Turnover and recycling of the murein sacculus in oligopeptide permease-negative strains of Escherichia coli: indirect evidence for an alternative permease system and for a monolayered sacculus

Bacterial peptidoglycan recycling

During growth and division, the bacterial cell wall is remodeled, resulting in the liberation of peptidoglycan (PG) fragments which are typically reinternalized and recycled. Recycling of PG has been studied in a few model species, but its importance and diversity are not yet well understood. Here, we review how bacteria transport and recycle the components of their PG, highlighting updates and new findings.

Escherichia coli CadB is capable of promiscuously transporting muropeptides and contributing to peptidoglycan recycling

The bacterial peptidoglycan (PG) cell wall is remodeled during growth and division, releasing fragments called muropeptides. Muropeptides can be internalized and reused in a process called PG recycling. Escherichia coli is highly devoted to recycling muropeptides and is known to have at least two transporters, AmpG and OppBCDF, that import them into the cytoplasm. While studying mutants lacking AmpG, we unintentionally isolated mutations that led to the altered expression of a third transporter, CadB. CadB is normally upregulated under acidic pH conditions and is an antiporter for lysine and cadaverine. Here, we explored if CadB was altering PG recycling to assist in the absence of AmpG. Surprisingly, CadB overexpression was able to restore PG recycling when both AmpG and OppBCDF were absent. CadB was found to import freed PG peptides, a subpopulation of muropeptides, through a promiscuous activity. Altogether, our data support that CadB is a third transporter capable of contributing to PG recycling. IMPORTANCE Bacteria produce a rigid mesh cell wall. During growth, the cell wall is remodeled, which releases cell wall fragments. If released into the extracellular environment, cell wall fragments can trigger inflammation by the immune system of a host. Gastrointestinal bacteria, like Escherichia coli, have dedicated pathways to recycle almost all cell wall fragments they produce. E. coli contains two known recycling transporters, AmpG and Opp, that we previously showed are optimized for growth in different environments. Here, we identify that a third transporter, CadB, can also contribute to cell wall recycling. This work expands our understanding of cell wall recycling and highlights the dedication of organisms like E. coli to ensure high recycling in multiple growth environments.

The hidden base of the iceberg: gut peptidoglycome dynamics is foundational to its influence on the host

The intestinal microbiota of humans includes a highly diverse range of bacterial species. All these bacteria possess a cell wall, composed primarily of the macromolecule peptidoglycan. As such, the gut also harbors an abundant and varied peptidoglycome. A remarkable range of host physiological pathways are regulated by peptidoglycan fragments that originate from the gut microbiota and enter the host system. Interactions between the host system and peptidoglycan can influence physiological development and homeostasis, promote health, or contribute to inflammatory disease. Underlying these effects is the interplay between microbiota composition and enzymatic processes that shape the intestinal peptidoglycome, dictating the types of peptidoglycan generated, that subsequently cross the gut barrier. In this review, we highlight and discuss the hidden and emerging functional aspects of the microbiome, i.e. the hidden base of the iceberg, that modulate the composition of gut peptidoglycan, and how these fundamental processes are drivers of physiological outcomes for the host.

Escherichia coli utilizes multiple peptidoglycan recycling permeases with distinct strategies of recycling

Bacteria produce a structural layer of peptidoglycan (PG) that enforces cell shape, resists turgor pressure, and protects the cell. As bacteria grow and divide, the existing layer of PG is remodeled and PG fragments are released. Enterics such as Escherichia coli go to great lengths to internalize and reutilize PG fragments. E. coli is estimated to break down one-third of its cell wall, yet only loses ~0 to 5% of meso-diaminopimelic acid, a PG-specific amino acid, per generation. Two transporters were identified early on to possibly be the primary permease that facilitates PG fragment recycling, i) AmpG and ii) the Opp ATP binding cassette transporter in conjunction with a PG-specific periplasmic binding protein, MppA. The contribution of each transporter to PG recycling has been debated. Here, we have found that AmpG and MppA/Opp are differentially regulated by carbon source and growth phase. In addition, MppA/Opp is uniquely capable of high-affinity scavenging of muropeptides from growth media, demonstrating that AmpG and MppA/Opp allow for different strategies of recycling PG fragments. Altogether, this work clarifies environmental contexts under which E. coli utilizes distinct permeases for PG recycling and explores how scavenging by MppA/Opp could be beneficial in mixed communities.

Bacterial virulence regulation through soluble peptidoglycan fragments sensing and response: knowledge gaps and therapeutic potential

Given the growing clinical-epidemiological threat posed by the phenomenon of antibiotic resistance, new therapeutic options are urgently needed, especially against top nosocomial pathogens such as those within the ESKAPE group. In this scenario, research is pushed to explore therapeutic alternatives and, among these, those oriented toward reducing bacterial pathogenic power could pose encouraging options. However, the first step in developing these antivirulence weapons is to find weak points in the bacterial biology to be attacked with the goal of dampening pathogenesis. In this regard, during the last decades some studies have directly/indirectly suggested that certain soluble peptidoglycan-derived fragments display virulence-regulatory capacities, likely through similar mechanisms to those followed to regulate the production of several β-lactamases: binding to specific transcriptional regulators and/or sensing/activation of two-component systems. These data suggest the existence of intra- and also intercellular peptidoglycan-derived signaling capable of impacting bacterial behavior, and hence likely exploitable from the therapeutic perspective. Using the well-known phenomenon of peptidoglycan metabolism-linked β-lactamase regulation as a starting point, we gather and integrate the studies connecting soluble peptidoglycan sensing with fitness/virulence regulation in Gram-negatives, dissecting the gaps in current knowledge that need filling to enable potential therapeutic strategy development, a topic which is also finally discussed.

Regulation of Immune Homeostasis via Muramyl Peptides-Low Molecular Weight Bioregulators of Bacterial Origin

Metabolites and fragments of bacterial cells play an important role in the formation of immune homeostasis. Formed in the course of evolution, symbiotic relationships between microorganisms and a macroorganism are manifested, in particular, in the regulation of numerous physiological functions of the human body by the innate immunity receptors. Low molecular weight bioregulators of bacterial origin have recently attracted more and more attention as drugs in the prevention and composition of complex therapy for a wide range of diseases of bacterial and viral etiology. Signaling networks show cascades of causal relationships of deterministic phenomena that support the homeostasis of multicellular organisms at different levels. To create networks, data from numerous biomedical and clinical research databases were used to prepare expert systems for use in pharmacological and biomedical research with an emphasis on muramyl dipeptides. Muramyl peptides are the fragments of the cell wall of Gram-positive and Gram-negative bacteria. Binding of muramyl peptides with intracellular NOD2 receptors is crucial for an immune response on pathogens. Depending on the microenvironment and duration of action, muramyl peptides possess positive or negative regulation of inflammation. Other factors, such as genetic, pollutions, method of application and stress also contribute and should be taken into account. A system biology approach should be used in order to systemize all experimental data for rigorous analysis, with the aim of understanding intrinsic pathways of homeostasis, in order to define precise medicine therapy and drug design.

Chemical Reporters for Bacterial Glycans: Development and Applications

Bacteria possess an extraordinary repertoire of cell envelope glycans that have critical physiological functions. Pathogenic bacteria have glycans that are essential for growth and virulence but are absent from humans, making them high-priority targets for antibiotic, vaccine, and diagnostic development. The advent of metabolic labeling with bioorthogonal chemical reporters and small-molecule fluorescent reporters has enabled the investigation and targeting of specific bacterial glycans in their native environments. These tools have opened the door to imaging glycan dynamics, assaying and inhibiting glycan biosynthesis, profiling glycoproteins and glycan-binding proteins, and targeting pathogens with diagnostic and therapeutic payload. These capabilities have been wielded in diverse commensal and pathogenic Gram-positive, Gram-negative, and mycobacterial species─including within live host organisms. Here, we review the development and applications of chemical reporters for bacterial glycans, including peptidoglycan, lipopolysaccharide, glycoproteins, teichoic acids, and capsular polysaccharides, as well as mycobacterial glycans, including trehalose glycolipids and arabinan-containing glycoconjugates. We cover in detail how bacteria-targeting chemical reporters are designed, synthesized, and evaluated, how they operate from a mechanistic standpoint, and how this information informs their judicious and innovative application. We also provide a perspective on the current state and future directions of the field, underscoring the need for interdisciplinary teams to create novel tools and extend existing tools to support fundamental and translational research on bacterial glycans.

Roles of lytic transglycosylases in biofilm formation and β-lactam resistance in methicillin-resistant Staphylococcus aureus

Staphylococcus aureus is responsible for numerous community outbreaks and is one of the most frequent causes of nosocomial infections with significant morbidity and mortality. While the function of lytic transglycosylases (LTs) in relation to cell division, biofilm formation, and antibiotic resistance has been determined for several bacteria, their role in S. aureus remains largely unknown. The only known LTs in S. aureus are immunodominant staphylococcal antigen A (IsaA) and Staphylococcus epidermidis D protein (SceD). Our study demonstrates that, in a strain of methicillin-resistant S. aureus (MRSA), IsaA and SceD contribute differently to biofilm formation and β-lactam resistance. Deletion of isaA, but not sceD, led to decreased biofilm formation. Additionally, in isaA-deleted strains, β-lactam resistance was significantly decreased compared to that of wild-type strains. Plasmid-based expression of mecA, a major determinant of β-lactam resistance in MRSA, in an isaA-deleted strain did not restore β-lactam resistance, demonstrating that the β-lactam susceptibility phenotype is exhibited by isaA mutant regardless of the production level of PBP2a. Overall, our results suggest that IsaA is a potential therapeutic target for MRSA infections.

A mechanism-based GlcNAc-inspired cyclophellitol inactivator of the peptidoglycan recycling enzyme NagZ reverses resistance to β-lactams in Pseudomonas aeruginosa

The development of a potent mechanism-based inactivator of NagZ, an enzyme critical to the production of inducible AmpC β-lactamase in Gram-negative bacteria, is presented. This inactivator significantly reduces MIC values for important β-lactams against a clinically relevant strain of Pseudomonas aeruginosa.

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.

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.

Pertussis: Microbiology, Disease, Treatment, and Prevention

Pertussis is a severe respiratory infection caused by Bordetella pertussis, and in 2008, pertussis was associated with an estimated 16 million cases and 195,000 deaths globally. Sizeable outbreaks of pertussis have been reported over the past 5 years, and disease reemergence has been the focus of international attention to develop a deeper understanding of pathogen virulence and genetic evolution of B. pertussis strains. During the past 20 years, the scientific community has recognized pertussis among adults as well as infants and children. Increased recognition that older children and adolescents are at risk for disease and may transmit B. pertussis to younger siblings has underscored the need to better understand the role of innate, humoral, and cell-mediated immunity, including the role of waning immunity. Although recognition of adult pertussis has increased in tandem with a better understanding of B. pertussis pathogenesis, pertussis in neonates and adults can manifest with atypical clinical presentations. Such disease patterns make pertussis recognition difficult and lead to delays in treatment. Ongoing research using newer tools for molecular analysis holds promise for improved understanding of pertussis epidemiology, bacterial pathogenesis, bioinformatics, and immunology. Together, these advances provide a foundation for the development of new-generation diagnostics, therapeutics, and vaccines.

Peptidoglycan Recycling

Peptidoglycan (PG) recycling allows Escherichia coli to reuse the massive amounts of sacculus components that are released during elongation. Goodell and Schwarz, in 1985, labeled E. coli cells with 3H-diaminopimelic acid (DAP) and chased. During the chase, the DAP pool dropped dramatically, whereas the precursor pool dropped only slightly. This could only occur if DAP from the sacculi was being used to produce more precursor. They calculated that the cells were recycling about 45% of their wall DAP (actually, 60% of the side walls, since the poles are stable). Thus, recycling was discovered. Goodell went on to show that the tripeptide, L-Ala-D-Glu-DAP, could be taken up via opp and used directly to form PG. It was subsequently shown that uptake was predominantly via a permease, AmpG, that was specific for GlcNAc-anhMurNAc with attached peptides. Eleven genes have been identified which appear to have as their sole function the recovery of degradation products from PG. PG represents only 2.5% of the cell mass, so the reason for this investment in recycling is obscure. Recycling enzymes exist that are specific for every bond in the principal product taken up by AmpG, namely, GlcNAc-anh-MurNAc-tetrapeptide. However, most of the tripeptide, L-Ala-D-Glu-DAP, is used by murein peptide ligase (Mpl) to form the precursor intermediate UDP-MurNAc-tripeptide. anh-MurNAc can be converted to GlcNAc by a two-step process and thus is available for use. Surprisingly, in the absence of AmpD, an enzyme that cleaves the anh-MurNAc-L-Ala bond, anh-MurNAc-tripeptide accumulates, resulting in induction of beta-lactamase. However, this has nothing to do with the induction of beta-lactamase by beta-lactam antibiotics. Uehara, Suefuji, and Park (unpublished data) have some evidence suggesting that murein pentapeptide may be involved. The presence of orthologs suggests that recycling also exists in many Gram-negative bacteria. Surprisingly, the ortholog search also revealed that all mammals may have an AmpG ortholog! Hence, mammalian AmpG may be involved in the process of innate immunity.

Coarse-grained simulations of bacterial cell wall growth reveal that local coordination alone can be sufficient to maintain rod shape

Bacteria are surrounded by a peptidoglycan (PG) cell wall that must be remodeled to allow cell growth. While many structural details and properties of PG and the individual enzymes involved are known, how the process is coordinated to maintain cell integrity and rod shape is not understood. We have developed a coarse-grained method to simulate how individual transglycosylases, transpeptidases, and endopeptidases could introduce new material into an existing unilayer PG network. We find that a simple model with no enzyme coordination fails to maintain cell wall integrity and rod shape. We then iteratively analyze failure modes and explore different mechanistic hypotheses about how each problem might be overcome by the macromolecules involved. In contrast to a current theory, which posits that long MreB filaments are needed to coordinate PG insertion sites, we find that local coordination of enzyme activities in individual complexes can be sufficient to maintain cell integrity and rod shape. We also present possible molecular explanations for the existence of monofunctional transpeptidases and glycosidases (glycoside hydrolases), trimeric peptide crosslinks, cell twisting during growth, and synthesis of new strands in pairs.

Is the exoproteome important for bacterial pathogenesis? Lessons learned from interstrain exoprotein diversity in Listeria monocytogenes grown at different temperatures

Bacterial exoproteomes vary in composition and quantity among species and within each species, depending on the environmental conditions to which the cells are exposed. This article critically reviews the literature available on exoproteins synthesized by the foodborne pathogenic bacterium Listeria monocytogenes grown at different temperatures. The main challenges posed for exoproteome analyses and the strategies that are being used to overcome these constraints are discussed. Over thirty exoproteins from L. monocytogenes are considered, and the multifunctionality of some of them is discussed. Thus, at the host temperature of 37°C, good examples are provided by Lmo0443, a potential marker for low virulence, and by the virulence factors internalin C (InlC) and listeriolysin O (LLO). Based on the reported LLO-induced mucin exocytosis, a model is proposed for the involvement of extracellular LLO in optimizing the conditions for InlC intervention in the invasion of intestinal epithelial cells. At lower growth temperatures, exoproteins such as flagellin (FlaA) and oligopeptide permease (OppA) may explain the persistence of particular strains in the food industry environment, eventually allowing the development of new tools to eradicate L. monocytogenes, a major concern for public health.

Bordetella pertussis pathogenesis: current and future challenges

Pertussis, also known as whooping cough, has recently re-emerged as a major public health threat despite high levels of vaccination against the aetiological agent Bordetella pertussis. In this Review, we describe the pathogenesis of this disease, with a focus on recent mechanistic insights into B. pertussis virulence-factor function. We also discuss the changing epidemiology of pertussis and the challenges facing vaccine development. Despite decades of research, many aspects of B. pertussis physiology and pathogenesis remain poorly understood. We highlight knowledge gaps that must be addressed to develop improved vaccines and therapeutic strategies.

The development of selective inhibitors of NagZ: increased susceptibility of Gram-negative bacteria to β-lactams

The increasing incidence of inducible chromosomal AmpC β-lactamases within the clinic is a growing concern because these enzymes deactivate a broad range of even the most recently developed β-lactam antibiotics. As a result, new strategies are needed to block the action of this antibiotic resistance enzyme. Presented here is a strategy to combat the action of inducible AmpC by inhibiting the β-glucosaminidase NagZ, which is an enzyme involved in regulating the induction of AmpC expression. A divergent route facilitating the rapid synthesis of a series of N-acyl analogues of 2-acetamido-2-deoxynojirimycin is reported here. Among these compounds are potent NagZ inhibitors that are selective against functionally related human enzymes. These compounds reduce minimum inhibitory concentration values for β-lactams against a clinically relevant Gram-negative bacterium bearing inducible chromosomal AmpC β-lactamase, Pseudomonas aeruginosa. The structure of a NagZ–inhibitor complex provides insight into the molecular basis for inhibition by these compounds.

Bacterial cell-wall recycling

Many Gram-negative and Gram-positive bacteria recycle a significant proportion of the peptidoglycan components of their cell walls during their growth and septation. In many--and quite possibly all--bacteria, the peptidoglycan fragments are recovered and recycled. Although cell-wall recycling is beneficial for the recovery of resources, it also serves as a mechanism to detect cell-wall-targeting antibiotics and to regulate resistance mechanisms. In several Gram-negative pathogens, anhydro-MurNAc-peptide cell-wall fragments regulate AmpC β-lactamase induction. In some Gram-positive organisms, short peptides derived from the cell wall regulate the induction of both β-lactamase and β-lactam-resistant penicillin-binding proteins. The involvement of peptidoglycan recycling with resistance regulation suggests that inhibitors of the enzymes involved in the recycling might synergize with cell-wall-targeted antibiotics. Indeed, such inhibitors improve the potency of β-lactams in vitro against inducible AmpC β-lactamase-producing bacteria. We describe the key steps of cell-wall remodeling and recycling, the regulation of resistance mechanisms by cell-wall recycling, and recent advances toward the discovery of cell-wall-recycling inhibitors.

Physics of bacterial morphogenesis

Bacterial cells utilize three-dimensional (3D) protein assemblies to perform important cellular functions such as growth, division, chemoreception, and motility. These assemblies are composed of mechanoproteins that can mechanically deform and exert force. Sometimes, small-nucleotide hydrolysis is coupled to mechanical deformations. In this review, we describe the general principle for an understanding of the coupling of mechanics with chemistry in mechanochemical systems. We apply this principle to understand bacterial cell shape and morphogenesis and how mechanical forces can influence peptidoglycan cell wall growth. We review a model that can potentially reconcile the growth dynamics of the cell wall with the role of cytoskeletal proteins such as MreB and crescentin. We also review the application of mechanochemical principles to understand the assembly and constriction of the FtsZ ring. A number of potential mechanisms are proposed, and important questions are discussed.

Peptidoglycan turnover and recycling in Gram-positive bacteria

Bacterial cells are protected by an exoskeleton, the stabilizing and shape-maintaining cell wall, consisting of the complex macromolecule peptidoglycan. In view of its function, it could be assumed that the cell wall is a static structure. In truth, however, it is steadily broken down by peptidoglycan-cleaving enzymes during cell growth. In this process, named cell wall turnover, in one generation up to half of the preexisting peptidoglycan of a bacterial cell is released from the wall. This would result in a massive loss of cell material, if turnover products were not be taken up and recovered. Indeed, in the Gram-negative model organism Escherichia coli, peptidoglycan recovery has been recognized as a complex pathway, named cell wall recycling. It involves about a dozen dedicated recycling enzymes that convey cell wall turnover products to peptidoglycan synthesis or energy pathways. Whether Gram-positive bacteria also recover their cell wall is currently questioned. Given the much larger portion of peptidoglycan in the cell wall of Gram-positive bacteria, however, recovery of the wall material would provide an even greater benefit in these organisms compared to Gram-negatives. Consistently, in many Gram-positives, orthologs of recycling enzymes were identified, indicating that the cell wall may also be recycled in these organisms. This mini-review provides a compilation of information about cell wall turnover and recycling in Gram-positive bacteria during cell growth and division, including recent findings relating to muropeptide recovery in Bacillus subtilis and Clostridium acetobutylicum from our group. Furthermore, the impact of cell wall turnover and recycling on biotechnological processes is discussed.

Specific labeling of peptidoglycan precursors as a tool for bacterial cell wall studies

Because of its importance for bacterial cell survival, the bacterial cell wall is an attractive target for new antibiotics in a time of great demand for new antibiotic compounds. Therefore, more knowledge about the diverse processes related to bacterial cell wall synthesis is needed. The cell wall is located on the exterior of the cell and consists mainly of peptidoglycan, a large macromolecule built up from a three-dimensional network of aminosugar strands interlinked with peptide bridges. The subunits of peptidoglycan are synthesized inside the cell before they are transported to the exterior in order to be incorporated into the growing peptidoglycan. The high flexibility of the cell wall synthesis machinery towards unnatural derivatives of these subunits enables research on the bacterial cell wall using labeled compounds. This review highlights the high potential of labeled cell wall precursors in various areas of cell wall research. Labeled precursors can be used in investigating direct cell wall-antibiotic interactions and in cell wall synthesis and localization studies. Moreover, these compounds can provide a powerful tool in the elucidation of the cell wall proteome, the "wallosome," and thus, might provide new targets for antibiotics.

Substrate-induced inactivation of the Escherichia coli AmiD N-acetylmuramoyl-L-alanine amidase highlights a new strategy to inhibit this class of enzyme

In the eubacterial cell, the peptidoglycan is perpetually hydrolyzed throughout the cell cycle by different enzymes such as lytic transglycosylases, endopeptidases, and amidases. In Escherichia coli, four N-acetylmuramoyl-l-alanine amidases, AmiA, -B, -C, and -D, are present in the periplasm. AmiA, -B, and -C are soluble enzymes, whereas AmiD is a lipoprotein anchored in the outer membrane. To determine more precisely the specificity and the kinetic parameters of AmiD, we overproduced and purified the native His-tagged AmiD in the presence of detergent and a soluble truncated form of this enzyme by removing its signal peptide and the cysteine residue responsible for its lipidic anchorage. AmiD is a zinc metalloenzyme and is inactivated by a metal chelator such as EDTA. Native His-tagged and truncated AmiD hydrolyzes peptidoglycan fragments that have at least three amino acids in their peptide chains, and the presence of an anhydro function on the N-acetylmuramic acid is not essential for its activity. The soluble truncated AmiD exhibits a biphasic kinetic time course that can be explained by the inactivation of the enzyme by the substrate. This behavior highlights a new strategy to inhibit this class of enzymes.

How bacteria consume their own exoskeletons (turnover and recycling of cell wall peptidoglycan)

SUMMARY

The phenomenon of peptidoglycan recycling is reviewed. Gram-negative bacteria such as Escherichia coli break down and reuse over 60% of the peptidoglycan of their side wall each generation. Recycling of newly made peptidoglycan during septum synthesis occurs at an even faster rate. Nine enzymes, one permease, and one periplasmic binding protein in E. coli that appear to have as their sole function the recovery of degradation products from peptidoglycan, thereby making them available for the cell to resynthesize more peptidoglycan or to use as an energy source, have been identified. It is shown that all of the amino acids and amino sugars of peptidoglycan are recycled. The discovery and properties of the individual proteins and the pathways involved are presented. In addition, the possible role of various peptidoglycan degradation products in the induction of beta-lactamase is discussed.

Growth of Escherichia coli: significance of peptidoglycan degradation during elongation and septation

We have found a striking difference between the modes of action of amdinocillin (mecillinam) and compound A22, both of which inhibit cell elongation. This was made possible by employment of a new method using an Escherichia coli peptidoglycan (PG)-recycling mutant, lacking ampD, to analyze PG degradation during cell elongation and septation. Using this method, we have found that A22, which is known to prevent MreB function, strongly inhibited PG synthesis during elongation. In contrast, treatment of elongating cells with amdinocillin, which inhibits penicillin-binding protein 2 (PBP2), allowed PG glycan synthesis to proceed at a nearly normal rate with concomitant rapid degradation of the new glycan strands. By treating cells with A22 to inhibit sidewall synthesis, the method could also be applied to study septum synthesis. To our surprise, over 30% of newly synthesized septal PG was degraded during septation. Thus, excess PG sufficient to form at least one additional pole was being synthesized and rapidly degraded during septation. We propose that during cell division, rapid removal of the excess PG serves to separate the new poles of the daughter cells. We have also employed this new method to demonstrate that PBP2 and RodA are required for the synthesis of glycan strands during elongation and that the periplasmic amidases that aid in cell separation are minor players, cleaving only one-sixth of the PG that is turned over by the lytic transglycosylases.

Muramylpeptide shedding modulates cell sensing of Shigella flexneri

Bacterial infections trigger the activation of innate immunity through the interaction of pathogen-associated molecular patterns (PAMPs) with pattern recognition molecules (PRMs). The nucleotide-binding oligomerization domain (Nod) proteins are intracellular PRMs that recognize muramylpeptides contained in peptidoglycan (PGN) of bacteria. It is still unclear how Nod1 physically interacts with PGN, a structure internal to the Gram-negative bacterial envelope. To contribute to the understanding of this process, we demonstrate that, like Escherichia coli, Bordetella pertussis and Neisseria gonorrheae, the Gram-negative pathogen Shigella spontaneously releases PGN fragments and that this process can be increased by inactivating either ampG or mppA, genes involved in PGN recycling. Both Shigella mutants, but especially the strain carrying the mppA deletion, trigger Nod1-mediated NF-kappaB activation to a greater extent than the wild-type strain. Likewise, muramylpeptides spontaneously shed by Shigella are able per se to trigger a Nod1-mediated response consistent with the relative amount. Finally, we found that qualitative changes in muramylpeptide shedding can alter in vivo host responses to Shigella infection. Our findings support the idea that muramylpeptides released by pathogens during infection could modulate the immune response through Nod proteins and thereby influence the outcome of disease.

Z-ring force and cell shape during division in rod-like bacteria

The life cycle of bacterial cells consists of repeated elongation, septum formation, and division. Before septum formation, a division ring called the Z-ring, which is made of a filamentous tubulin analog, FtsZ, is seen at the mid cell. Together with several other proteins, FtsZ is essential for cell division. Visualization of strains with GFP-labeled FtsZ shows that the Z-ring contracts before septum formation and pinches the cell into two equal halves. Thus, the Z-ring has been postulated to act as a force generator, although the magnitude of the contraction force is unknown. In this article, we develop a mathematical model to describe the process of growth and Z-ring contraction in rod-like bacteria. The elasticity and growth of the cell wall is incorporated in the model to predict the contraction speed, the cell shape, and the contraction force. With reasonable parameters, the model shows that a small force from the Z-ring (8 pN in Escherichia coli) is sufficient to accomplish division.

Small Molecule Inhibitors of a Glycoside Hydrolase Attenuate Inducible AmpC-mediated β-Lactam Resistance

The increasing spread of plasmid-borne ampC-ampR operons is of considerable medical importance, since the AmpC beta-lactamases they encode confer high level resistance to many third generation cephalosporins. Induction of AmpC beta-lactamase from endogenous or plasmid-borne ampC-ampR operons is mediated by a catabolic inducer molecule, 1,6-anhydro-N-acetylmuramic acid (MurNAc) tripeptide, an intermediate of the cell wall recycling pathway derived from the peptidoglycan. Here we describe a strategy for attenuating the antibiotic resistance associated with the ampC-ampR operon by blocking the formation of the inducer molecule using small molecule inhibitors of NagZ, the glycoside hydrolase catalyzing the formation of this inducer molecule. The structure of the NagZ-inhibitor complex provides insight into the molecular basis for inhibition and enables the development of inhibitors with 100-fold selectivity for NagZ over functionally related human enzymes. These PUGNAc-derived inhibitors reduce the minimal inhibitory concentration (MIC) values for several clinically relevant cephalosporins in both wild-type and AmpC-hyperproducing strains lacking functional AmpD.

Murein (peptidoglycan) structure, architecture and biosynthesis in Escherichia coli

The periplasmic murein (peptidoglycan) sacculus is a giant macromolecule made of glycan strands cross-linked by short peptides completely surrounding the cytoplasmic membrane to protect the cell from lysis due to its internal osmotic pressure. More than 50 different muropeptides are released from the sacculus by treatment with a muramidase. Escherichia coli has six murein synthases which enlarge the sacculus by transglycosylation and transpeptidation of lipid II precursor. A set of twelve periplasmic murein hydrolases (autolysins) release murein fragments during cell growth and division. Recent data on the in vitro murein synthesis activities of the murein synthases and on the interactions between murein synthases, hydrolases and cell cycle related proteins are being summarized. There are different models for the architecture of murein and for the incorporation of new precursor into the sacculus. We present a model in which morphogenesis of the rod-shaped E. coli is driven by cytoskeleton elements competing for the control over the murein synthesis multi-enzyme complexes.

The Critical Roles of Polyamines in Regulating ColE7 Production and Restricting ColE7 Uptake of the Colicin-producing Escherichia coli

The ColE7 operon is an SOS response regulon, which encodes bacteriocin ColE7 to kill susceptible Escherichia coli and its related enterobacteria under conditions of stress. We have observed for the first time that polyamines confer limited resistance against ColE7 on E. coli cells. Thus, this study aims to investigate the role of polyamines in modulating the protective effect of the E. coli cells against colicin. In the experiments, we surprisingly found that endogenous polyamines are also essential for ColE7 production, and the rate of polyamine synthesis is directly related to the SOS response. Our experimental results further indicated that exogenous polyamines suppress the expression of TolA, BtuB, OmpF, and OmpC proteins that are responsible for ColE7 uptake. Moreover, two-dimensional gel electrophoresis revealed that the production of two periplasmic proteins, PotD and OppA, is increased in E. coli cells under ColE7 exposure. Based on these observations, we propose that endogenous polyamines may play a dual role in the ColE7 system. Polyamines may participate in initiating the expression of the SOS response of the ColE7 operon and simultaneously down-regulate proteins that are essential for colicin uptake, thus conferring a survival advantage on colicin-producing E. coli under stress conditions in the natural environment.

Bacterial cell wall synthesis: new insights from localization studies

In order to maintain shape and withstand intracellular pressure, most bacteria are surrounded by a cell wall that consists mainly of the cross-linked polymer peptidoglycan (PG). The importance of PG for the maintenance of bacterial cell shape is underscored by the fact that, for various bacteria, several mutations affecting PG synthesis are associated with cell shape defects. In recent years, the application of fluorescence microscopy to the field of PG synthesis has led to an enormous increase in data on the relationship between cell wall synthesis and bacterial cell shape. First, a novel staining method enabled the visualization of PG precursor incorporation in live cells. Second, penicillin-binding proteins (PBPs), which mediate the final stages of PG synthesis, have been localized in various model organisms by means of immunofluorescence microscopy or green fluorescent protein fusions. In this review, we integrate the knowledge on the last stages of PG synthesis obtained in previous studies with the new data available on localization of PG synthesis and PBPs, in both rod-shaped and coccoid cells. We discuss a model in which, at least for a subset of PBPs, the presence of substrate is a major factor in determining PBP localization.

Hexose/Pentose and Hexitol/Pentitol Metabolism

Escherichia coli and Salmonella enterica serovar Typhimurium exhibit a remarkable versatility in the usage of different sugars as the sole source of carbon and energy, reflecting their ability to make use of the digested meals of mammalia and of the ample offerings in the wild. Degradation of sugars starts with their energy-dependent uptake through the cytoplasmic membrane and is carried on further by specific enzymes in the cytoplasm, destined finally for degradation in central metabolic pathways. As variant as the different sugars are, the biochemical strategies to act on them are few. They include phosphorylation, keto-enol isomerization, oxido/reductions, and aldol cleavage. The catabolic repertoire for using carbohydrate sources is largely the same in E. coli and in serovar Typhimurium. Nonetheless, significant differences are found, even among the strains and substrains of each species. We have grouped the sugars to be discussed according to their first step in metabolism, which is their active transport, and follow their path to glycolysis, catalyzed by the sugar-specific enzymes. We will first discuss the phosphotransferase system (PTS) sugars, then the sugars transported by ATP-binding cassette (ABC) transporters, followed by those that are taken up via proton motive force (PMF)-dependent transporters. We have focused on the catabolism and pathway regulation of hexose and pentose monosaccharides as well as the corresponding sugar alcohols but have also included disaccharides and simple glycosides while excluding polysaccharide catabolism, except for maltodextrins.

Recycling of the anhydro-N-acetylmuramic acid derived from cell wall murein involves a two-step conversion to N-acetylglucosamine-phosphate

Escherichia coli breaks down over 60% of the murein of its side wall and reuses the component amino acids to synthesize about 25% of the cell wall for the next generation. The amino sugars of the murein are also efficiently recycled. Here we show that the 1,6-anhydro-N-acetylmuramic acid (anhMurNAc) is returned to the biosynthetic pathway by conversion to N-acetylglucosamine-phosphate (GlcNAc-P). The sugar is first phosphorylated by anhydro-N-acetylmuramic acid kinase (AnmK), yielding MurNAc-P, and this is followed by action of an etherase which cleaves the bond between D-lactic acid and the N-acetylglucosamine moiety of MurNAc-P, yielding GlcNAc-P. The kinase gene has been identified by a reverse genetics method. The enzyme was overexpressed, purified, and characterized. The cell extract of an anmK deletion mutant totally lacked activity on anhMurNAc. Surprisingly, in the anmK mutant, anhMurNAc did not accumulate in the cytoplasm but instead was found in the medium, indicating that there was rapid efflux of free anhMurNAc.

Identification of genes required for avian Escherichia coli septicemia by signature-tagged mutagenesis

Infections with avian pathogenic Escherichia coli (APEC) cause colibacillosis, an acute and largely systemic disease resulting in significant economic losses in poultry industry worldwide. Although various virulence-associated genes have been identified in APEC, their actual role in pathogenesis is still not fully understood, and, furthermore, certain steps of the infection process have not been related to previously identified factors. Here we describe the application of a signature-tagged transposon mutagenesis (STM) approach to identify critical genes required for APEC infections in vivo. Twenty pools of about 1,800 IMT5155 (O2:H5) mutants were screened in an infection model using 5-week-old chickens, and potentially attenuated mutants were subjected to a secondary screen and in vivo competition assays to confirm their attenuation. A total of 28 genes required for E. coli septicemia in chickens were identified as candidates for further characterization. Among these disrupted genes, six encode proteins involved in biosynthesis of extracellular polysaccharides and lipopolysaccharides; two encode iron transporters that have not been previously characterized in APEC in in vivo studies, and four showed similarity to membrane or periplasmic proteins. In addition, several metabolic enzymes, putative proteins with unknown function, and open reading frames with no similarity to other database entries were identified. This genome-wide analysis has identified both novel and previously known factors potentially involved in pathogenesis of APEC infection.

Susceptibility to antibiotics and beta-lactamase induction in murein hydrolase mutants of Escherichia coli

The antibiotic susceptibilities and capabilities to induce beta-lactamases were studied in multiple Escherichia coli murein (peptidoglycan) hydrolase mutants. E. coli mutants lacking either three amidases, three amidases and one lytic transglycosylase, or six lytic transglycosylases showed higher levels of susceptibility to bacitracin, erythromycin, gallidermin, and vancomycin than the wild type. Mutant cells without three amidases lost viability in the presence of vancomycin and gallidermin, whereas the wild type was resistant to both antibiotics. Beta-lactamase induction was studied after introduction of a plasmid carrying the ampC and ampR genes. Upon addition of cefoxitin to the growth medium, the wild type as well as a mutant lacking all known amidases and DD-endopeptidases induced beta-lactamase, whereas a mutant lacking all known lytic transglycosylases was unable to induce beta-lactamase, showing that lytic transglycosylase activity is essential for beta-lactamase induction. Consequently, cells lacking lytic transglycosylase activity lysed in the presence of penicillin, despite the presence of the inducible beta-lactamase system. We discuss the potential of murein hydrolase inhibitors for antibiotic therapy.

Identification of genes required for avian Escherichia coli septicemia by signature-tagged mutagenesis

Infections with avian pathogenic Escherichia coli (APEC) cause colibacillosis, an acute and largely systemic disease resulting in significant economic losses in poultry industry worldwide. Although various virulence-associated genes have been identified in APEC, their actual role in pathogenesis is still not fully understood, and, furthermore, certain steps of the infection process have not been related to previously identified factors. Here we describe the application of a signature-tagged transposon mutagenesis (STM) approach to identify critical genes required for APEC infections in vivo. Twenty pools of about 1,800 IMT5155 (O2:H5) mutants were screened in an infection model using 5-week-old chickens, and potentially attenuated mutants were subjected to a secondary screen and in vivo competition assays to confirm their attenuation. A total of 28 genes required for E. coli septicemia in chickens were identified as candidates for further characterization. Among these disrupted genes, six encode proteins involved in biosynthesis of extracellular polysaccharides and lipopolysaccharides; two encode iron transporters that have not been previously characterized in APEC in in vivo studies, and four showed similarity to membrane or periplasmic proteins. In addition, several metabolic enzymes, putative proteins with unknown function, and open reading frames with no similarity to other database entries were identified. This genome-wide analysis has identified both novel and previously known factors potentially involved in pathogenesis of APEC infection.

Identification of a phosphotransferase system of Escherichia coli required for growth on N-acetylmuramic acid

We report here that wild-type Escherichia coli grows on N-acetylmuramic acid (MurNAc) as the sole source of carbon and energy. Analysis of mutants defective in N-acetylglucosamine (GlcNAc) catabolism revealed that the catabolic pathway for MurNAc merges into the GlcNAc pathway on the level of GlcNAc 6-phosphate. Furthermore, analysis of mutants defective in components of the phosphotransferase system (PTS) revealed that a PTS is essential for growth on MurNAc. However, neither the glucose-, mannose/glucosamine-, nor GlcNAc-specific PTS (PtsG, ManXYZ, and NagE, respectively) was found to be necessary. Instead, we identified a gene at 55 min on the E. coli chromosome that is responsible for MurNAc uptake and growth. It encodes a single polypeptide consisting of the EIIB and C domains of a so-far-uncharacterized PTS that was named murP. MurP lacks an EIIA domain and was found to require the activity of the crr-encoded enzyme IIA-glucose (EIIA(Glc)), a component of the major glucose transport system for growth on MurNAc. murP deletion mutants were unable to grow on MurNAc as the sole source of carbon; however, growth was rescued by providing murP in trans expressed from an isopropylthiogalactopyranoside-inducible plasmid. A functional His(6) fusion of MurP was constructed, isolated from membranes, and identified as a polypeptide with an apparent molecular mass of 37 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis. Close homologs of MurP were identified in the genome of several bacteria, and we believe that these organisms might also be able to utilize MurNAc.

Identification of genes involved in the switch between the smooth and rugose phenotypes of Vibrio cholerae

Vibrio cholerae can switch to a 'rugose' phenotype characterized by an exopolysaccharide (EPS) matrix, wrinkled colony morphology, increased biofilm formation and increased survival under specific conditions. The vps gene cluster responsible for the biosynthesis of the rugose EPS (rEPS) is positively regulated by VpsR. We recently identified media (APW#3) promoting EPS production and the rugose phenotype and found epidemic strains switch at a higher frequency than non-pathogenic strains, suggesting this switch and the rugose phenotype are important in cholera epidemiology. In this study, transposon mutagenesis on a smooth V. cholerae strain was used to identify mutants that were unable to shift to the rugose phenotype under inducing conditions to better understand the molecular basis of the switch. We identified vpsR, galE and vps previously associated with the rugose phenotype, and also identified genes not previously associated with the phenotype, including rfbD and rfbE having roles in LPS (lipopolysaccharide) synthesis and aroB and aroK with roles in aromatic amino acid synthesis. Additionally, a mutation in amiB encoding N-acetylmuramoyl-L-alanine amidase caused defects in the switch, motility and cell morphology. We also found that a gene encoding a novel regulatory protein we termed RocS (regulation of cell signaling) containing a GGDEF and EAL domains and associated with c-di-GMP levels is important for the rugose phenotype, EPS, biofilm formation and motility. We propose that modulation of cyclic dinucleotide (e.g. c-di-GMP) levels might have application in regulating various phenotypes of prokaryotes. Our study shows the molecular complexity of the switch between the smooth and rugose phenotypes of V. cholerae and may be relevant to similar phenotypes in other species.

Mutational analysis of the catalytic centre of the Citrobacter freundii AmpD N-acetylmuramyl-L-alanine amidase

Citrobacter freundii AmpD is an intracellular 1,6-anhydro-N-acetylmuramyl-L-alanine amidase involved in both peptidoglycan recycling and beta-lactamase induction. AmpD exhibits a strict specificity for 1,6-anhydromuropeptides and requires zinc for enzymic activity. The AmpD three-dimensional structure exhibits a fold similar to that of another Zn2+ N-acetylmuramyl-L-alanine amidase, the T7 lysozyme, and these two enzymes define a new family of Zn-amidases which can be related to the eukaryotic PGRP (peptidoglycan-recognition protein) domains. In an attempt to assign the different zinc ligands and to probe the catalytic mechanism of AmpD amidase, molecular modelling based on the NMR structure and site-directed mutagenesis were performed. Mutation of the two residues presumed to act as zinc ligands into alanine (H34A and D164A) yielded inactive proteins which had also lost their ability to bind zinc. By contrast, the active H154N mutant retained the capacity to bind the metal ion. Three other residues which could be involved in the AmpD catalytic mechanism have been mutated (Y63F, E116A, K162H and K162Q). The E116A mutant was inactive, but on the basis of the molecular modelling this residue is not directly involved in the catalytic mechanism, but rather in the binding of the zinc by contributing to the correct orientation of His-34. The K162H and K162Q mutants retained very low activity (0.7 and 0.2% of the wild-type activity respectively), whereas the Y63F mutant showed 16% of the wild-type activity. These three latter mutants exhibited a good affinity for Zn ions and the substituted residues are probably involved in the binding of the substrate. We also describe a new method for generating the N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-tripeptide AmpD substrate from purified peptidoglycan by the combined action of two hydrolytic enzymes.

NMR structure of Citrobacter freundii AmpD, comparison with bacteriophage T7 lysozyme and homology with PGRP domains

AmpD is a bacterial amidase involved in the recycling of cell-wall fragments in Gram-negative bacteria. Inactivation of AmpD leads to derepression of beta-lactamase expression, presenting a major pathway for the acquisition of constitutive antibiotic resistance. Here, we report the NMR structure of AmpD from Citrobacter freundii (PDB accession code 1J3G). A deep substrate-binding pocket explains the observed specificity for low molecular mass substrates. The fold is related to that of bacteriophage T7 lysozyme. Both proteins bind zinc at a conserved site and require zinc for amidase activity, although the enzymatic mechanism seems to differ in detail. The structure-based sequence alignment identifies conserved features that are also conserved in the eukaryotic peptidoglycan recognition protein (PGRP) domains, including the zinc-coordination site in several of them. PGRP domains thus belong to the same fold family and, where zinc-binding residues are conserved, may have amidase activity. This hypothesis is supported by the observation that human serum N-acetylmuramyl-L-alanine amidase seems to be identical with a soluble form of human PGRP-L.

Role of the murein precursor UDP-N-acetylmuramyl-L-Ala-gamma-D-Glu-meso-diaminopimelic acid-D-Ala-D-Ala in repression of beta-lactamase induction in cell division mutants

Certain beta-lactam antibiotics induce the chromosomal ampC beta-lactamase of many gram-negative bacteria. The natural inducer, though not yet unequivocally identified, is a cell wall breakdown product which enters the cell via the AmpG permease component of the murein recycling pathway. Surprisingly, it has been reported that beta-lactamase is not induced by cefoxitin in the absence of FtsZ, which is required for cell division, or in the absence of penicillin-binding protein 2 (PBP2), which is required for cell elongation. Since these results remain unexplained, we examined an ftsZ mutant and other cell division mutants (ftsA, ftsQ, and ftsI) and a PBP2 mutant for induction of beta-lactamase. In all mutants, beta-lactamase was not induced by cefoxitin, which confirms the initial reports. The murein precursor, UDP-N-acetylmuramyl-L-Ala-gamma-D-Glu-meso-diaminopimelic acid-D-Ala-D-Ala (UDP-MurNAc-pentapeptide), has been shown to serve as a corepressor with AmpR to repress beta-lactamase expression in vitro. Our results suggest that beta-lactamase is not induced because the fts mutants contain a greatly increased amount of corepressor which the inducer cannot displace. In the PBP2(Ts) mutant, in addition to accumulation of corepressor, cell wall turnover and recycling were greatly reduced so that little or no inducer was available. Hence, in both cases, a high ratio of repressor to inducer presumably prevents induction.

Identification of a dedicated recycling pathway for anhydro-N-acetylmuramic acid and N-acetylglucosamine derived from Escherichia coli cell wall murein

Turnover and recycling of the cell wall murein represent a major metabolic pathway of Escherichia coli. It is known that E. coli efficiently reuses, i.e., recycles, its murein tripeptide, L-alanyl-gamma-D-glutamyl-meso-diaminopimelate, to form new murein. However, the question of whether the cells also recycle the amino sugar moieties of cell wall murein has remained unanswered. It is demonstrated here that E. coli recycles the N-acetylglucosamine present in cell wall murein degradation products for de novo murein and lipopolysaccharide synthesis. Furthermore, E. coli also recycles the anhydro-N-acetylmuramic acid moiety by first converting it into N-acetylglucosamine. Based on the results obtained by studying mutants unable to recycle amino sugars, the pathway for recycling is revealed.

Characterization of a beta -N-acetylglucosaminidase of Escherichia coli and elucidation of its role in muropeptide recycling and beta -lactamase induction

Using the known mapping position the gene encoding a beta-1, 4-N-acetylglucosaminidase needed for the degradation of muropeptides could be identified. nagZ encodes a cytosolic enzyme active on N-actylglucosamyl-beta-1,4-(1,6)-anhydromuramic acid containing muropeptides. These degradation products of the peptidoglycan are formed during the enlargement of the murein sacculus as a consequence of a growth mechanism, which couples the controlled degradation of the cell wall polymer with the insertion of new material. NagZ is needed for the formation of monosaccharides from the released disaccharides during the cytosolic steps of the muropeptide-recycling pathway. The formation of intracellular 1, 6-anhydro-N-acetylmuramyl-peptides is important for the expression control of the inducible beta-lactamases of the AmpC type. A mutant lacking active NagZ cannot establish AmpC mediated beta-lactam resistance. The biochemical characterization of the enzyme showed its activity on different muropeptides and inhibitors of enzyme activity could be identified. This observation might be important for designing inhibitors of NagZ that could prevent the establishment of beta-lactam resistance of Enterobacteria possessing inducible beta-lactamases.

Inactivation of the ampDE operon increases transcription of algD and affects morphology and encystment of Azotobacter vinelandii

Transcription of algD, encoding GDP-mannose dehydrogenase, the key enzyme in the alginate biosynthetic pathway, is highly regulated in Azotobacter vinelandii. We describe here the characterization of a Tn5 insertion mutant (AC28) which shows a higher level of expression of an algD::lacZ fusion. AC28 cells were morphologically abnormal and unable to encyst. The cloning and nucleotide sequencing of the Tn5-disrupted locus in AC28 revealed an operon homologous to the Escherichia coli ampDE operon. Tn5 was located within the ampD gene, encoding a cytosolic N-acetyl-anhydromuramyl-L-alanine amidase that participates in the intracellular recycling of peptidoglycan fragments. The ampE gene encodes a transmembrane protein, but the function of the protein is not known. We constructed strains carrying ampD or ampE mutations and one with an ampDE deletion. The strain with a deletion of the ampDE operon showed a phenotype similar to that of mutant AC28. The present work demonstrates that both alginate production and bacterial encystment are greatly influenced by the bacterial ability to recycle its cell wall.

Ralstonia solanacearum AmpD is required for wild-type bacterial wilt virulence

Abstract A gene resembling enterobacterial ampD was identified in the bacterial wilt pathogen, Ralstonia solanacearum. The gene lies 13 bp 3' of pehSR, a two-component positive regulator of virulence factors such as plant cell wall-degrading polygalacturonases and bacterial motility. AmpD, an N-acetylmuramyl-l-alanine amidase, degrades and recycles bacterial cell wall components and also plays a role in the induction of beta-lactamase, which confers ampicillin resistance. AmpD is probably not involved in beta-lactamase regulation in R. solanacearum, because the species produces no detectable beta-lactamase activity and is not ampicillin resistant. However, the R. solanacearum ampD gene restores inducible beta-lactamase activity to an Escherichia coli ampD mutant, demonstrating that the gene encodes an AmpD protein that can function in a heterologous background. An R. solanacearumampD chromosomal mutant was motile, produced wild-type levels of polygalacturonase activity and had wild-type cell and colony morphology. This mutant also grew normally in minimal medium and in plant tissue. Nonetheless, the ampD mutant was significantly reduced in bacterial wilt virulence on eggplant and tomato, suggesting a previously unsuspected role for N-acetylmuramyl-l-alanine amidase in plant pathogenesis.

beta-Lactamase induction in gram-negative bacteria is intimately linked to peptidoglycan recycling

A number of Gram-negative organisms normally express a chromosomally mediated class C beta-lactamase that is inducible by beta-lactam antibiotics. Data have recently emerged suggesting a close link between beta-lactamase induction and the recycling of released muramyl peptides from the bacterial peptidoglycan. Thus the AmpG transporter is responsible for the uptake into the cell of GlcNAc-anhMurNAc-tripeptide. A mutant unable to express AmpG is therefore unable to recycle the cell wall and is at the same time not possible to induce by a beta-lactam. Once inside the cytosol the above muramyl peptide and its derivative anhMurNAc-tripeptide is degraded by the cytosolic AmpD amidase that specifically releases the tripeptide from cytosolic muramyl peptides brought into the cell via AmpG. Mutants unable to produce AmpD are blocked in a cytosolic step for cell wall recycling and accumulate large amounts of cytosolic anhMurNAc-tripeptide. It is believed that cytosolic muramyl peptides can act as ligands for the beta-lactamase regulator AmpR to activate expression of beta-lactamase. AmpD mutants, therefore, constitutively overproduce the chromosomal beta-lactamase and are beta-lactam resistant. In wild-type strains beta-lactams that result in an increased cell wall breakdown will cause an increase in the cytosol of muramyl peptides leading to beta-lactamase induction. Mutants affected in the ampD gene arise readily during treatment with third-generation cephalosporins. Since these mutants lack a functional cell wall recycling system they may be at a disadvantage in the absence of selection. However, since muramyl peptides may act as cytotoxins, especially for respiratory epithelial cells, ampD mutants due to their large accumulation of anhMurNAc-tripeptide may be altered in their pathogenic properties as compared to wild-type cells possessing a normal cell wall recycling system.

The convergence of murein recycling research with beta-lactamase research

The story of how investigation of Escherichia coli cell wall elongation evolved into a study of murein recycling and how this led to the discovery that ampG and ampD were required for both murein recycling and beta-lactamase regulation is chronicled. Preliminary information on two other genes believed to be involved in recycling, nagZ, the structural gene for beta-N-acetylglucosaminidase, and tpl, the presumed structural gene for the hypothetical tripeptide-adding enzyme, is presented. The possibility that recycling of murein fragments serves a signaling function is discussed.

Synergistic epithelial responses to endotoxin and a naturally occurring muramyl peptide

We have investigated the synergistic interactions of a naturally occurring peptidoglycan fragment (muramyl peptide) and bacterial endotoxin in the induction of inflammatory processes within respiratory epithelial cells, at the levels of both signal transduction events and ultimate cellular metabolic effects. The source of the muramyl peptide is Bordetella pertussis, the causative agent of the respiratory disease pertussis. During log-phase growth, B. pertussis releases the muramyl peptide tracheal cytotoxin (TCT), which has the structure N - acetylglucosaminyl - 1,6 - anhydro - N - acetylmuramyl - (L) - alanyl - gamma - (D) - glutamyl - meso - diaminopimelyl - (D) - alanine, equivalent to a monomeric subunit of gram-negative bacterial peptidoglycan. When applied to hamster trachea epithelial (HTE) cells, TCT and endotoxin were found to be highly synergistic in the induction of interleukin-1alpha (IL-1alpha), type II (inducible) nitric oxide synthase (iNOS), nitric oxide production, and inhibition of DNA synthesis. Neither molecule alone significantly triggered these responses. The serine/threonine protein kinase inhibitor H7 blocked induction of both IL-1alpha and iNOS. More selective inhibitors of protein kinase C, cyclic AMP-dependent protein kinase, and cyclic GMP-dependent protein kinase were not capable of blocking the effects of TCT and endotoxin, suggesting that the H7-inhibited component in this pathway is not among the commonly described kinase targets of H7. Treatment of HTE cells with exogenous IL-1 reproduced the induction of iNOS and DNA synthesis inhibition caused by TCT and endotoxin. H7 was not capable of interfering with effects caused by exogenous IL-1, implying that the H7-sensitive step in the pathway is upstream of IL-1 protein production. Similar assays with the phorbol ester phorbol myristate acetate indicate that it could effectively synergize with endotoxin but not with TCT, suggesting that TCT and endotoxin induce different signal transduction events that combine synergistically. The synergy observed with TCT and endotoxin in epithelial cells is significantly different from their interaction with other cell types, revealing a unique inflammatory response by epithelial cells to these natural bacterial products.

Interference with murein turnover has no effect on growth but reduces beta-lactamase induction in Escherichia coli

Physiological studies of a mutant of Escherichia coli lacking the three lytic transglycosylases Slt70, MltA, and MltB revealed that interference with murein turnover can prevent AmpC beta-lactamase induction. The triple mutant, although growing normally, shows a dramatically reduced rate of murein turnover. Despite the reduction in the formation of low-molecular-weight murein turnover products, neither the rate of murein synthesis nor the amount of murein per cell was increased. This might be explained by assuming that during growth in the absence of the major lytic transglycosylases native murein strands are excised by the action of endopeptidases and directly reused without further breakdown to muropeptides. The reduced rate of murein turnover could be correlated with lowered cefoxitin-induced expression of beta-lactamase, present on a plasmid carrying the ampC and ampR genes from Enterobacter cloacae. Overproduction of MltB stimulated beta-lactamase induction, whereas specific inhibition of Slt70 by bulgecin repressed ampC expression. Thus, specific inhibitors of lytic transglycosylases can increase the potency of penicillins and cephalosporins against bacteria inducing AmpC-like beta-lactamases.

Comparison of the D-glutamate-adding enzymes from selected gram-positive and gram-negative bacteria

The biochemical properties of the D-glutamate-adding enzymes (MurD) from Escherichia coli, Haemophilus influenzae, Enterococcus faecalis, and Staphylococcus aureus were investigated to detect any differences in the activity of this enzyme between gram-positive and gram-negative bacteria. The genes (murD) that encode these enzymes were cloned into pMAL-c2 fusion vector and overexpressed as maltose-binding protein-MurD fusion proteins. Each fusion protein was purified to homogeneity by affinity to amylose resin. Proteolytic treatments of the fusion proteins with factor Xa regenerated the individual MurD proteins. It was found that these fusion proteins retain D-glutamate-adding activity and have Km and Vmax values similar to those of the regenerated MurDs, except for the H. influenzae enzyme. Substrate inhibition by UDP-N-acetylmuramyl-L-alanine, the acceptor substrate, was observed at concentrations greater than 15 and 30 microM for E. coli and H. influenzae MurD, respectively. Such substrate inhibition was not observed with the E. faecalis and S. aureus enzymes, up to a substrate concentration of 1 to 2 mM. In addition, the two MurDs of gram-negative origin were shown to require monocations such as NH4+ and/or K+, but not Na+, for optimal activity, while anions such as Cl- and SO4(2-) had no effect on the enzyme activities. The activities of the two MurDs of gram-positive origin, on the other hand, were not affected by any of the ions tested. All four enzymes required Mg2+ for the ligase activity and exhibited optimal activities around pH 8. These differences observed between the gram-positive and gram-negative MurDs indicated that the two gram-negative bacteria may apply a more stringent regulation of cell wall biosynthesis at the early stage of peptidoglycan biosynthesis pathway than do the two gram-positive bacteria. Therefore, the MurD-catalyzed reaction may constitute a fine-tuning step necessary for the gram-negative bacteria to optimally maintain its relatively thin yet essential cell wall structure during all stages of growth.