Bacterial secretion of D-arginine controls environmental microbial biodiversity

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

Determinants of Bacterial Morphology: From Fundamentals to Possibilities for Antimicrobial Targeting

Bacterial morphology is extremely diverse. Specific shapes are the consequence of adaptive pressures optimizing bacterial fitness. Shape affects critical biological functions, including nutrient acquisition, motility, dispersion, stress resistance and interactions with other organisms. Although the characteristic shape of a bacterial species remains unchanged for vast numbers of generations, periodical variations occur throughout the cell (division) and life cycles, and these variations can be influenced by environmental conditions. Bacterial morphology is ultimately dictated by the net-like peptidoglycan (PG) sacculus. The species-specific shape of the PG sacculus at any time in the cell cycle is the product of multiple determinants. Some morphological determinants act as a cytoskeleton to guide biosynthetic complexes spatiotemporally, whereas others modify the PG sacculus after biosynthesis. Accumulating evidence supports critical roles of morphogenetic processes in bacteria-host interactions, including pathogenesis. Here, we review the molecular determinants underlying morphology, discuss the evidence linking bacterial morphology to niche adaptation and pathogenesis, and examine the potential of morphological determinants as antimicrobial targets.

Ultra-Sensitive, High-Resolution Liquid Chromatography Methods for the High-Throughput Quantitative Analysis of Bacterial Cell Wall Chemistry and Structure

High-performance liquid chromatography (HPLC) analysis has been critical for determining the structural and chemical complexity of the cell wall. However this method is very time consuming in terms of sample preparation and chromatographic separation. Here we describe (1) optimized methods for peptidoglycan isolation from both Gram-negative and Gram-positive bacteria that dramatically reduce the sample preparation time, and (2) the application of the fast and highly efficient ultra-performance liquid chromatography (UPLC) technology to muropeptide separation and quantification. The advances in both analytical instrumentation and stationary-phase chemistry have allowed for evolved protocols which cut run time from hours (2-3 h) to minutes (10-20 min), and sample demands by at least one order of magnitude. Furthermore, development of methods based on organic solvents permits in-line mass spectrometry (MS) of the UPLC-resolved muropeptides. Application of these technologies to high-throughput analysis will expedite the better understanding of the cell wall biology.

A novel peptidoglycan D,L-endopeptidase induced by Salmonella inside eukaryotic cells contributes to virulence

Bacteria remodel peptidoglycan structure in response to environmental changes. Many enzymes are involved in peptidoglycan metabolism; however, little is known about their responsiveness in a defined environment or the modes they assist bacteria to adapt to new niches. Here, we focused in peptidoglycan enzymes that intracellular bacterial pathogens use inside eukaryotic cells. We identified a peptidoglycan enzyme induced by Salmonella enterica serovar Typhimurium in fibroblasts and epithelial cells. This enzyme, which shows γ-D-glutamyl-meso-diaminopimelic acid D,L-endopeptidase activity, is also produced by the pathogen in media with limited nutrients and in resting conditions. The enzyme, termed EcgA for endopeptidase responding to cessation of growth', is encoded in a S. Typhimurium genomic island absent in Escherichia coli. EcgA production is strictly dependent on the virulence regulator PhoP in extra- and intracellular environments. Consistent to this regulation, a mutant lacking EcgA is attenuated in the mouse typhoid model. These findings suggest that specialised peptidoglycan enzymes, such as EcgA, might facilitate Salmonella adaptation to the intracellular lifestyle. Moreover, they indicate that readjustment of peptidoglycan metabolism inside the eukaryotic cell is essential for host colonisation.

High-throughput, Highly Sensitive Analyses of Bacterial Morphogenesis Using Ultra Performance Liquid Chromatography

The bacterial cell wall is a network of glycan strands cross-linked by short peptides (peptidoglycan); it is responsible for the mechanical integrity of the cell and shape determination. Liquid chromatography can be used to measure the abundance of the muropeptide subunits composing the cell wall. Characteristics such as the degree of cross-linking and average glycan strand length are known to vary across species. However, a systematic comparison among strains of a given species has yet to be undertaken, making it difficult to assess the origins of variability in peptidoglycan composition. We present a protocol for muropeptide analysis using ultra performance liquid chromatography (UPLC) and demonstrate that UPLC achieves resolution comparable with that of HPLC while requiring orders of magnitude less injection volume and a fraction of the elution time. We also developed a software platform to automate the identification and quantification of chromatographic peaks, which we demonstrate has improved accuracy relative to other software. This combined experimental and computational methodology revealed that peptidoglycan composition was approximately maintained across strains from three Gram-negative species despite taxonomical and morphological differences. Peptidoglycan composition and density were maintained after we systematically altered cell size in Escherichia coli using the antibiotic A22, indicating that cell shape is largely decoupled from the biochemistry of peptidoglycan synthesis. High-throughput, sensitive UPLC combined with our automated software for chromatographic analysis will accelerate the discovery of peptidoglycan composition and the molecular mechanisms of cell wall structure determination.

Structural constraints and dynamics of bacterial cell wall architecture

The peptidoglycan wall (PG) is a unique structure which confers physical strength and defined shape to bacteria. It consists of a net-like macromolecule of peptide interlinked glycan chains overlying the cell membrane. The structure and layout of the PG dictates that the wall has to be continuously modified as bacteria go through division, morphological differentiation, and adaptive responses. The PG is poorly known in structural terms. However, to understand morphogenesis a precise knowledge of glycan strand arrangement and of local effects of the different kinds of subunits is essential. The scarcity of data led to a conception of the PG as a regular, highly ordered structure which strongly influenced growth models. Here, we review the structure of the PG to define a more realistic conceptual framework. We discuss the consequences of the plasticity of murein architecture in morphogenesis and try to define a set of minimal structural constraints that must be fulfilled by any model to be compatible with present day information.

Bile-induced peptidoglycan remodelling in Salmonella enterica

Changes in the peptidoglycan (PG) structure of Salmonella enterica are detected in the presence of a sublethal concentration of sodium deoxycholate (DOC): (i) lower proportions of Braun lipoprotein (Lpp)-bound muropeptides; (ii) reduced levels of muropeptides cross-linked by L(meso)-diaminopimelyl-D(meso)-diaminopimelic acid (L-D) peptide bridges (3-3 cross-links). Similar structural changes are found in S. enterica cultures adapted to grow in the presence of a lethal concentration of DOC, suggesting that reduced anchoring of Braun protein to PG and low occurrence of 3-3 cross-links may increase S. enterica resistance to bile. This view is further supported by additional observations: (i) A triple mutant lacking L,D-transpeptidases YbiS, ErfK, and YcfS, which does not contain Lpp anchored to PG, is hyper-resistant to bile; (ii) enhanced 3-3 cross-linking upon overexpression of YnhG transpeptidase causes a decrease in bile resistance. These observations suggest that remodelling of the cell wall may be added to the list of adaptive responses that permit survival of S. enterica in the presence of bile.

Peptidoglycan remodeling by the coordinated action of multispecific enzymes

The peptidoglycan (PG) cell wall constitutes the main defense barrier of bacteria against environmental insults and acts as communication interface. The biochemistry of this macromolecule has been well characterized throughout the years but recent discoveries have unveiled its chemical plasticity under environmental stresses. Non-canonical D-amino acids (NCDAA) are produced and released to the extracellular media by diverse bacteria. Such molecules govern cell wall adaptation to challenging environments through their incorporation into the polymer, a widespread capability among bacteria that reveals the inherent catalytic plasticity of the enzymes involved in the cell wall metabolism. Here, we analyze the recent structural and biochemical characterization of Bsr, a new family of broad spectrum racemases able to generate a wide range of NCDAA. We also discuss the necessity of a coordinated action of PG multispecific enzymes to generate adequate levels of modification in the murein sacculus. Finally, we also highlight how this catalytic plasticity of NCDAA-incorporating enzymes has allowed the development of new revolutionary methodologies for the study of PG modes of growth and in vivo dynamics.

Peptidoglycan plasticity in bacteria: emerging variability of the murein sacculus and their associated biological functions

The peptidoglycan (PG) sacculus once thought to be just a reinforcing, static and uniform structure, is fast becoming recognized as a dynamic cell constituent involved in every aspect of bacterial physiology. Recent advances showed that in addition to 'classical' tasks-as an essential element to define bacterial shape, size, division and resistance to osmotic stress-the sacculus plays very important roles in many other fields. The very few chemical and structural changes that were once considered as bizarre, or maybe exotic exceptions, are now universally accepted as fundamental pieces in bacterial cell wall adaptation to different kinds of environmental stresses; immune response; intra-specific and inter-specific signalling and antibiotics, just to mention a few. Most, if not all, of these implications are a consequence of the enormous adaptability of PG metabolism to cope with changing conditions, a characteristic for which the term plasticity is proposed. Here we overview and comment on a number of recent contributions on the cell wall adaptive responses to environmental challenges that has greatly impacted the already high complexity of the PG biology field. These new evidences have revived the interest in PG plasticity as an exciting and trendy topic in current microbiology which considers this variability as the trustworthy picture of bacterial PG in nature.

Isolation and preparation of bacterial cell walls for compositional analysis by ultra performance liquid chromatography

The bacterial cell wall is critical for the determination of cell shape during growth and division, and maintains the mechanical integrity of cells in the face of turgor pressures several atmospheres in magnitude. Across the diverse shapes and sizes of the bacterial kingdom, the cell wall is composed of peptidoglycan, a macromolecular network of sugar strands crosslinked by short peptides. Peptidoglycan's central importance to bacterial physiology underlies its use as an antibiotic target and has motivated genetic, structural, and cell biological studies of how it is robustly assembled during growth and division. Nonetheless, extensive investigations are still required to fully characterize the key enzymatic activities in peptidoglycan synthesis and the chemical composition of bacterial cell walls. High Performance Liquid Chromatography (HPLC) is a powerful analytical method for quantifying differences in the chemical composition of the walls of bacteria grown under a variety of environmental and genetic conditions, but its throughput is often limited. Here, we present a straightforward procedure for the isolation and preparation of bacterial cell walls for biological analyses of peptidoglycan via HPLC and Ultra Performance Liquid Chromatography (UPLC), an extension of HPLC that utilizes pumps to deliver ultra-high pressures of up to 15,000 psi, compared with 6,000 psi for HPLC. In combination with the preparation of bacterial cell walls presented here, the low-volume sample injectors, detectors with high sampling rates, smaller sample volumes, and shorter run times of UPLC will enable high resolution and throughput for novel discoveries of peptidoglycan composition and fundamental bacterial cell biology in most biological laboratories with access to an ultracentrifuge and UPLC.

Structural basis for the broad specificity of a new family of amino-acid racemases

Broad-spectrum amino-acid racemases (Bsrs) enable bacteria to generate noncanonical D-amino acids, the roles of which in microbial physiology, including the modulation of cell-wall structure and the dissolution of biofilms, are just beginning to be appreciated. Here, extensive crystallographic, mutational, biochemical and bioinformatic studies were used to define the molecular features of the racemase BsrV that enable this enzyme to accommodate more diverse substrates than the related PLP-dependent alanine racemases. Conserved residues were identified that distinguish BsrV and a newly defined family of broad-spectrum racemases from alanine racemases, and these residues were found to be key mediators of the multispecificity of BrsV. Finally, the structural analysis of an additional Bsr that was identified in the bioinformatic analysis confirmed that the distinguishing features of BrsV are conserved among Bsr family members.

Modes of cell wall growth differentiation in rod-shaped bacteria

A bacterial cell takes on the challenge to preserve and reproduce its shape at every generation against a substantial internal pressure by surrounding itself with a mechanical support, a peptidoglycan cell wall. The enlargement of the cell wall via net incorporation of precursors into the pre-existing wall conditions bacterial growth and morphology. However, generation, reproduction and/or modification of a specific shape requires that the incorporation takes place at precise locations for a defined time period. Much has been learnt in the past few years about the biochemistry of the peptidoglycan synthesis process, but topological approaches to the understanding of shape generation have been hindered by a lack of appropriate techniques. Recent technological advances are paving the way for substantial progress in understanding the mechanisms of bacterial morphogenesis. Here we review the latest developments, focusing on the impact of new techniques on the precise mapping of cell wall growth sites.

In Situ probing of newly synthesized peptidoglycan in live bacteria with fluorescent D-amino acids

Tracking a bug's life: Peptidoglycan (PG) of diverse bacteria is labeled by exploiting the tolerance of cells for incorporating different non-natural D-amino acids. These nontoxic D-amino acids preferably label the sites of active PG synthesis, thereby enabling fine spatiotemporal tracking of cell-wall dynamics in phylogenetically and morphologically diverse bacteria. HCC = 7-hydroxycoumarin, NBD = 7-nitrobenzofurazan, TAMRA = carboxytetramethylrhodamine.

Homogeneous incorporation of secondary cell wall polysaccharides to the cell wall of Thermus thermophilus HB27

Regular surface protein layers (S-layers) from most Gram-positive bacteria and from the ancestral bacterium Thermus thermophilus attach to pyruvylated polysaccharides (SCWP) covalently bound to the peptidoglycan through their SLH domain. However, it is not known whether the synthesis of SCWP and S-layer is coordinated enough as to follow a similar pattern of incorporation to the cell wall during growth. In this work we analyse the localization of newly synthesized SCWP on the cell wall of T. thermophilus by immunoelectron microscopy. For this, we obtained mutants with a reduced amount of pyruvylated SCWP through mutation of the csaB gene encoding the SCWP-pyruvylating activity, and its upstream gene csaA, a putative sugar transporter. We hypothesized that CsaA would be required for the synthesis of the SCWP. However, we found that csaA mutants showed only a minor decrease in the amount of SCWP immunodetected on the cell walls in comparison with csaB mutants, revealing its irrelevance in the process. Complementation experiments of csaB mutants with CsaB expressed from inducible promoters revealed that newly synthesized SCWP was homogeneously distributed along the cell wall. Fusions with thermostable fluorescent protein revealed that CsaB was distributed also in homogeneous pattern associated with the membrane. These data support that synthesis of SCWP takes place in disperse and homogeneous form all over the cell surface, in contrast to the zonal incorporation at the cell centre recently demonstrated for SlpA.

Peptidoglycan plasticity in bacteria: stress-induced peptidoglycan editing by noncanonical D-amino acids

It has been generally assumed that the role of D-amino acids in bacterial physiology is rather limited. However, recent new evidence demonstrated that millimolar concentrations of noncanonical D-amino acids are synthesized and released to the environment by bacteria from diverse phyla. These D-amino acids help bacteria adapt to environmental challenges by modulating the structure and composition of the peptidoglycan (PG). This regulation, which appears to be well conserved among bacterial species, occurs principally through the incorporation of the D-amino acids into the terminus of the peptide moiety of muropeptides. These findings revived interest in studies investigating D-amino acids as an exciting and trendy topic in current microbiology, which considers them as fundamental players in different aspects of bacterial physiology. In this article, we provide an overview of the origins of research on the effects of D-amino acids in the biology of bacterial cell walls, including their recent implication as key factors for stress-associated PG remodeling.

Escherichia coli low-molecular-weight penicillin-binding proteins help orient septal FtsZ, and their absence leads to asymmetric cell division and branching

Escherichia coli cells lacking low-molecular-weight penicillin-binding proteins (LMW PBPs) exhibit morphological alterations that also appear when the septal protein FtsZ is mislocalized, suggesting that peptidoglycan modification and division may work together to produce cell shape. We found that in strains lacking PBP5 and other LMW PBPs, higher FtsZ concentrations increased the frequency of branched cells and incorrectly oriented Z rings by 10- to 15-fold. Invagination of these rings produced improperly oriented septa, which in turn gave rise to asymmetric cell poles that eventually elongated into branches. Branches always originated from the remnants of abnormal septation events, cementing the relationship between aberrant cell division and branch formation. In the absence of PBP5, PBP6 and DacD localized to nascent septa, suggesting that these PBPs can partially substitute for the loss of PBP5. We propose that branching begins when mislocalized FtsZ triggers the insertion of inert peptidoglycan at unusual positions during cell division. Only later, after normal cell wall elongation separates the patches, do branches become visible. Thus, a relationship between the LMW PBPs and cytoplasmic FtsZ ultimately affects cell division and overall shape.

Complete genome sequence of Hirschia baltica type strain (IFAM 1418(T))

The family Hyphomonadaceae within the Alphaproteobacteria is largely comprised of bacteria isolated from marine environments with striking morphologies and an unusual mode of cell growth. Here, we report the complete genome sequence Hirschia baltica, which is only the second a member of the Hyphomonadaceae with a published genome sequence. H. baltica is of special interest because it has a dimorphic life cycle and is a stalked, budding bacterium. The 3,455,622 bp long chromosome and 84,492 bp plasmid with a total of 3,222 protein-coding and 44 RNA genes were sequenced as part of the DOE Joint Genome Institute Program CSP 2008.

Emerging knowledge of regulatory roles of D-amino acids in bacteria

The d-enantiomers of amino acids have been thought to have relatively minor functions in biological processes. While l-amino acids clearly predominate in nature, d-amino acids are sometimes found in proteins that are not synthesized by ribosomes, and d-Ala and d-Glu are routinely found in the peptidoglycan cell wall of bacteria. Here, we review recent findings showing that d-amino acids have previously unappreciated regulatory roles in the bacterial kingdom. Many diverse bacterial phyla synthesize and release d-amino acids, including d-Met and d-Leu, which were not previously known to be made. These noncanonical d-amino acids regulate cell wall remodeling in stationary phase and cause biofilm dispersal in aging bacterial communities. Elucidating the mechanisms by which d-amino acids govern cell wall remodeling and biofilm disassembly will undoubtedly reveal new paradigms for understanding how extracytoplasmic processes are regulated as well as lead to development of novel therapeutics.

D-amino acids govern stationary phase cell wall remodeling in bacteria

In all known organisms, amino acids are predominantly thought to be synthesized and used as their L-enantiomers. Here, we found that bacteria produce diverse D-amino acids as well, which accumulate at millimolar concentrations in supernatants of stationary phase cultures. In Vibrio cholerae, a dedicated racemase produced D-Met and D-Leu, whereas Bacillus subtilis generated D-Tyr and D-Phe. These unusual D-amino acids appear to modulate synthesis of peptidoglycan, a strong and elastic polymer that serves as the stress-bearing component of the bacterial cell wall. D-Amino acids influenced peptidoglycan composition, amount, and strength, both by means of their incorporation into the polymer and by regulating enzymes that synthesize and modify it. Thus, synthesis of D-amino acids may be a common strategy for bacteria to adapt to changing environmental conditions.

FtsZ directs a second mode of peptidoglycan synthesis in Escherichia coli

Certain penicillin binding protein mutants of Escherichia coli grow with spirillum-like morphologies when the FtsZ protein is inhibited, suggesting that FtsZ might govern aspects of cell wall growth other than those strictly associated with septation. While investigating the mechanism of spiral cell formation, we discovered conditions for visualizing this second function of FtsZ. Normally, inhibiting the cytoskeleton protein MreB forces E. coli cells to grow as smoothly enlarging spheres from which the poles disappear, yielding coccoid or lemon-shaped forms. However, when FtsZ and MreB were inhibited simultaneously in a strain lacking PBP 5 and PBP 7, the resulting cells ballooned outward but retained conspicuous rod-shaped extensions at sites representing the original poles. This visual phenotype was paralleled by the biochemistry of sacculus growth. Muropeptides are usually inserted homogeneously into the lateral cell walls, but when FtsZ polymerization was inhibited, the incorporation of new material occurred mainly in the central regions of cells and was significantly lower in those portions of side walls abutting a pole. Thus, reduced precursor incorporation into side walls near the poles explained why these regions retained their rod-like morphology while the rest of the cell grew spherically. Also, inhibiting FtsZ increased the amount of pentapeptides in sacculi by about one-third. Finally, the MreB protein directed the helical or diagonal incorporation of new peptidoglycan into the wall, but the location of that incorporation depended on whether FtsZ was active. In sum, the results indicate that in addition to nucleating cell septation in E. coli, FtsZ can direct the insertion of new peptidoglycan into portions of the lateral wall.

Role of peptidoglycan amidases in the development and morphology of the division septum in Escherichia coli

Escherichia coli contains multiple peptidoglycan-specific hydrolases, but their physiological purposes are poorly understood. Several mutants lacking combinations of hydrolases grow as chains of unseparated cells, indicating that these enzymes help cleave the septum to separate daughter cells after cell division. Here, we confirm previous observations that in the absence of two or more amidases, thickened and dark bands, which we term septal peptidoglycan (SP) rings, appear at division sites in isolated sacculi. The formation of SP rings depends on active cell division, and they apparently represent a cell division structure that accumulates because septal synthesis and hydrolysis are uncoupled. Even though septal constriction was incomplete, SP rings exhibited two properties of mature cell poles: they behaved as though composed of inert peptidoglycan, and they attracted the IcsA protein. Despite not being separated by a completed peptidoglycan wall, adjacent cells in these chains were often compartmentalized by the inner membrane, indicating that cytokinesis could occur in the absence of invagination of the entire cell envelope. Finally, deletion of penicillin-binding protein 5 from amidase mutants exacerbated the formation of twisted chains, producing numerous cells having septa with abnormal placements and geometries. The results suggest that the amidases are necessary for continued peptidoglycan synthesis during cell division, that their activities help create a septum having the appropriate geometry, and that they may contribute to the development of inert peptidoglycan.

Effect of the morphogenebolAon the permeability of theEscherichia coliouter membrane

Escherichia coli bolA is a morphogene involved in stress response and cell division. Overexpression of bolA induces biofilm formation and affects the levels of carboxypeptidases PBP5, PBP6 and beta-lactamase AmpC. In this study, we have shown that changes in the expression of bolA result in alterations in the properties of the outer membrane. The sensitivity to detergents and vancomycin was reduced when bolA was overexpressed and fluorescent probes indicated that different levels of bolA had an effect on outer membrane protein accessibility. Moreover, bolA was shown to be involved in the modulation of the OmpF/OmpC balance.

Restricted Mobility of Cell Surface Proteins in the Polar Regions of Escherichia coli

The polar regions of the Escherichia coli murein sacculus are metabolically inert and stable in time. Because the sacculus and the outer membrane are tightly associated, we investigated whether polar inert murein could restrict the mobility of other cell envelope elements. Cells were covalently labeled with a fluorescent reagent, chased in dye-free medium, and observed by microscopy. Fluorescent material was more efficiently retained at the cell poles than at any other location. The boundary between high and low fluorescence intensity areas was rather sharp. Labeled material consisted mostly of cell envelope proteins, among them the free and murein-bound forms of Braun's lipoprotein. Our results indicate that the mobility of at least some cell envelope proteins is restrained at regions in correspondence with underlying areas of inert murein.

Resistance of rumen bacteria murein to bovine gastric lysozyme

BACKGROUND

Lysozymes, enzymes mostly associated with defence against bacterial infections, are mureinolytic. Ruminants have evolved a gastric c type lysozyme as a digestive enzyme, and profit from digestion of foregut bacteria, after most dietary components, including protein, have been fermented in the rumen. In this work we characterized the biological activities of bovine gastric secretions against membranes, purified murein and bacteria.

RESULTS

Bovine gastric extract (BGE) was active against both G+ and G- bacteria, but the effect against Gram- bacteria was not due to the lysozyme, since purified BGL had only activity against Gram+ bacteria. We were unable to find small pore forming peptides in the BGE, and found that the inhibition of Gram negative bacteria by BGE was due to an artefact caused by acetate. We report for first time the activity of bovine gastric lysozyme (BG lysozyme) against pure bacterial cultures, and the specific resistance of some rumen Gram positive strains to BGL.

CONCLUSIONS

Some Gram+ rumen bacteria showed resistance to abomasum lysozyme. We discuss the implications of this finding in the light of possible practical applications of such a stable antimicrobial peptide.

Binding to pyruvylated compounds as an ancestral mechanism to anchor the outer envelope in primitive bacteria

Electron microscopy of isolated cell walls of the ancient bacterium Thermus thermophilus revealed that most of the peptidoglycan (PG) surface, apart from the septal region, was shielded against specific alphaPG antibodies. On the other hand, an antiserum raised against S-layer-attached cell wall fragments (alphaSAC) bound to most of the surface except for the septal regions. Treatments with alpha-amylase and pronase E made the entire cell wall surface uniformly accessible to alphaPG and severely decreased the binding of alphaSAC. We concluded that a layer of strongly bound secondary cell wall polymers (SCWPs) covers most of the cell wall surface in this ancient bacterium. A preliminary analysis revealed that such SCWPs constitute 14% of the cell wall and are essentially composed of sugars. Enzyme treatments of the cell walls revealed that SCWP was required in vitro for the binding of the S-layer protein through the S-layer homology (SLH) motif. The csaB gene was necessary for the attachment of the S-layer-outer membrane (OM) complex to the cell wall in growing cells of T. thermophilus. In vitro experiments confirmed that cell walls from a csaB mutant bound to the S-layer with a much lower affinity ( approximately 1/10) than that of the wild type. CsaB was found to be required for pyruvylation of components of the SCWP and for immunodetection with alpha-SAC antiserum. Therefore, the S-layer-OM complex of T. thermophilus binds to the cell wall through the SLH motif of the S-layer protein via a strong interaction with a highly immunogenic pyruvylated component of the SCWP. Immuno-cross-reactive compounds were detected with alphaSAC on cell walls of other Thermus spp. and in the phylogenetically related microorganism Deinococcus radiodurans. These results imply that the interaction between the SLH motif and pyruvylated components of the cell wall arose early during bacterial evolution as an ancestral mechanism for anchoring proteins and outer membranes to the cell walls of primitive bacteria.

Branching of Escherichia coli cells arises from multiple sites of inert peptidoglycan

Some strains of Escherichia coli defective for dacA, the gene coding for penicillin-binding protein 5, exhibit a strong branching phenotype when cell division is blocked. Since such branch formation implies a differentiation of polar caps at ectopic locations in the cell envelope, we analyzed murein segregation and observed a strong correlation between areas of inert murein and these morphological anomalies. In particular, the tips of branches exhibited the same properties as those described for polar caps of wild-type cells, i.e., the synthesis and turnover of murein were inhibited. Also, the mobility of cell envelope proteins was apparently constrained in areas with morphological defects. Polar regions of branching cells and sacculi had aberrant morphologies with a very high frequency. Of special interest was that areas of inert murein at polar caps were often split by areas of active synthesis, a situation unlike that observed in wild-type cells. These observations suggest that in dacA mutants, branches and other morphological anomalies may arise from split polar caps or by de novo generation of new poles built around inert peptidoglycan patches in the side walls of the cell.

The periplasmic space in Thermus thermophilus: evidence from a regulation-defective S-layer mutant overexpressing an alkaline phosphatase

The presence of a periplasmic space within the cell envelope of Thermus thermophilus was analyzed in a mutant (HB8(Delta)UTR1) defective in the regulation of its S-layer (surface crystalline layer). This mutant forms round multicellular bodies (MBs) surrounded by a common envelope as the culture approaches the stationary phase. Confocal microscopy revealed that the origin of the MBs is the progressive detachment of the external layers coupled with the accumulation of NH(2)-containing material between the external envelopes and the peptidoglycan. A specific pattern of proteins was found as soluble components of the intercellular space of the MBs by a single fractionation procedure, suggesting that they are periplasmic-like components. To demonstrate this, we cloned a gene ( phoA) from T. thermophilus HB8 encoding a signal peptide-wearing alkaline phosphatase (AP), and engineered it to be overexpressed in the mutant from a shuttle vector. Most of the AP activity (>80%) was found as a soluble component of the MBs' intercellular fraction. All these data indicate that Thermus thermophilus actually has a periplasmic space which is functionally similar to that of Proteobacteria. The potential application of the HB8(Delta)UTR1 mutant for the overexpression of periplasmic thermophilic proteins is discussed.