface. Meanwhile, peptidoglycan cross-linkage increased from 36.8% (F44) to 41.9% (F44A), and intracellular pH can be better maintained by blocking extracellular H⁺ due to the maintenance of peptidoglycan integrity, which probably resulted from the action of inhibiting hydrolases activity. The inference was further supported by the acmC-overexpression strain F44C, which was characterized by uncontrolled peptidoglycan hydrolase activity. Our results provided a novel strategy for enhancing nisin yield through cell wall remodeling, which contributed to both continuous nisin synthesis and less nisin adsorption in acidic fermentation (dual enhancement).

Diamide Inhibitors of the Bacillus subtilis N-Acetylglucosaminidase LytG That Exhibit Antibacterial Activity

N-Acetylglucosaminidases (GlcNAcases) play an important role in the remodeling and recycling of bacterial peptidoglycan by degrading the polysaccharide backbone. Genetic deletions of autolysins can impair cell division and growth, suggesting an opportunity for using small molecule autolysin inhibitors both as tools for studying the chemical biology of autolysins and also as antibacterial agents. We report here the synthesis and evaluation of a panel of diamides that inhibit the growth of Bacillus subtilis. Two compounds, fgkc (21) and fgka (5), were found to be potent inhibitors (MIC 3.8 ± 1.0 and 21.3 ± 0.1 μM, respectively). These compounds inhibit the B. subtilis family 73 glycosyl hydrolase LytG, an exo GlcNAcase. Phenotypic analysis of fgkc (21)-treated cells demonstrates a propensity for cells to form linked chains, suggesting impaired cell growth and division.

Catalytic Cycle of the N-Acetylglucosaminidase NagZ from Pseudomonas aeruginosa

The N-acetylglucosaminidase NagZ of Pseudomonas aeruginosa catalyzes the first cytoplasmic step in recycling of muropeptides, cell-wall-derived natural products. This reaction regulates gene expression for the β-lactam resistance enzyme, β-lactamase. The enzyme catalyzes hydrolysis of N-acetyl-β-d-glucosamine-(1→4)-1,6-anhydro-N-acetyl-β-d-muramyl-peptide (1) to N-acetyl-β-d-glucosamine (2) and 1,6-anhydro-N-acetyl-β-d-muramyl-peptide (3). The structural and functional aspects of catalysis by NagZ were investigated by a total of seven X-ray structures, three computational models based on the X-ray structures, molecular-dynamics simulations and mutagenesis. The structural insights came from the unbound state and complexes of NagZ with the substrate, products and a mimetic of the transient oxocarbenium species, which were prepared by synthesis. The mechanism involves a histidine as acid/base catalyst, which is unique for glycosidases. The turnover process utilizes covalent modification of D244, requiring two transition-state species and is regulated by coordination with a zinc ion. The analysis provides a seamless continuum for the catalytic cycle, incorporating large motions by four loops that surround the active site.

Regulation and Molecular Basis of Environmental Muropeptide Uptake and Utilization in Fastidious Oral Anaerobe Tannerella forsythia

Tannerella forsythia is a Gram-negative oral anaerobe associated with periodontitis. This bacterium is auxotrophic for the peptidoglycan amino sugar N-acetylmuramic (MurNAc) and likely relies on scavenging peptidoglycan fragments (muropeptides) released by cohabiting bacteria during their cell wall recycling. Many Gram-negative bacteria utilize an inner membrane permease, AmpG, to transport peptidoglycan fragments into their cytoplasm. In the T. forsythia genome, the Tanf_08365 ORF has been identified as a homolog of AmpG permease. In order to confirm the functionality of Tanf_08365, a reporter system in an Escherichia coli host was generated that could detect AmpG-dependent accumulation of cytosolic muropeptides via a muropeptide-inducible β-lactamase reporter gene. In trans complementation of this reporter strain with a Tanf_08365 containing plasmid caused significant induction of β-lactamase activity compared to that with an empty plasmid control. These data indicated that Tanf_08365 acted as a functional muropeptide permease causing accumulation of muropeptides in E. coli and thus suggested that it is a permease involved in muropeptide scavenging in T. forsythia. Furthermore, we showed that the promoter regulating the expression of Tanf_08365 was activated significantly by a hybrid two-component system regulatory protein, GppX. We also showed that compared to the parental T. forsythia strain a mutant lacking GppX in which the expression of AmpG was reduced significantly attenuated in utilizing free muropeptides. In summary, we have uncovered the mechanism by which this nutritionally fastidious microbe accesses released muropeptides in its environment, opening up the possibility of targeting this activity to reduce its numbers in periodontitis patients with potential benefits in the treatment of disease.

Identification of MupP as a New Peptidoglycan Recycling Factor and Antibiotic Resistance Determinant in Pseudomonas aeruginosa

Peptidoglycan (PG) is an essential cross-linked polymer that surrounds most bacterial cells to prevent osmotic rupture of the cytoplasmic membrane. Its synthesis relies on penicillin-binding proteins, the targets of beta-lactam antibiotics. Many Gram-negative bacteria, including the opportunistic pathogen Pseudomonas aeruginosa, are resistant to beta-lactams because of a chromosomally encoded beta-lactamase called AmpC. In P. aeruginosa, expression of the ampC gene is tightly regulated and its induction is linked to cell wall stress. We reasoned that a reporter gene fusion to the ampC promoter would allow us to identify mutants defective in maintaining cell wall homeostasis and thereby uncover new factors involved in the process. A library of transposon-mutagenized P. aeruginosa was therefore screened for mutants with elevated ampC promoter activity. As an indication that the screen was working as expected, mutants with transposons disrupting the dacB gene were isolated. Defects in DacB have previously been implicated in ampC induction and clinical resistance to beta-lactam antibiotics. The screen also uncovered murU and PA3172 mutants that, upon further characterization, displayed nearly identical drug resistance and sensitivity profiles. We present genetic evidence that PA3172, renamed mupP, encodes the missing phosphatase predicted to function in the MurU PG recycling pathway that is widely distributed among Gram-negative bacteria.

The cell wall biogenesis pathway is the target of many of our best antibiotics, including penicillin and related beta-lactam drugs. Resistance to these therapies is on the rise, particularly among Gram-negative species like Pseudomonas aeruginosa, a problematic opportunistic pathogen. To better understand how these organisms resist cell wall-targeting antibiotics, we screened for P. aeruginosa mutants defective in maintaining cell wall homeostasis. The screen identified a new factor, called MupP, involved in the recycling of cell wall turnover products. Characterization of MupP and other components of the pathway revealed that cell wall recycling plays important roles in both the resistance and the sensitivity of P. aeruginosa to cell wall-targeting antibiotics.

The N-Acetylmuramic Acid 6-Phosphate Phosphatase MupP Completes the Pseudomonas Peptidoglycan Recycling Pathway Leading to Intrinsic Fosfomycin Resistance

Bacterial cells are encased in and stabilized by a netlike peptidoglycan (PGN) cell wall that undergoes turnover during bacterial growth. PGN turnover fragments are frequently salvaged by the cells via a pathway referred to as PGN recycling. Two different routes for the recycling of the cell wall sugar N-acetylmuramic acid (MurNAc) have been recognized in bacteria. In Escherichia coli and related enterobacteria, as well as in most Gram-positive bacteria, MurNAc is recovered via a catabolic route requiring a MurNAc 6-phosphate etherase (MurQ in E. coli) enzyme. However, many Gram-negative bacteria, including Pseudomonas species, lack a MurQ ortholog and use an alternative, anabolic recycling route that bypasses the de novo biosynthesis of uridyldiphosphate (UDP)-MurNAc, the first committed precursor of PGN. Bacteria featuring the latter pathway become intrinsically resistant to the antibiotic fosfomycin, which targets the de novo biosynthesis of UDP-MurNAc. We report here the identification and characterization of a phosphatase enzyme, named MupP, that had been predicted to complete the anabolic recycling pathway of Pseudomonas species but has remained unknown so far. It belongs to the large haloacid dehalogenase family of phosphatases and specifically converts MurNAc 6-phosphate to MurNAc. A ΔmupP mutant of Pseudomonas putida was highly susceptible to fosfomycin, accumulated large amounts of MurNAc 6-phosphate, and showed lower levels of UDP-MurNAc than wild-type cells, altogether consistent with a role for MupP in the anabolic PGN recycling route and as a determinant of intrinsic resistance to fosfomycin.

Many Gram-negative bacteria, but not E. coli, make use of a cell wall salvage pathway that contributes to the pool of UDP-MurNAc, the first committed precursor of cell wall synthesis in bacteria. This salvage pathway is of particular interest because it confers intrinsic resistance to the antibiotic fosfomycin, which blocks de novo UDP-MurNAc biosynthesis. Here we identified and characterized a previously missing enzyme within the salvage pathway, the MurNAc 6-phosphate phosphatase MupP of P. putida. MupP, together with the other enzymes of the anabolic recycling pathway, AnmK, AmgK, and MurU, yields UDP-MurNAc, renders bacteria intrinsically resistant to fosfomycin, and thus may serve as a novel drug target for antimicrobial therapy.

From Genome to Proteome to Elucidation of Reactions for All Eleven Known Lytic Transglycosylases from Pseudomonas aeruginosa

An enzyme superfamily, the lytic transglycosylases (LTs), occupies the space between the two membranes of Gram-negative bacteria. LTs catalyze the non-hydrolytic cleavage of the bacterial peptidoglycan cell-wall polymer. This reaction is central to the growth of the cell wall, for excavating the cell wall for protein insertion, and for monitoring the cell wall so as to initiate resistance responses to cell-wall-acting antibiotics. The nefarious Gram-negative pathogen Pseudomonas aeruginosa encodes eleven LTs. With few exceptions, their substrates and functions are unknown. Each P. aeruginosa LT was expressed as a soluble protein and evaluated with a panel of substrates (both simple and complex mimetics of their natural substrates). Thirty-one distinct products distinguish these LTs with respect to substrate recognition, catalytic activity, and relative exolytic or endolytic ability. These properties are foundational to an understanding of the LTs as catalysts and as antibiotic targets.

Carbohydrate recognition and lysis by bacterial peptidoglycan hydrolases

The major component of bacterial cell wall is peptidoglycan (PG), a complex polymer formed by long glycan chains cross-linked by peptide stems. PG is in constant equilibrium requiring well-orchestrated coordination between synthesis and degradation. The resulting cell-wall fragments can be recycled, act as messengers for bacterial communication, as effector molecules in immune response or as signaling molecules triggering antibiotics resistance. Tailoring and recycling of PG requires the cleavage of different covalent bonds of the PG sacculi by a diverse set of specific enzymes whose activities are strictly regulated. Here, we review the molecular mechanisms that govern PG remodeling focusing on the structural information available for the bacterial lytic enzymes and the mechanisms by which they recognize their substrates.

Muropeptide Binding and the X-ray Structure of the Effector Domain of the Transcriptional Regulator AmpR of Pseudomonas aeruginosa

A complex link exists between cell-wall recycling/repair and the manifestation of resistance to β-lactam antibiotics in many Enterobacteriaceae and Pseudomonas aeruginosa. This process is mediated by specific cell-wall-derived muropeptide products. These muropeptides are internalized into the cytoplasm and bind to the transcriptional regulator AmpR, which controls the cytoplasmic events that lead to expression of β-lactamase, an antibiotic-resistance determinant. The effector-binding domain (EBD) of AmpR was purified to homogeneity. We document that the EBD exists exclusively as a dimer, even at a concentration as low as 1 μM. The EBD binds to the suppressor ligand UDP-N-acetyl-β-d-muramyl-l-Ala-γ-d-Glu-meso-DAP-d-Ala-d-Ala and binds to two activator muropeptides, N-acetyl-β-d-glucosamine-(1→4)-1,6-anhydro-N-acetyl-β-d-muramyl-l-Ala-γ-d-Glu-meso-DAP-d-Ala-d-Ala and 1,6-anhydro-N-acetyl-β-d-muramyl-l-Ala-γ-d-Glu-meso-DAP-d-Ala-d-Ala, as assessed by non-denaturing mass spectrometry. The EBD does not bind to 1,6-anhydro-N-acetyl-β-d-muramyl-l-Ala-γ-d-Glu-meso-DAP. This binding selectivity revises the dogma in the field. The crystal structure of the EBD dimer was solved to 2.2 Å resolution. The EBD crystallizes in a "closed" conformation, in contrast to the "open" structure required to bind the muropeptides. Structural issues of this ligand recognition are addressed by molecular dynamics simulations, which reveal significant differences among the complexes with the effector molecules.

Potential of semiarid soil from Caatinga biome as a novel source for mining lignocellulose-degrading enzymes

The litterfall is the major organic material deposited in soil of Brazilian Caatinga biome, thus providing the ideal conditions for plant biomass-degrading microorganisms to thrive. Herein, the phylogenetic composition and lignocellulose-degrading capacity have been explored for the first time from a fosmid library dataset of Caatinga soil by sequence-based screening. A complex bacterial community dominated by Proteobacteria and Actinobacteria was unraveled. SEED subsystems-based annotations revealed a broad range of genes assigned to carbohydrate and aromatic compounds metabolism, indicating microbial ability to utilize plant-derived material. CAZy-based annotation identified 7275 genes encoding 37 glycoside hydrolases (GHs) families related to hydrolysis of cellulose, hemicellulose, oligosaccharides and other lignin-modifying enzymes. Taxonomic affiliation of genes showed high genetic potential of the phylum Acidobacteria for hemicellulose degradation, whereas Actinobacteria members appear to play an important role in celullose hydrolysis. Additionally, comparative analyses revealed greater GHs profile similarity among soils as compared to the digestive tract of animals capable of digesting plant biomass, particularly in the hemicellulases content. Combined results suggest a complex synergistic interaction of community members required for biomass degradation into fermentable sugars. This large repertoire of lignocellulolytic enzymes opens perspectives for mining potential candidates of biochemical catalysts for biofuels production from renewable resources and other environmental applications.

Bacteria sensing mechanisms in Drosophila gut: Local and systemic consequences

All insects are colonized by microorganisms on their exoskeleton, their gut and even in some cases within their own somatic and germ line cells. This microbiota that can represent up to a few percent of the insect biomass may have a pervasive impact on many aspects of insect biology including physiology, nutrient acquisition, ageing, behaviour and resistance to infection. Mainly through ingestion of contaminated food, the mouth-gut axis represents the first and principal access of external bacteria to the host. Soon after ingestion, the feeding insect needs to rapidly and accurately identify the ingested microbes and decide whether to preserve them if beneficial or neutral, or to eliminate them if potentially harmful. We will review here the recent data acquired in Drosophila on the mechanisms that invertebrate enterocytes rely on to detect the presence of bacteria in the gut. We will compare these modes of bacteria sensing to those in other immune competent tissues and try to rationalize differences that may exist. We will also analyse the physiological consequences of bacteria detection not only locally for the gut itself but also for remote tissues. Finally, we will describe the physiological disorders that can occur due to inaccurate bacteria identification by the gut epithelium.

Impact of AmpC Derepression on Fitness and Virulence: the Mechanism or the Pathway?

Understanding the interplay between antibiotic resistance and bacterial fitness and virulence is essential to guide individual treatments and improve global antibiotic policies. A paradigmatic example of a resistance mechanism is the intrinsic inducible chromosomal β-lactamase AmpC from multiple Gram-negative bacteria, including Pseudomonas aeruginosa, a major nosocomial pathogen. The regulation of ampC expression is intimately linked to peptidoglycan recycling, and AmpC-mediated β-lactam resistance is frequently mediated by inactivating mutations in ampD, encoding an N-acetyl-anhydromuramyl-l-alanine amidase, affecting the levels of ampC-activating muropeptides. Here we dissect the impact of the multiple pathways causing AmpC hyperproduction on P. aeruginosa fitness and virulence. Through a detailed analysis, we demonstrate that the lack of all three P. aeruginosa AmpD amidases causes a dramatic effect in fitness and pathogenicity, severely compromising growth rates, motility, and cytotoxicity; the latter effect is likely achieved by repressing key virulence factors, such as protease LasA, phospholipase C, or type III secretion system components. We also show that ampC overexpression is required but not sufficient to confer the growth-motility-cytotoxicity impaired phenotype and that alternative pathways leading to similar levels of ampC hyperexpression and resistance, such as those involving PBP4, had no fitness-virulence cost. Further analysis indicated that fitness-virulence impairment is caused by overexpressing ampC in the absence of cell wall recycling, as reproduced by expressing ampC from a plasmid in an AmpG (muropeptide permease)-deficient background. Thus, our findings represent a major step in the understanding of β-lactam resistance biology and its interplay with fitness and pathogenesis.

IMPORTANCE

Understanding the impact of antibiotic resistance mechanisms on bacterial pathogenesis is critical to curb the spread of antibiotic resistance. A particularly noteworthy antibiotic resistance mechanism is the β-lactamase AmpC, produced by Pseudomonas aeruginosa, a major pathogen causing hospital-acquired infections. The regulation of AmpC is linked to the cell wall recycling pathways, and frequently, resistance to β-lactams is caused by mutation of several of the components of the cell wall recycling pathways such as AmpD. Here we dissect the impact of the pathways for AmpC hyperproduction on virulence, showing that the lack of all three P. aeruginosa AmpD amidases causes a major effect in fitness and pathogenicity, compromising growth, motility, and cytotoxicity. Further analysis indicated that fitness-virulence impairment is specifically caused by the hyperproduction of AmpC in the absence of cell wall recycling. Our work provides valuable information for delineating future strategies for combating P. aeruginosa infections by simultaneously targeting virulence and antibiotic resistance.

Peptidoglycan Recycling in Gram-Positive Bacteria Is Crucial for Survival in Stationary Phase

Peptidoglycan recycling is a metabolic process by which Gram-negative bacteria reutilize up to half of their cell wall within one generation during vegetative growth. Whether peptidoglycan recycling also occurs in Gram-positive bacteria has so far remained unclear. We show here that three Gram-positive model organisms, Staphylococcus aureus, Bacillus subtilis, and Streptomyces coelicolor, all recycle the sugar N-acetylmuramic acid (MurNAc) of their peptidoglycan during growth in rich medium. They possess MurNAc-6-phosphate (MurNAc-6P) etherase (MurQ in E. coli) enzymes, which are responsible for the intracellular conversion of MurNAc-6P to N-acetylglucosamine-6-phosphate and d-lactate. By applying mass spectrometry, we observed accumulation of MurNAc-6P in MurNAc-6P etherase deletion mutants but not in either the isogenic parental strains or complemented strains, suggesting that MurQ orthologs are required for the recycling of cell wall-derived MurNAc in these bacteria. Quantification of MurNAc-6P in ΔmurQ cells of S. aureus and B. subtilis revealed small amounts during exponential growth phase (0.19 nmol and 0.03 nmol, respectively, per ml of cells at an optical density at 600 nm [OD₆₀₀] of 1) but large amounts during transition (0.56 nmol and 0.52 nmol) and stationary (0.53 nmol and 1.36 nmol) phases. The addition of MurNAc to ΔmurQ cultures greatly increased the levels of intracellular MurNAc-6P in all growth phases. The ΔmurQ mutants of S. aureus and B. subtilis showed no growth deficiency in rich medium compared to the growth of the respective parental strains, but intriguingly, they had a severe survival disadvantage in late stationary phase. Thus, although peptidoglycan recycling is apparently not essential for the growth of Gram-positive bacteria, it provides a benefit for long-term survival.

IMPORTANCE

The peptidoglycan of the bacterial cell wall is turned over steadily during growth. As peptidoglycan fragments were found in large amounts in spent medium of exponentially growing Gram-positive bacteria, their ability to recycle these fragments has been questioned. We conclusively showed recycling of the peptidoglycan component MurNAc in different Gram-positive model organisms and revealed that a MurNAc-6P etherase (MurQ or MurQ ortholog) enzyme is required in this process. We further demonstrated that recycling occurs predominantly during the transition to stationary phase in S. aureus and B. subtilis, explaining why peptidoglycan fragments are found in the medium during exponential growth. We quantified the intracellular accumulation of recycling products in MurNAc-6P etherase gene mutants, revealing that about 5% and 10% of the MurNAc of the cell wall per generation is recycled in S. aureus and B. subtilis, respectively. Importantly, we showed that MurNAc recycling and salvaging does not sustain growth in these bacteria but is used to enhance survival during late stationary phase.

In vivo functional and molecular characterization of the Penicillin-Binding Protein 4 (DacB) of Pseudomonas aeruginosa

BACKGROUND

Community and nosocomial infections by Pseudomonas aeruginosa still create a major therapeutic challenge. The resistance of this opportunist pathogen to β-lactam antibiotics is determined mainly by production of the inactivating enzyme AmpC, a class C cephalosporinase with a regulation system more complex than those found in members of the Enterobacteriaceae family. This regulatory system also participates directly in peptidoglycan turnover and recycling. One of the regulatory mechanisms for AmpC expression, recently identified in clinical isolates, is the inactivation of LMM-PBP4 (Low-Molecular-Mass Penicillin-Binding Protein 4), a protein whose catalytic activity on natural substrates has remained uncharacterized until now.

RESULTS

We carried out in vivo activity trials for LMM-PBP4 of Pseudomonas aeruginosa on macromolecular peptidoglycan of Escherichia coli and Pseudomonas aeruginosa. The results showed a decrease in the relative quantity of dimeric, trimeric and anhydrous units, and a smaller reduction in monomer disaccharide pentapeptide (M5) levels, validating the occurrence of D,D-carboxypeptidase and D,D-endopeptidase activities. Under conditions of induction for this protein and cefoxitin treatment, the reduction in M5 is not fully efficient, implying that LMM-PBP4 of Pseudomonas aeruginosa presents better behaviour as a D,D-endopeptidase. Kinetic evaluation of the direct D,D-peptidase activity of this protein on natural muropeptides M5 and D45 confirmed this bifunctionality and the greater affinity of LMM-PBP4 for its dimeric substrate. A three-dimensional model for the monomeric unit of LMM-PBP4 provided structural information which supports its catalytic performance.

CONCLUSIONS

LMM-PBP4 of Pseudomonas aeruginosa is a bifunctional enzyme presenting both D,D-carboxypeptidase and D,D-endopeptidase activities; the D,D-endopeptidase function is predominant. Our study provides unprecedented functional and structural information which supports the proposal of this protein as a potential hydrolase-autolysin associated with peptidoglycan maturation and recycling. The fact that mutant PBP4 induces AmpC, may indicate that a putative muropeptide-subunit product of the DD-EPase activity of PBP4 could be a negative regulator of the pathway. This data contributes to understanding of the regulatory aspects of resistance to β-lactam antibiotics in this bacterial model.

Lytic transglycosylases LtgA and LtgD perform distinct roles in remodeling, recycling and releasing peptidoglycan in Neisseria gonorrhoeae

Neisseria gonorrhoeae releases peptidoglycan (PG) fragments during infection that provoke a large inflammatory response and, in pelvic inflammatory disease, this response leads to the death and sloughing of ciliated cells of the Fallopian tube. We characterized the biochemical functions and localization of two enzymes responsible for the release of proinflammatory PG fragments. The putative lytic transglycosylases LtgA and LtgD were shown to create the 1,6-anhydromuramyl moieties, and both enzymes were able to digest a small, synthetic tetrasaccharide dipeptide PG fragment into the cognate 1,6-anhydromuramyl-containing reaction products. Degradation of tetrasaccharide PG fragments by LtgA is the first demonstration of a family 1 lytic transglycosylase exhibiting this activity. Pulse-chase experiments in gonococci demonstrated that LtgA produces a larger amount of PG fragments than LtgD, and a vast majority of these fragments are recycled. In contrast, LtgD was necessary for wild-type levels of PG precursor incorporation and produced fragments predominantly released from the cell. Additionally, super-resolution microscopy established that LtgA localizes to the septum, whereas LtgD is localized around the cell. This investigation suggests a model where LtgD produces PG monomers in such a way that these fragments are released, whereas LtgA creates fragments that are mostly taken into the cytoplasm for recycling.

On the phosphorylase activity of GH3 enzymes: A β-N-acetylglucosaminidase from Herbaspirillum seropedicae SmR1 and a glucosidase from Saccharopolyspora erythraea

A phosphorolytic activity has been reported for beta-N-acetylglucosaminidases from glycoside hydrolase family 3 (GH3) giving an interesting explanation for an unusual histidine as catalytic acid/base residue and suggesting that members from this family may be phosphorylases [J. Biol. Chem. 2015, 290, 4887]. Here, we describe the characterization of Hsero1941, a GH3 beta-N-acetylglucosaminidase from the endophytic nitrogen-fixing bacterium Herbaspirillum seropedicae SmR1. The enzyme has significantly higher activity against pNP-beta-D-GlcNAcp (K_m = 0.24 mM, k_cat = 1.2 s^-1, k_cat/K_m = 5.0 mM^-1s^-1) than pNP-beta-D-Glcp (K_m = 33 mM, k_cat = 3.3 × 10^-3 s^-1, k_cat/K_m = 9 × 10^-4 mM^-1s^-1). The presence of phosphate failed to significantly modify the kinetic parameters of the reaction. The enzyme showed a broad aglycone site specificity, being able to hydrolyze sugar phosphates beta-D-GlcNAc 1P and beta-D-Glc 1P, albeit at a fraction of the rate of hydrolysis of aryl glycosides. GH3 beta-glucosidase EryBI, that does not have a histidine as the general acid/base residue, also hydrolyzed beta-D-Glc 1P, at comparable rates to Hsero1941. These data indicate that Hsero1941 functions primarily as a hydrolase and that phosphorolytic activity is likely adventitious. The prevalence of histidine as a general acid/base residue is not predictive, nor correlative, with GH3 beta-N-acetylglucosaminidases having phosphorolytic activity.

Bulgecin A as a β-lactam enhancer for carbapenem-resistant Pseudomonas aeruginosa and carbapenem-resistant Acinetobacter baumannii clinical isolates containing various resistance mechanisms

Genetic screening of Pseudomonas aeruginosa (PSDA) and Acinetobacter baumannii (ACB) reveals genes that confer increased susceptibility to β-lactams when disrupted, suggesting novel drug targets. One such target is lytic transglycosylase. Bulgecin A (BlgA) is a natural product of Pseudomonas mesoacidophila and a lytic transglycosolase inhibitor that works synergistically with β-lactams targeting PBP3 for Enterobacteriaceae. BlgA also weakly inhibits di-Zn²⁺ metallo-β-lactamases like L1 of Stenotrophomonas maltophilia. We hypothesized that because of its unique mechanism of action, BlgA could restore susceptibility to carbapenems in carbapenem-resistant PSDA (CR-PSDA) and carbapenem-resistant ACB, as well as ACB resistant to sulbactam. A BlgA-containing extract was prepared using a previously published protocol. CR-PSDA clinical isolates demonstrating a variety of carbapenem resistance mechanisms (VIM-2 carbapenemases, efflux mechanisms, and AmpC producer expression) were characterized with agar dilution minimum inhibitory concentration (MIC) testing and polymerase chain reaction. Growth curves using these strains were prepared using meropenem, BlgA extract, and meropenem plus BlgA extract. A concentrated Blg A extract combined with low concentrations of meropenem, was able to inhibit the growth of clinical strains of CR-PSDA for strains that had meropenem MICs ≥8 mg/L by agar dilution, and a clinical strain of an OXA-24 producing ACB that had a meropenem MIC >32 mg/L and intermediate ampicillin/sulbactam susceptibility. Similar experiments were conducted on a TEM-1 producing ACB strain resistant to sulbactam. BlgA with ampicillin/sulbactam inhibited the growth of this organism. As in Enterobacteriaceae, BlgA appears to restore the efficacy of meropenem in suppressing the growth of CR-PSDA and carbapenem-resistant ACB strains with a variety of common carbapenem resistance mechanisms. BlgA extract also inhibits VIM-2 β-lactamase in vitro. BlgA may prove to be an exciting adjunctive compound to extend the life of carbapenems against these vexing pathogens.

A Fluorescent Transport Assay Enables Studying AmpG Permeases Involved in Peptidoglycan Recycling and Antibiotic Resistance

Inducible AmpC β-lactamases deactivate a broad-spectrum of β-lactam antibiotics and afford antibiotic resistance in many Gram-negative bacteria. The disturbance of peptidoglycan recycling caused by β-lactam antibiotics leads to accumulation of GlcNAc-1,6-anhydroMurNAc-peptides, which are transported by AmpG to the cytoplasm where they are processed into AmpC inducers. AmpG transporters are poorly understood; however, their loss restores susceptibility toward β-lactam antibiotics, highlighting AmpG as a potential target for resistance-attenuating therapeutics. We prepare a GlcNAc-1,6-anhydroMurNAc-fluorophore conjugate and, using live E. coli spheroplasts, quantitatively analyze its transport by AmpG and inhibition of this process by a competing substrate. Further, we use this transport assay to evaluate the function of two AmpG homologues from Pseudomonas aeruginosa and show that P. aeruginosa AmpG (Pa-AmpG) but not AmpP (Pa-AmpP) transports this probe substrate. We corroborate these results by AmpC induction assays with Pa-AmpG and Pa-AmpP. This fluorescent AmpG probe and spheroplast-based transport assay will enable improved understanding of PG recycling and of permeases from the major facilitator superfamily of transport proteins and may aid in identification of AmpG antagonists that combat AmpC-mediated resistance toward β-lactam antibiotics.

On the translocation of bacteria and their lipopolysaccharides between blood and peripheral locations in chronic, inflammatory diseases: the central roles of LPS and LPS-induced cell death

We have recently highlighted (and added to) the considerable evidence that blood can contain dormant bacteria. By definition, such bacteria may be resuscitated (and thus proliferate). This may occur under conditions that lead to or exacerbate chronic, inflammatory diseases that are normally considered to lack a microbial component. Bacterial cell wall components, such as the endotoxin lipopolysaccharide (LPS) of Gram-negative strains, are well known as potent inflammatory agents, but should normally be cleared. Thus, their continuing production and replenishment from dormant bacterial reservoirs provides an easy explanation for the continuing, low-grade inflammation (and inflammatory cytokine production) that is characteristic of many such diseases. Although experimental conditions and determinants have varied considerably between investigators, we summarise the evidence that in a great many circumstances LPS can play a central role in all of these processes, including in particular cell death processes that permit translocation between the gut, blood and other tissues. Such localised cell death processes might also contribute strongly to the specific diseases of interest. The bacterial requirement for free iron explains the strong co-existence in these diseases of iron dysregulation, LPS production, and inflammation. Overall this analysis provides an integrative picture, with significant predictive power, that is able to link these processes via the centrality of a dormant blood microbiome that can resuscitate and shed cell wall components.

Activation by Allostery in Cell-Wall Remodeling by a Modular Membrane-Bound Lytic Transglycosylase from Pseudomonas aeruginosa

Bacteria grow and divide without loss of cellular integrity. This accomplishment is notable, as a key component of their cell envelope is a surrounding glycopeptide polymer. In Gram-negative bacteria this polymer-the peptidoglycan-grows by the difference between concurrent synthesis and degradation. The regulation of the enzymatic ensemble for these activities is poorly understood. We report herein the structural basis for the control of one such enzyme, the lytic transglycosylase MltF of Pseudomonas aeruginosa. Its structure comprises two modules: an ABC-transporter-like regulatory module and a catalytic module. Occupancy of the regulatory module by peptidoglycan-derived muropeptides effects a dramatic and long-distance (40 Å) conformational change, occurring over the entire protein structure, to open its active site for catalysis. This discovery of the molecular basis for the allosteric control of MltF catalysis is foundational to further study of MltF within the complex enzymatic orchestration of the dynamic peptidoglycan.

Three Yersinia enterocolitica AmpD Homologs Participate in the Multi-Step Regulation of Chromosomal Cephalosporinase, AmpC

In many gram negative bacilli, AmpD plays a key role in both cell well-recycling pathway and β-lactamase regulation, inactivation of the ampD causes the accumulation of 1,6-anhydromuropeptides, and results in the ampC overproduction. In Yersinia enterocolitica, the regulation of ampC expression may also rely on the ampR-ampC system, the role of AmpD in this species is still unknown. In this study, three AmpD homologs (AmpD1, AmpD2, and AmpD3) have been identified in complete sequence of strain Y. enterocolitica subsp. palearctica 105.5R(r). To understand the role of three AmpD homologs, several mutant strains were constructed and analyzed where a rare ampC regulation mechanism was observed: low-effective ampD2 and ampD3 cooperate with the high-effective ampD1 in the three levels regulation of ampC expression. Enterobacteriaceae was used to be supposed to regulate ampC expression by two steps, three steps regulation was only observed in Pseudomonas aeruginosa. In this study, we first reported that Enterobacteriaceae Y. enterocolitica can also possess a three steps stepwise regulation mechanism, regulating the ampC expression precisely.

Structural Studies on a Glucosamine/Glucosaminide N-Acetyltransferase

Glucosamine/glucosaminide N-acetyltransferase or GlmA catalyzes the transfer of an acetyl group from acetyl CoA to the primary amino group of glucosamine. The enzyme from Clostridium acetobutylicum is thought to be involved in cell wall rescue. In addition to glucosamine, GlmA has been shown to function on di- and trisaccharides of glucosamine as well. Here we present a structural and kinetic analysis of the enzyme. For this investigation, eight structures were determined to resolutions of 2.0 Å or better. The overall three-dimensional fold of GlmA places it into the tandem GNAT superfamily. Each subunit of the dimer folds into two distinct domains which exhibit high three-dimensional structural similarity. Whereas both domains bind acetyl CoA, it is the C-terminal domain that is catalytically competent. On the basis of the various structures determined in this investigation, two amino acid residues were targeted for further study: Asp 287 and Tyr 297. Although their positions in the active site suggested that they may play key roles in catalysis by functioning as active site bases and acids, respectively, this was not borne out by characterization of the D287N and Y297F variants. The kinetic properties revealed that both residues were important for substrate binding but had no critical roles as acid/base catalysts. Kinetic analyses also indicated that GlmA follows an ordered mechanism with acetyl CoA binding first followed by glucosamine. The product N-acetylglucosamine is then released prior to CoA. The investigation described herein provides significantly new information on enzymes belonging to the tandem GNAT superfamily.

Corynebacterium glutamicum possesses β-N-acetylglucosaminidase

BACKGROUND

In Gram-positive Corynebacterium glutamicum and other members of the suborder Corynebacterianeae, which includes mycobacteria, cell elongation and peptidoglycan biosynthesis is mainly due to polar growth. C. glutamicum lacks an uptake system for the peptidoglycan constituent N-acetylglucosamine (GlcNAc), but is able to catabolize GlcNAc-6-phosphate. Due to its importance in white biotechnology and in order to ensure more sustainable processes based on non-food renewables and to reduce feedstock costs, C. glutamicum strains have previously been engineered to produce amino acids from GlcNAc. GlcNAc also is a constituent of chitin, but it is unknown if C. glutamicum possesses chitinolytic enzymes.

RESULTS

Chitin was shown here not to be growth substrate for C. glutamicum. However, its genome encodes a putative N-acetylglucosaminidase. The nagA 2 gene product was active as β-N-acetylglucosaminidase with 0.27 mM 4-nitrophenyl N,N'-diacetyl-β-D-chitobioside as substrate supporting half-maximal activity. NagA2 was secreted into the culture medium when overproduced with TAT and Sec dependent signal peptides, while it remained cytoplasmic when overproduced without signal peptide. Heterologous expression of exochitinase gene chiB from Serratia marcescens resulted in chitinolytic activity and ChiB secretion was enhanced when a signal peptide from C. glutamicum was used. Colloidal chitin did not support growth of a strain secreting exochitinase ChiB and β-N-acetylglucosaminidase NagA2.

CONCLUSIONS

C. glutamicum possesses β-N-acetylglucosaminidase. In the wild type, β-N-acetylglucosaminidase activity was too low to be detected. However, overproduction of the enzyme fused to TAT or Sec signal peptides led to secretion of active β-N-acetylglucosaminidase. The finding that concomitant secretion of endogenous NagA2 and exochitinase ChiB from S. marcescens did not entail growth with colloidal chitin as sole or combined carbon source, may indicate the requirement for higher or additional enzyme activities such as processive chitinase or endochitinase activities.

Renew or die: The molecular mechanisms of peptidoglycan recycling and antibiotic resistance in Gram-negative pathogens

Antimicrobial resistance is one of the most serious health threats. Cell-wall remodeling processes are tightly regulated to warrant bacterial survival and in some cases are directly linked to antibiotic resistance. Remodeling produces cell-wall fragments that are recycled but can also act as messengers for bacterial communication, as effector molecules in immune response and as signaling molecules triggering antibiotic resistance. This review is intended to provide state-of-the-art information about the molecular mechanisms governing this process and gather structural information of the different macromolecular machineries involved in peptidoglycan recycling in Gram-negative bacteria. The growing body of literature on the 3D structures of the corresponding macromolecules reveals an extraordinary complexity. Considering the increasing incidence and widespread emergence of Gram-negative multidrug-resistant pathogens in clinics, structural information on the main actors of the recycling process paves the way for designing novel antibiotics disrupting cellular communication in the recycling-resistance pathway.

Orthologous and Paralogous AmpD Peptidoglycan Amidases from Gram-Negative Bacteria

Cell wall recycling and β-lactam antibiotic resistance are linked in Enterobacteriaceae and in Pseudomonas aeruginosa. This process involves a large number of murolytic enzymes, among them a cytoplasmic peptidoglycan amidase AmpD, which plays an essential role by cleaving the peptide stem from key intermediates en route to the β-lactamase production (a resistance mechanism) and cell wall recycling. Uniquely, P. aeruginosa has two additional paralogues of AmpD, designated AmpDh2 and AmpDh3, which are periplasmic enzymes. Despite the fact that AmpDh2 and AmpDh3 share a common motif for their respective catalytic domains, they are each comprised of multidomain architectures and exhibit distinct oligomerization properties. We review herein the structural and biochemical properties of orthologous and paralogous AmpD proteins and discuss their implication in cell wall recycling and antibiotic resistance processes.