Turnover of the Cell Wall of Gram-positive Bacteria

The purpose and ubiquity of turnover

Turnover-constant component production and destruction-is ubiquitous in biology. Turnover occurs across organisms and scales, including for RNAs, proteins, membranes, macromolecular structures, organelles, cells, hair, feathers, nails, antlers, and teeth. For many systems, turnover might seem wasteful when degraded components are often fully functional. Some components turn over with shockingly high rates and others do not turn over at all, further making this process enigmatic. However, turnover can address fundamental problems by yielding powerful properties, including regeneration, rapid repair onset, clearance of unpredictable damage and errors, maintenance of low constitutive levels of disrepair, prevention of stable hazards, and transitions. I argue that trade-offs between turnover benefits and metabolic costs, combined with constraints on turnover, determine its presence and rates across distinct contexts. I suggest that the limits of turnover help explain aging and that turnover properties and the basis for its levels underlie this fundamental component of life.

Peptide transport in Bacillus subtilis - structure and specificity in the extracellular solute binding proteins OppA and DppE

Peptide transporters play important nutritional and cell signalling roles in Bacillus subtilis, which are pronounced during stationary phase adaptations and development. Three high-affinity ATP-binding cassette (ABC) family transporters are involved in peptide uptake - the oligopeptide permease (Opp), another peptide permease (App) and a less well-characterized dipeptide permease (Dpp). Here we report crystal structures of the extracellular substrate binding proteins, OppA and DppE, which serve the Opp and Dpp systems, respectively. The structure of OppA was determined in complex with endogenous peptides, modelled as Ser-Asn-Ser-Ser, and with the sporulation-promoting peptide Ser-Arg-Asn-Val-Thr, which bind with K _d values of 0.4 and 2 µM, respectively, as measured by isothermal titration calorimetry. Differential scanning fluorescence experiments with a wider panel of ligands showed that OppA has highest affinity for tetra- and penta-peptides. The structure of DppE revealed the unexpected presence of a murein tripeptide (MTP) ligand, l-Ala-d-Glu-meso-DAP, in the peptide binding groove. The mode of MTP binding in DppE is different to that observed in the murein peptide binding protein, MppA, from Escherichia coli, suggesting independent evolution of these proteins from an OppA-like precursor. The presence of MTP in DppE points to a role for Dpp in the uptake and recycling of cell wall peptides, a conclusion that is supported by analysis of the genomic context of dpp, which revealed adjacent genes encoding enzymes involved in muropeptide catabolism in a gene organization that is widely conserved in Firmicutes.

Use of Capillary Zone Electrophoresis Coupled to Electrospray Mass Spectrometry for the Detection and Absolute Quantitation of Peptidoglycan-Derived Peptides in Bacterial Cytoplasmic Extracts

Peptidoglycan (PGN) is an essential structure found in the bacterial cell wall. During the bacterial life cycle, PGN continuously undergoes biosynthesis and degradation to ensure bacterial growth and division. The resulting PGN fragments (muropeptides and peptides), which are generated by the bacterial autolytic system, are usually transported into the cytoplasm to be recycled. On the other hand, PGN fragments can act as messenger molecules involved in the bacterial cell wall stress response as in the case of β-lactamase induction in the presence of β-lactam antibiotic or in triggering mammalian innate immune response. During their cellular life, bacteria modulate their PGN degradation by their autolytic system or their recognition by the mammalian innate immune system by chemically modifying their PGN. Among these modifications, the amidation of the ε-carboxyl group of meso-diaminopimelic acid present in the PGN peptide chain is frequently observed. Currently, the detection and quantitation of PGN-derived peptides is still challenging because of the difficulty in separating these highly hydrophilic molecules by RP-HPLC as these compounds are eluted closely after the column void volume or coeluted in many cases. Here, we report the use of capillary zone electrophoresis coupled via an electrospray-based CE-MS interface to high-resolution mass spectrometry for the quantitation of three PGN peptides of interest and their amidated derivatives in bacterial cytoplasmic extracts. The absolute quantitation of the tripeptide based on the [¹³C,¹⁵N] isotopically labeled standard was also performed in crude cytoplasmic extracts of bacteria grown in the presence or absence of a β-lactam antibiotic (cephalosporin C). Despite the high complexity of the samples, the repeatability of the CZE-MS quantitation results was excellent, with relative standard deviations close to 1%. The global reproducibility of the method including biological handling was better than 20%.

Gene loss through pseudogenization contributes to the ecological diversification of a generalist Roseobacter lineage

Ecologically relevant genes generally show patchy distributions among related bacterial genomes. This is commonly attributed to lateral gene transfer, whereas the opposite mechanism-gene loss-has rarely been explored. Pseudogenization is a major mechanism underlying gene loss, and pseudogenes are best characterized by comparing closely related genomes because of their short life spans. To explore the role of pseudogenization in microbial ecological diversification, we apply rigorous methods to characterize pseudogenes in the 279 newly sequenced Ruegeria isolates of the globally abundant Roseobacter group collected from two typical coastal habitats in Hong Kong, the coral Platygyra acuta and the macroalga Sargassum hemiphyllum. Pseudogenes contribute to ~16% of the accessory genomes of these strains. Ancestral state reconstruction reveals that many pseudogenization events are correlated with ancestral niche shifts. Specifically, genes related to resource scavenging and energy acquisition were often pseudogenized when roseobacters inhabiting carbon-limited and energy-poor coral skeleton switched to other resource-richer niches. For roseobacters inhabiting the macroalgal niches, genes for nitrogen regulation and carbohydrate utilization were important but became dispensable upon shift to coral skeleton where nitrate is abundant but carbohydrates are less available. Whereas low-energy-demanding secondary transporters are more favorable in coral skeleton, ATP-driven primary transporters are preferentially kept in the energy-replete macroalgal niches. Moreover, a large proportion of these families mediate organismal interactions, suggesting their rapid losses by pseudogenization as a potential response to host and niche shift. These findings illustrate an important role of pseudogenization in shaping genome content and driving ecological diversification of marine roseobacters.

Cell wall peptidoglycan in Mycobacterium tuberculosis: An Achilles' heel for the TB-causing pathogen

Tuberculosis (TB), caused by the intracellular pathogen Mycobacterium tuberculosis, remains one of the leading causes of mortality across the world. There is an urgent requirement to build a robust arsenal of effective antimicrobials, targeting novel molecular mechanisms to overcome the challenges posed by the increase of antibiotic resistance in TB. Mycobacterium tuberculosis has a unique cell envelope structure and composition, containing a peptidoglycan layer that is essential for maintaining cellular integrity and for virulence. The enzymes involved in the biosynthesis, degradation, remodelling and recycling of peptidoglycan have resurfaced as attractive targets for anti-infective drug discovery. Here, we review the importance of peptidoglycan, including the structure, function and regulation of key enzymes involved in its metabolism. We also discuss known inhibitors of ATP-dependent Mur ligases, and discuss the potential for the development of pan-enzyme inhibitors targeting multiple Mur ligases.

The Gram-Positive Bacterial Cell Wall

The chapter about the Gram-positive bacterial cell wall gives a brief historical background on the discovery of Gram-positive cell walls and their constituents and microscopic methods applied for studying the Gram-positive cell envelope. Followed by the description of the different chemical building blocks of peptidoglycan and the biosynthesis of the peptidoglycan layers and high turnover of peptidoglycan during bacterial growth. Lipoteichoic acids and wall teichoic acids are highlighted as major components of the cell wall. Characterization of capsules and the formation of extracellular vesicles by Gram-positive bacteria close the section on cell envelopes which have a high impact on bacterial pathogenesis. In addition, the specialized complex and unusual cell wall of mycobacteria is introduced thereafter. Next a short back view is given on the development of electron microscopic examinations for studying bacterial cell walls. Different electron microscopic techniques and methods applied to examine bacterial cell envelopes are discussed in the view that most of the illustrated methods should be available in a well-equipped life sciences orientated electron microscopic laboratory. In addition, newly developed and mostly well-established cryo-methods like high-pressure freezing and freeze-substitution (HPF-FS) and cryo-sections of hydrated vitrified bacteria (CEMOVIS, Cryo-electron microscopy of vitreous sections) are described. At last, modern cryo-methods like cryo-electron tomography (CET) and cryo-FIB-SEM milling (focus ion beam-scanning electron microscopy) are introduced which are available only in specialized institutions, but at present represent the best available methods and techniques to study Gram-positive cell walls under close-to-nature conditions in great detail and at high resolution.

Role of Major Toxin Virulence Factors in Pertussis Infection and Disease Pathogenesis

Bordetella pertussis produces several toxins that affect host-pathogen interactions. Of these, the major toxins that contribute to pertussis infection and disease are pertussis toxin, adenylate cyclase toxin-hemolysin and tracheal cytotoxin. Pertussis toxin is a multi-subunit protein toxin that inhibits host G protein-coupled receptor signaling, causing a wide array of effects on the host. Adenylate cyclase toxin-hemolysin is a single polypeptide, containing an adenylate cyclase enzymatic domain coupled to a hemolysin domain, that primarily targets phagocytic cells to inhibit their antibacterial activities. Tracheal cytotoxin is a fragment of peptidoglycan released by B. pertussis that elicits damaging inflammatory responses in host cells. This chapter describes these three virulence factors of B. pertussis, summarizing background information and focusing on the role of each toxin in infection and disease pathogenesis, as well as their role in pertussis vaccination.

Identification of Two Phosphate Starvation-induced Wall Teichoic Acid Hydrolases Provides First Insights into the Degradative Pathway of a Key Bacterial Cell Wall Component

The cell wall of most Gram-positive bacteria contains equal amounts of peptidoglycan and the phosphate-rich glycopolymer wall teichoic acid (WTA). During phosphate-limited growth of the Gram-positive model organism Bacillus subtilis 168, WTA is lost from the cell wall in a response mediated by the PhoPR two-component system, which regulates genes involved in phosphate conservation and acquisition. It has been thought that WTA provides a phosphate source to sustain growth during starvation conditions; however, WTA degradative pathways have not been described for this or any condition of bacterial growth. Here, we uncover roles for the Bacillus subtilis PhoP regulon genes glpQ and phoD as encoding secreted phosphodiesterases that function in WTA metabolism during phosphate starvation. Unlike the parent 168 strain, ΔglpQ or ΔphoD mutants retained WTA and ceased growth upon phosphate limitation. Characterization of GlpQ and PhoD enzymatic activities, in addition to X-ray crystal structures of GlpQ, revealed distinct mechanisms of WTA depolymerization for the two enzymes; GlpQ catalyzes exolytic cleavage of individual monomer units, and PhoD catalyzes endo-hydrolysis at nonspecific sites throughout the polymer. The combination of these activities appears requisite for the utilization of WTA as a phosphate reserve. Phenotypic characterization of the ΔglpQ and ΔphoD mutants revealed altered cell morphologies and effects on autolytic activity and antibiotic susceptibilities that, unexpectedly, also occurred in phosphate-replete conditions. Our findings offer novel insight into the B. subtilis phosphate starvation response and implicate WTA hydrolase activity as a determinant of functional properties of the Gram-positive cell envelope.

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.

The Invertebrate Lysozyme Effector ILYS-3 Is Systemically Activated in Response to Danger Signals and Confers Antimicrobial Protection in C. elegans

Little is known about the relative contributions and importance of antibacterial effectors in the nematode C. elegans, despite extensive work on the innate immune responses in this organism. We report an investigation of the expression, function and regulation of the six ilys (invertebrate-type lysozyme) genes of C. elegans. These genes exhibited a surprising variety of tissue-specific expression patterns and responses to starvation or bacterial infection. The most strongly expressed, ilys-3, was investigated in detail. ILYS-3 protein was expressed constitutively in the pharynx and coelomocytes, and dynamically in the intestine. Analysis of mutants showed that ILYS-3 was required for pharyngeal grinding (disruption of bacterial cells) during normal growth and consequently it contributes to longevity, as well as being protective against bacterial pathogens. Both starvation and challenge with Gram-positive pathogens resulted in ERK-MAPK-dependent up-regulation of ilys-3 in the intestine. The intestinal induction by pathogens, but not starvation, was found to be dependent on MPK-1 activity in the pharynx rather than in the intestine, demonstrating unexpected communication between these two tissues. The coelomocyte expression appeared to contribute little to normal growth or immunity. Recombinant ILYS-3 protein was found to exhibit appropriate lytic activity against Gram-positive cell wall material.

Innate immune defenses against bacterial pathogenesis depend on the activation of antibacterial factors. We examined the expression and relative importance of a gene family encoding six invertebrate-type lysozymes in the much-studied nematode C. elegans. The ilys genes exhibit distinct patterns of tissue-specific expression and response to pathogenic challenge and/or starvation. The most abundantly expressed, ilys-3, exhibits constitutive pharyngeal expression, which we show is essential for efficient disruption of bacteria under non-pathogenic growth conditions, and consequently it contributes to normal longevity. ilys-3 is also strongly up-regulated in intestinal cells after starvation or exposure to Gram-positive pathogens such as Microbacterium nematophilum and acts as a ‘slow-effector’ in limiting pathogenic damage from intestinal infections. We show that this induction by pathogens depends on the action of an ERK-MAPK cascade, which acts in pharyngeal rather than intestinal cells; this implies communication between pharynx and intestine. Tagged ILYS-3 protein was detected mainly in recycling endosomes of intestinal cells and in the intestinal lumen after starvation. ILYS-3 was also expressed in coelomocytes (scavenger cells) but we found that these cells make little or no contribution to defense. We examined the enzymatic properties of recombinant ILYS-3 protein, finding that it has lytic activity against M. nematophilum cell-walls.

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.

Characterization of Wall Teichoic Acid Degradation by the Bacteriophage ϕ29 Appendage Protein GP12 Using Synthetic Substrate Analogs

The genetics and enzymology of the biosynthesis of wall teichoic acid have been the extensively studied, however, comparatively little is known regarding the enzymatic degradation of this biological polymer. The GP12 protein from the Bacillus subtilis bacteriophage ϕ29 has been implicated as a wall teichoic acid hydrolase. We have studied the wall teichoic acid hydrolase activity of pure, recombinant GP12 using chemically defined wall teichoic acid analogs. The GP12 protein had potent wall teichoic acid hydrolytic activity in vitro and demonstrated ∼13-fold kinetic preference for glycosylated poly(glycerol phosphate) teichoic acid compared with non-glycosylated. Product distribution patterns suggested that the degradation of glycosylated polymers proceeded from the hydroxyl terminus of the polymer, whereas hydrolysis occurred at random sites in the non-glycosylated polymer. In addition, we present evidence that the GP12 protein possesses both phosphodiesterase and phosphomonoesterase activities.

Peptidoglycan perception--sensing bacteria by their common envelope structure

Most Eubacteria possess peptidoglycan (PGN) or murein that surrounds the cytoplasmic membrane. While on the one hand this PGN sacculus is a very protective shield that provides resistance to the internal turgor and adverse effects of the environment, it serves on the other hand as a major pattern of recognition due to its unique structure. Eukaryotes harness this particular bacterial macromolecule to perceive (pathogenic) microorganisms and initiate their immune defence. PGN fragments are generated by bacteria as turnover products during bacterial cell wall growth and these fragments can be sensed by plants and animals to assess a potential bacterial threat. To increase the sensitivity the concentration of PGN fragments can be amplified by host hydrolytic enzymes such as lysozyme or amidase. But also bacteria themselves are able to perceive information about the state of their cell wall by sensing small soluble fragments released from its PGN, which eventually leads to the induction of antibiotic responses or cell differentiation. How PGN is sensed by bacteria, plants and animals, and how the antibacterial defence is modulated by PGN perception is the issue of this review.

The medium is the message: interspecies and interkingdom signaling by peptidoglycan and related bacterial glycans

Peptidoglycan serves as a key structure of the bacterial cell by determining cell shape and providing resistance to internal turgor pressure. However, in addition to these essential and well-studied functions, bacterial signaling by peptidoglycan fragments, or muropeptides, has been demonstrated by recent work. Actively growing bacteria release muropeptides as a consequence of cell wall remodeling during elongation and division. Therefore, the presence of muropeptide synthesis is indicative of growth-promoting conditions and may serve as a broadly conserved signal for nongrowing cells to reinitiate growth. In addition, muropeptides serve as signals between bacteria and eukaryotic organisms during both pathogenic and symbiotic interactions. The increasingly appreciated role of the microbiota in metazoan organisms suggests that muropeptide signaling likely has important implications for homeostatic mammalian physiology.

Mechanical consequences of cell-wall turnover in the elongation of a Gram-positive bacterium

A common feature of walled organisms is their exposure to osmotic forces that challenge the mechanical integrity of cells while driving elongation. Most bacteria rely on their cell wall to bear osmotic stress and determine cell shape. Wall thickness can vary greatly among species, with Gram-positive bacteria having a thicker wall than Gram-negative bacteria. How wall dimensions and mechanical properties are regulated and how they affect growth have not yet been elucidated. To investigate the regulation of wall thickness in the rod-shaped Gram-positive bacterium Bacillus subtilis, we analyzed exponentially growing cells in different media. Using transmission electron and epifluorescence microscopy, we found that wall thickness and strain were maintained even between media that yielded a threefold change in growth rate. To probe mechanisms of elongation, we developed a biophysical model of the Gram-positive wall that balances the mechanical effects of synthesis of new material and removal of old material through hydrolysis. Our results suggest that cells can vary their growth rate without changing wall thickness or strain by maintaining a constant ratio of synthesis and hydrolysis rates. Our model also indicates that steady growth requires wall turnover on the same timescale as elongation, which can be driven primarily by hydrolysis rather than insertion. This perspective of turnover-driven elongation provides mechanistic insight into previous experiments involving mutants whose growth rate was accelerated by the addition of lysozyme or autolysin. Our approach provides a general framework for deconstructing shape maintenance in cells with thick walls by integrating wall mechanics with the kinetics and regulation of synthesis and turnover.

Novel regulator PgrR for switch control of peptidoglycan recycling in Escherichia coli

Peptidoglycan (PG), also designated as murein, forms a skeletal mesh within the periplasm of bacterial membrane. PG is a metabolically stable cell architecture in Escherichia coli, but under as yet ill-defined conditions, a portion of PG is degraded, of which both amino sugar and peptide moieties are either recycled or used as self-generated nutrients for cell growth. At present, the control of PG degradation remains uncharacterized. Using the Genomic SELEX screening system, we identified an uncharacterized transcription factor YcjZ is a repressor of the expression of the initial step enzymes for PG peptide degradation. Under nutrient starvation, the genes encoding the enzymes for PG peptide degradation are derepressed so as to generate amino acids but are tightly repressed at high osmotic conditions so as to maintain the rigid membrane for withstanding the turgor. Taken together, we propose to rename YcjZ as PgrR (regulator of peptide glycan recycling).

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.

Anti-peptidoglycan antibodies and Fcγ receptors are the key mediators of inflammation in Gram-positive sepsis

Gram-positive bacteria are an important public health problem, but it is unclear how they cause systemic inflammation in sepsis. Our previous work showed that peptidoglycan (PGN) induced proinflammatory cytokines in human cells by binding to an unknown extracellular receptor, followed by phagocytosis leading to the generation of NOD ligands. In this study, we used flow cytometry to identify host factors that supported PGN binding to immune cells. PGN binding required plasma, and plasma from all tested healthy donors contained IgG recognizing PGN. Plasma depleted of IgG or of anti-PGN Abs did not support PGN binding or PGN-triggered cytokine production. Adding back intact but not F(ab')₂ IgG restored binding and cytokine production. Transfection of HEK293 cells with FcγRIIA enabled PGN binding and phagocytosis. These data establish a key role for anti-PGN IgG and FcγRs in supporting inflammation to a major structural element of Gram-positive bacteria and suggest that anti-PGN IgG contributes to human pathology in Gram-positive sepsis.

Messenger functions of the bacterial cell wall-derived muropeptides

Bacterial muropeptides are soluble peptidoglycan structures central to recycling of the bacterial cell wall and messengers in diverse cell signaling events. Bacteria sense muropeptides as signals that antibiotics targeting cell-wall biosynthesis are present, and eukaryotes detect muropeptides during the innate immune response to bacterial infection. This review summarizes the roles of bacterial muropeptides as messengers, with a special emphasis on bacterial muropeptide structures and the relationship of structure to the biochemical events that the muropeptides elicit. Muropeptide sensing and recycling in both Gram-positive and Gram-negative bacteria are discussed, followed by muropeptide sensing by eukaryotes as a crucial event in the innate immune response of insects (via peptidoglycan-recognition proteins) and mammals (through Nod-like receptors) to bacterial invasion.

Intracellular sensors of extracellular bacteria

Initial recognition of bacteria by the innate immune system is thought to occur primarily by germline-encoded pattern recognition receptors (PRRs). These receptors are present in multiple compartments of host cells and are thus capable of surveying both the intracellular and extracellular milieu for bacteria. It has generally been presumed that the cellular location of these receptors dictates what type of bacteria they respond to: extracellular bacteria being recognized by cell surface receptors, such as certain Toll-like receptors, and bacteria that are capable of breaching the plasma membrane and entering the cytoplasm, being sensed by cytoplasmic receptors, including the Nod-like receptors (NLRs). Increasingly, it is becoming apparent that this is a false dichotomy and that extracellular bacteria can be sensed by cytoplasmic PRRs and this is crucial for controlling the levels of these bacteria. In this review, we discuss the role of two NLRs, Nod1 and Nod2, in the recognition of and response to extracellular bacteria.

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.

Pseudomonas aeruginosa enhances production of an antimicrobial in response to N-acetylglucosamine and peptidoglycan

Pseudomonas aeruginosa is an opportunistic pathogen often associated with chronic lung infections in individuals with the genetic disease cystic fibrosis (CF). Previous work from our laboratory revealed that five genes predicted to be important for catabolism of N-acetylglucosamine (GlcNAc) are induced during in vitro growth in CF lung secretions (sputum). Here, we demonstrate that these genes comprise an operon (referred to as the nag operon) and that NagE, a putative component of the GlcNAc phosphotransferase system, is required for growth on and uptake of GlcNAc. Using primer extension analysis, the promoter of the nag operon was mapped and shown to be inducible by GlcNAc and regulated by the transcriptional regulator NagR. Transcriptome analysis revealed that in addition to induction of the nag operon, several P. aeruginosa genes encoding factors critical for extracellular antimicrobial production are also induced by GlcNAc. Finally, we show that the GlcNAc-containing polymer peptidoglycan induces production of the antimicrobial pyocyanin. Based on this data, we propose a model in which P. aeruginosa senses surrounding bacteria by monitoring exogenous peptidoglycan and responds to this cue through enhanced production of an antimicrobial.

Muropeptide rescue in Bacillus subtilis involves sequential hydrolysis by beta-N-acetylglucosaminidase and N-acetylmuramyl-L-alanine amidase

We identified a pathway in Bacillus subtilis that is used for recovery of N-acetylglucosamine (GlcNAc)-N-acetylmuramic acid (MurNAc) peptides (muropeptides) derived from the peptidoglycan of the cell wall. This pathway is encoded by a cluster of six genes, the first three of which are orthologs of Escherichia coli genes involved in N-acetylmuramic acid dissimilation and encode a MurNAc-6-phosphate etherase (MurQ), a MurNAc-6-phosphate-specific transcriptional regulator (MurR), and a MurNAc-specific phosphotransferase system (MurP). Here we characterized two other genes of this cluster. The first gene was shown to encode a cell wall-associated beta-N-acetylglucosaminidase (NagZ, formerly YbbD) that cleaves the terminal nonreducing N-acetylglucosamine of muropeptides and also accepts chromogenic or fluorogenic beta-N-acetylglucosaminides. The second gene was shown to encode an amidase (AmiE, formerly YbbE) that hydrolyzes the N-acetylmuramyl-L-Ala bond of MurNAc peptides but not this bond of muropeptides. Hence, AmiE requires NagZ, and in conjunction these enzymes liberate MurNAc by sequential hydrolysis of muropeptides. NagZ expression was induced at late exponential phase, and it was 6-fold higher in stationary phase. NagZ is noncovalently associated with lysozyme-degradable particulate material and can be released from it with salt. A nagZ mutant accumulates muropeptides in the spent medium and displays a lytic phenotype in late stationary phase. The evidence for a muropeptide catabolic pathway presented here is the first evidence for cell wall recovery in a Gram-positive organism, and this pathway is distinct from the cell wall recycling pathway of E. coli and other Gram-negative bacteria.

Inflammatory cytokine response to Bacillus anthracis peptidoglycan requires phagocytosis and lysosomal trafficking

During advanced stages of inhalation anthrax, Bacillus anthracis accumulates at high levels in the bloodstream of the infected host. This bacteremia leads to sepsis during late-stage anthrax; however, the mechanisms through which B. anthracis-derived factors contribute to the pathology of infected hosts are poorly defined. Peptidoglycan, a major component of the cell wall of Gram-positive bacteria, can provoke symptoms of sepsis in animal models. We have previously shown that peptidoglycan of B. anthracis can induce the production of proinflammatory cytokines by cells in human blood. Here, we show that biologically active peptidoglycan is shed from an active culture of encapsulated B. anthracis strain Ames in blood. Peptidoglycan is able to bind to surfaces of responding cells, and internalization of peptidoglycan is required for the production of inflammatory cytokines. We also show that the peptidoglycan traffics to lysosomes, and lysosomal function is required for cytokine production. We conclude that peptidoglycan of B. anthracis is initially bound by an unknown extracellular receptor, is phagocytosed, and traffics to lysosomes, where it is degraded to a product recognized by an intracellular receptor. Binding of the peptidoglycan product to the intracellular receptor causes a proinflammatory response. These findings provide new insight into the mechanism by which B. anthracis triggers sepsis during a critical stage of anthrax disease.

Induction and regulation of a secreted peptidoglycan hydrolase by a membrane Ser/Thr kinase that detects muropeptides

Here, we report that the model Gram-positive organism, Bacillus subtilis, expresses and secretes a muralytic enzyme, YocH, in response to cell wall-derived muropeptides derived from growing cells but not lysed cells. This induction is dependent on PrkC, a membrane Ser/Thr kinase that binds to peptidoglycan and that belongs to a broadly conserved family including the essential PknB kinase of M. tuberculosis. YocH stimulates its own expression in a PrkC-dependent manner demonstrating the presence of an autoregulatory loop during growth. Cells lacking YocH display a survival defect in stationary phase but enzymes secreted by other cells in the culture rescue this defect. The essential translation factor EF-G is an in vivo substrate of PrkC and this phosphorylation occurs in response to muropeptides. Therefore, we hypothesize that YocH is used by the bacterium to digest peptidoglycan released by other bacteria in the milieu and that the presence of these fragments is detected by a membrane kinase that modifies a key regulator of translation as well as to stimulate its own expression.

A eukaryotic-like Ser/Thr kinase signals bacteria to exit dormancy in response to peptidoglycan fragments

Bacteria can respond to adverse environmental conditions by drastically reducing or even ceasing metabolic activity. They must then determine that conditions have improved before exiting dormancy, and one indication of such a change is the growth of other bacteria in the local environment. Growing bacteria release muropeptide fragments of the cell wall into the extracellular milieu, and we report here that these muropeptides are potent germinants of dormant Bacillus subtilis spores. The ability of a muropeptide to act as a germinant is determined by the identity of a single amino acid. A well-conserved, eukaryotic-like Ser/Thr membrane kinase containing an extracellular domain capable of binding peptidoglycan is necessary for this response, and a small molecule that stimulates related eukaryotic kinases is sufficient to induce germination. Another small molecule, staurosporine, that inhibits related eukaryotic kinases blocks muropeptide-dependent germination. Thus, in contrast to traditional antimicrobials that inhibit metabolically active cells, staurosporine acts by blocking germination of dormant spores.

Dormant spores receive an unexpected wake-up call

Germination of spores of Bacillus bacteria can be triggered by nutrients acting through receptors on the spore's inner membrane. Shah et al. (2008) now report that cell wall peptidoglycan fragments can also trigger spore germination by binding to an inner membrane-bound protein kinase.

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.

Excretion of a phosphorus-containing carbohydrate by Streptomyces sp. A50

A new phosphorus-containing compound (1) was detected by (31)P NMR spectroscopy in Streptomyces sp. A50. Compound 1, 1(alpha)-O-methyl-2-(N-acetyl)glucoseamine-6-O-phosphate-1(alpha)-2-(N-acetyl)glucosamine, exhibited a pK(a) value around zero. The compound was found both in the extracellular culture broth and in the cells. While very low concentrations of 1 were found in the culture broth of other species of Streptomyces, its presence in high concentrations was specific to Streptomyces sp. A50. The highly acidic compound was isolated from the broth, and its structure was elucidated by a combination of 1D-, 2D-homonuclear, and inverse heteronuclear NMR techniques and mass spectroscopy.

Determination of the gram-positive bacterial content of soils and sediments by analysis of teichoic acid components

Many gram-positive bacteria form substituted polymers of glycerol and ribitol phosphate esters known as teichoic acids. Utilizing the relative specificity of cold concentrated hydrofluoric acid in the hydrolysis of polyphosphate esters it proved possible to quantitatively assay the teichoic acid-derived glycerol and ribitol from gram-positive bacteria added to various soils and sediments. The lipids are first removed from the soils or sediments with a one phase chloroform-methanol extraction and the lipid extracted residue is hydrolyzed with cold concentrated hydrofluoric acid. To achieve maximum recovery of the teichoic acid ribitol, a second acid hydrolysis of the aqueous extract is required. The glycerol and ribitol are then acetylated after neutralization and analyzed by capillary gas-liquid chromatography. This technique together with measures of the total phospholipid, the phospholipid fatty acid, the muramic acid and the hydroxy fatty acids of the lipopolysaccharide lipid A of the gram-negative bacteria makes it possible to describe the community structure environmental samples. The proportion of gram-positive bacteria measured as the teichoic acid glycerol and ribitol is higher in soils than in sediments and increases with depth in both.

Stoichiometric growth model for riboflavin-producing Bacillus subtilis

Rate equations for measured extracellular rates and macromolecular composition data were combined with a stoichiometric model to describe riboflavin production with an industrial Bacillus subtilis strain using errors in variables regression analysis. On the basis of this combined stoichiometric growth model, we explored the topological features of the B. subtilis metabolic reaction network that was assembled from a large amount of literature. More specifically, we simulated maximum theoretical yields of biomass and riboflavin, including the associated flux regimes. Based on the developed model, the importance of experimental data on building block requirements for maximum yield and flux calculations were investigated. These analyses clearly show that verification of macromolecular composition data is important for optimum flux calculations.

Formation of the glycan chains in the synthesis of bacterial peptidoglycan

The main structural features of bacterial peptidoglycan are linear glycan chains interlinked by short peptides. The glycan chains are composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), all linkages between sugars being beta,1-->4. On the outside of the cytoplasmic membrane, two types of activities are involved in the polymerization of the peptidoglycan monomer unit: glycosyltransferases that catalyze the formation of the linear glycan chains and transpeptidases that catalyze the formation of the peptide cross-bridges. Contrary to the transpeptidation step, for which there is an abundant literature that has been regularly reviewed, the transglycosylation step has been studied to a far lesser extent. The aim of the present review is to summarize and evaluate the molecular and cellullar data concerning the formation of the glycan chains in the synthesis of peptidoglycan. Early work concerned the use of various in vivo and in vitro systems for the study of the polymerization steps, the attachment of newly made material to preexisting peptidoglycan, and the mechanism of action of antibiotics. The synthesis of the glycan chains is catalyzed by the N-terminal glycosyltransferase module of class A high-molecular-mass penicillin-binding proteins and by nonpenicillin-binding monofunctional glycosyltransferases. The multiplicity of these activities in a given organism presumably reflects a variety of in vivo functions. The topological localization of the incorporation of nascent peptidoglycan into the cell wall has revealed that bacteria have at least two peptidoglycan-synthesizing systems: one for septation, the other one for elongation or cell wall thickening. Owing to its location on the outside of the cytoplasmic membrane and its specificity, the transglycosylation step is an interesting target for antibacterials. Glycopeptides and moenomycins are the best studied antibiotics known to interfere with this step. Their mode of action and structure-activity relationships have been extensively studied. Attempts to synthesize other specific transglycosylation inhibitors have recently been made.

An extracellular protease of Streptococcus gordonii hydrolyzes type IV collagen and collagen analogues

Streptococcus gordonii is a frequent cause of infective bacterial endocarditis, but its mechanisms of virulence are not well defined. In this study, streptococcal proteases were recovered from spent chemically defined medium (CDM) and fractionated by ammonium sulfate precipitation and by ion-exchange and gel filtration column chromatography. Three proteases were distinguished by their different solubilities in ammonium sulfate and their specificities for synthetic peptides. One of the enzymes cleaved collagen analogs Gly-Pro 4-methoxy-beta-naphthylamide, 2-furanacryloyl-Leu-Gly-Pro-Ala (FALGPA), and p-phenylazobenzyloxycarbonyl-Pro-Leu-Gly-Pro-Arg (pZ-peptide) and was released from the streptococci while complexed to peptidoglycan fragments. Treatment of this protease with mutanolysin reduced its 180- to 200-kDa mass to 98 kDa without loss of enzymatic activity. The purified protease cleaved bovine gelatin, human placental type IV collagen, and the Aalpha chain of fibrinogen but not albumin, fibronectin, laminin, or myosin. Enzyme activity was inhibited by phenylmethylsulfonyl fluoride, indicating that it is a serine-type protease. Maximum production of the 98-kDa protease occurred during growth of S. gordonii CH1 in CDM containing 0.075% total amino acids at pH 7.0 with minimal aeration. Higher initial concentrations of amino acids prevented the release of the protease without reducing cell-associated enzyme levels, and the addition of an amino acid mixture to an actively secreting culture stopped further enzyme release. The purified protease was stored frozen at -20 degreesC for several months or heated at 50 degreesC for 10 min without loss of activity. These data indicate that S. gordonii produces an extracellular gelatinase/type IV collagenase during growth in medium containing minimal concentrations of free amino acids. Thus, the extracellular enzyme is a potential virulence factor in the amino acid-stringent, thrombotic, valvular lesions of bacterial endocarditis.

Analysis of outgrowth of Bacillus subtilis spores lacking penicillin-binding protein 2a

The loss of Bacillus subtilis penicillin-binding protein (PBP) 2a, encoded by pbpA, was previously shown to slow spore outgrowth and result in an increased diameter of the outgrowing spore. Further analyses to define the defect in pbpA spore outgrowth have shown that (i) outgrowing pbpA spores exhibited only a slight defect in the rate of peptidoglycan (PG) synthesis compared to wild-type spores, but PG turnover was significantly slowed during outgrowth of pbpA spores; (ii) there was no difference in the location of PG synthesis in outgrowing wild-type and pbpA spores once cell elongation had been initiated; (iii) outgrowth and elongation of pbpA spores were dramatically affected by the levels of monovalent or divalent cations in the medium; (iv) there was a partial redundancy of function between PBP2a and PBP1 or -4 during spore outgrowth; and (v) there was no difference in the structure of PG from outgrowing wild-type spores or spores lacking PBP2a or PBP2a and -4; but also (vi) PG from outgrowing spores lacking PBP1 and -2a had transiently decreased cross-linking compared to PG from outgrowing wild-type spores, possibly due to the loss of transpeptidase activity.

In vivo immobilization of enzymatically active polypeptides on the cell surface of Staphylococcus carnosus

Many surface proteins of Gram-positive bacteria are covalently anchored to the cell wall by a ubiquitous mechanism, involving a specific, C-terminal sorting signal. To achieve cell-wall immobilization of a normally secreted enzyme in vivo, we constructed a hybrid protein consisting of Staphylococcus hyicus lipase and the C-terminal region of Staphylococcus aureus fibronectin binding protein B (FnBPB). This region comprised the authentic cell-wall-spanning region and cell-wall sorting signal of FnBPB. Expression of the hybrid protein in Staphylococcus carnosus resulted in efficient cell-wall anchoring of enzymatically active lipase. The cell-wall-immobilized lipase (approximately 10,000 molecules per cell) retained more than 80% of the specific activity, compared to the C-terminally unmodified S. hyicus lipase secreted by S.carnosus cells. After releasing the hybrid protein from the cell wall by lysostaphin treatment. Its specific activity was indistinguishable from that of the unmodified lipase. Thus, the C-terminal region of FnBPB per se was fully compatible with folding of the lipase to an active conformation. To study the Influence of the distance between the cell-wall sorting signal and the C-terminus of the lipase on the activity of the immobilized lipase, the length of this spacer region was varied. Reduction of the spacer length gradually reduced the activity of the surface-immobilized lipase. On the other hand, elongation of this spacer did not stimulate the activity of the immobilized lipase, indicating that the spacer must exceed a critical length of approx. 90 amino acids to allow efficient folding of the enzyme, which probably can only be achieved outside the peptidoglycan web of the cell wall. When the lipase was replaced by another enzyme, the Escherichia coli beta-lactamase, the resulting hybrid was also efficiently anchored in an active conformation to the cell wall of S. carnosus. These results demonstrate that it is possible to immobilize normality soluble enzymes on the cell wall of S. carnosus-without radically altering their catalytic activity-by fusing them to a cell-wall-immobilization unit, consisting of a suitable cell-wall-spanning region and a standard cell-wall sorting signal.

Anomalies in cell wall turnover associated with the growth temperature of Bacillus subtilis

Cell wall turnover appeared to be anomalously fast in Bacillus subtilis when the cells were grown at temperatures below 29 degrees C. Turnover rates k(generation-1), of exponential cultures at 25 degrees were approximately double those of cells grown at 37 degrees C. When autolysin levels were assayed in cell walls, it was found that the enzyme activities were constant between 25 degrees C and 40 degrees C, suggesting that there was no greater synthesis of autolysin at the lower temperature. Analyses of walls for individual components, extent of aminosugar substitution and extent of crosslinking, did not reveal significant differences between samples obtained from 25 degrees C or 37 degrees C cultures. The N-acetylmuramoyl-L-alanine amidase was stable over the temperature range studied. Lysis of cells, induced by carbonylcyanide-m-chlorophenylhydrazone, occurred at a faster rate for cells obtained at 25 degrees C than for cells obtained at 37 degrees C. In addition, the lysis of cells by hen egg white lysozyme was slightly faster when the cells were obtained from 25 degrees C cultures than from 37 degrees C cultures. It is possible the autolysin(s) responsible for cell wall turnover are cold-activated.

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

Turnover of murein in oligopeptide permease-negative Escherichia coli cells appeared to be minimal or nonexistent. In one strain in which it was possible to measure turnover during the first generation of chase, it was found that the rate of turnover was constant throughout a chase of three generations. This result suggests that an "inside-to-outside" mode of growth of the sacculus does not occur in E. coli. Turnover, though minimal, was significantly higher from cells labeled uniformly than from cells labeled only in the lateral wall, suggesting that a significant portion of the observed turnover is related to cell separation. Actually, turnover only appeared to be minimal in opp mutant strains. Tripeptides were being released by turnover at a rate of about 50% per generation and then were efficiently recycled. This suggests that in addition to opp, a low-affinity uptake system for tripeptide derived from the sacculus may exist.

Isolation, and morphological and chemical properties of an autolysis-deficient mutant of Clostridium botulinum type A

An autolysis-deficient mutant was isolated from Clostridium botulinum type A 190L by treatment with ethyl methanesulfonate. The cell wall prepared from the mutant autolyzed at much slower rate than that from the parent strain, accompanying with much less liberation of both amino terminals and reducing groups. Electron microscopic observation revealed that the mutant strain was converted to short rod or curved spherical form with thickened cell walls when the growth temperature was shifted from 37 to 45 C. The mutant had a significantly larger amount of non-peptidoglycan-carbohydrate complexes than did the parent strain and became markedly resistant to the autolysin partially purified from the parent, compared with the parent strain. Furthermore, the mutant was fairly tolerant to killing by penicillin. These results suggest that the autolysis deficiency of the mutant was due not only to the deficient production of autolysin but also to the excess accumulation of carbohydrate in the cell wall.

Fermentation products, amino acid utilization, maintenance energies and growth yields for the fibrillar Streptococcus salivarius HB and a non-fibrillar mutant HB-B grown in continuous culture under glucose limitation

The fibrillar strain Streptococcus salivarius HB and a non-fibrillar mutant, strain HB-B, were grown in a defined medium under glucose limitation in a chemostat. Fermentation balances were produced for both strains in batch culture and at growth rates between 0.1/h and 1.1/h. In batch culture both strains fermented glucose to lactate, but in continuous culture glucose was fermented to formate, acetate and ethanol with increasing amounts of lactate as the growth rate was increased. Lactate never became the major fermentation product even at the highest growth rate. Amino acid analysis showed that only lysine was more than 50% utilized, while proline and tyrosine showed net production. The non-fibrillar strain HB-B showed, in general, a reduced utilization of amino acids compared with the fibrillar strain HB. Calculated growth yields and maintenance energies for the two strains showed that there was a reduction in the true growth yield and the maintenance energy coefficient of the non-fibrillar strain HB-B when compared with the fibrillar strain HB. The increase in the maintenance energy of the fibrillar strain HB (1.382 mmol/g/h) when compared with the non-fibrillar strain HB-B (0.546 mmol/g/h) of 153% is proposed to be the energy required for the maintenance of the fibrillar surface of the cell.

Lipoteichoic acid from Bacillus subtilis subsp. niger WM: isolation and effects on cell wall autolysis and turnover

Lipoteichoic acid (LTA) was extracted by means of hot aqueous phenol from Bacillus subtilis subsp. niger WM cells grown under various conditions in chemostat culture. The extracts were partially purified by nuclease treatment and gel permeation chromatography. Chemical analyses revealed a composition consistent with a polyglycerol phosphate polymer. The influence on autolysis of the LTAs thus obtained was studied with both whole cells and autolysin-containing native walls of B. subtilis subsp. niger WM. Lysis rates of phosphate-limited cells could be reduced to about 40% of the control rate by the addition of LTA, whereas lysis of cells grown under phosphate-sufficient conditions was affected to a much lesser extent. The lysis of native walls prepared from variously grown cells proved to be fairly insensitive to the addition of LTA. The effect of LTA on wall turnover was studied by following the release of radioactively labeled wall material during exponential growth. The most obvious effect of LTA was a lowered first-order rate of release of labeled wall material; calculations according to the model for cell wall turnover in Bacillus spp. formulated by De Boer et al. (W. R. De Boer, F. J. Kruyssen, and J. T. M. Wouters, J. Bacteriol. 145:50-60, 1981) revealed changes in wall geometry and not in turnover rate in the presence of LTA.

The functions of autolysins in the growth and division of Bacillus subtilis

Some bacteria, such as streptococci, exhibit growth from discrete and well-defined zones. In Streptococcus faecalis, growth zones can be observed in the electron microscope, and the position of the zone can be used as a marker for cell cycle events. Growth of the cell surface of Bacillus subtilis appears to be by a much different mechanism from that of streptococci. Cell elongation takes place by the insertion at many sites in the cell cylinder of peptidoglycan components. The insertion occurs on the inner face of the wall, and upon cross linking, the new wall material becomes stress bearing and older wall is pushed to the surface. When old wall reaches the surface, it becomes susceptible to excision by autolysins, resulting in wall turnover; cell elongation, due to the stretching of the cross-linked peptidoglycan, therefore, accompanies turnover and does not require a specialized growth zone.

Release of cell wall peptides into culture medium by exponentially growing Escherichia coli

Escherichia coli W7 cells were found to release three different muropeptides into the culture medium: tetrapeptide (L-Ala-D-Glu-meso-diaminopimelic acid-D-Ala), tripeptide (L-Ala-D-Glu-meso-diaminopimelic acid), and a previously undescribed dipeptide (meso-diaminopimelic acid-D-Ala). From the rate of release of these three peptides, it was calculated that 6 to 8% of the murein in the sacculus was lost per generation.

Monovalent cations enable cell wall turnover of the turnover-deficient lyt-15 mutant of Bacillus subtilis

A lyt-15 mutant reported to be unable to turn over the cell wall exhibited the same rate of wall turnover as the standard strain if the medium contained 0.2 M NaCl, which did not affect growth. Cell wall autolysis was also optimal at 0.2 M NaCl.

Cell wall turnover in lyt mutants of Bacillus subtilis

One of the reported phenotypes of Bacillus subtilis strains containing the lyt mutations is a deficiency in cell wall turnover. However, it has recently been shown in our laboratory that these mutant cells can turn over their walls under certain conditions (Vitkovic et al. [2, 11] and Abstr. K3, Annu. Meet. Am. Soc. Microbiol., 1984, p. 147). I show in this paper that the long lag in wall turnover of lyt-1 (FJ3) and lyt-2 (FJ6) strains coincided with the time it took cells (which grow in long chains) to reach the end of exponential growth and to begin separating. At this time, the specific activity of N-acetylmuramyl-L-alanine amidase in the mutant strains reached the same level as in the standard strain in mid-exponential growth phase. Wall turnover was therefore probably caused by increased amidase activity. The turnover in strain lyt-15 (Nil5) was initiated by addition of NaCl and its rate reached that of the standard strain at 0.2 M NaCl. The amidase activities isolated from the lyt-15 and the standard strains varied similarly with NaCl concentration. When walls were probed with amidases and lysozyme in the presence of increasing concentrations of NaCl, the lyt-15 cell wall appeared to be very different from the walls of the standard strain or M. luteus (a standard lysozyme substrate). The results suggest that turnover deficiency in the lyt-15 strain may be due to a change in its wall resulting in a reversible resistance to turnover.

Isolation and characterization of soluble peptidoglycan from several strains of Streptococcus faecium

Two phenotypically autolysis-deficient strains of Streptococcus faecium ATCC 9790 were shown to produce high-molecular-weight, soluble, linear, uncross-linked peptidoglycan when incubated with benzylpenicillin in a wall medium which permits cell wall synthesis (wall thickening) but not balanced growth. This high-molecular-weight s-peptidoglycan was shown to have a molecular weight of 46,000 to 54,000, lack peptide cross-links, and be virtually devoid of accessory wall polymers. It was hydrolyzed by hen egg white lysozyme and the endogenous, autolytic N-acetylmuramidase of S. faecium, but was not attacked by proteinases. Chemical analyses of the polymer are consistent with the following structure, where n is the number of repeating disaccharide units: (formula; see text).

Effect of decoyinine on peptidoglycan synthesis and turnover in Bacillus subtilis

The sporulation of Bacillus subtilis can be induced in the presence of amino acids and glucose by partially depriving the cells of guanine nucleotides. This can be achieved, e.g., by the addition of decoyinine, a specific inhibitor of GMP synthetase. To determine the effect of this and other inhibitors on cell wall synthesis, we measured in their presence the incorporation of acetylglucosamine into acid-precipitable material. The rate of wall synthesis decreased by 50% within 5 min after decoyinine addition; this decrease was prevented by the presence of guanosine. A comparison with the effects of other inhibitors of cell wall synthesis indicated that decoyinine inhibited the final portion of the cell wall biosynthetic pathway, i.e., after the steps inhibited by bacitracin or vancomycin. Decoyinine addition also prevented cellular autolysis and cell wall turnover. It is not known whether these two effects of decoyinine on cell wall synthesis are causally related.