Electromechanical interactions in cell walls of gram-positive cocci

Differentiating interactions of antimicrobials with Gram-negative and Gram-positive bacterial cell walls using molecular dynamics simulations

Developing molecular models to capture the complex physicochemical architecture of the bacterial cell wall and to study the interaction with antibacterial molecules is an important aspect of assessing and developing novel antimicrobial molecules. We carried out molecular dynamics simulations using an atomistic model of peptidoglycan to represent the architecture for Gram-positive S. aureus. The model is developed to capture various structural features of the Staphylococcal cell wall, such as the peptide orientation, area per disaccharide, glycan length distribution, cross-linking, and pore size. A comparison of the cell wall density and electrostatic potentials is made with a previously developed cell wall model of Gram-negative bacteria, E. coli, and properties for both single and multilayered structures of the Staphylococcal cell wall are studied. We investigated the interactions of the antimicrobial peptide melittin with peptidoglycan structures. The depth of melittin binding to peptidoglycan is more pronounced in E. coli than in S. aureus, and consequently, melittin has greater contacts with glycan units of E. coli. Contacts of melittin with the amino acids of peptidoglycan are comparable across both the strains, and the D-Ala residues, which are sites for transpeptidation, show enhanced interactions with melittin. A low energetic barrier is observed for translocation of a naturally occurring antimicrobial thymol with the four-layered peptidoglycan model. The molecular model developed for Gram-positive peptidoglycan allows us to compare and contrast the cell wall penetrating properties with Gram-negative strains and assess for the first time binding and translocation of antimicrobial molecules for Gram-positive cell walls.

Spore Peptidoglycan

Bacterial endospores possess multiple integument layers, one of which is the cortex peptidoglycan wall. The cortex is essential for the maintenance of spore core dehydration and dormancy and contains structural modifications that differentiate it from vegetative cell peptidoglycan and determine its fate during spore germination. Following the engulfment stage of sporulation, the cortex is synthesized within the intermembrane space surrounding the forespore. Proteins responsible for cortex synthesis are produced in both the forespore and mother cell compartments. While some of these proteins also contribute to vegetative cell wall synthesis, others are sporulation specific. In order for the bacterial endospore to germinate and resume metabolism, the cortex peptidoglycan must first be degraded through the action of germination-specific lytic enzymes. These enzymes are present, yet inactive, in the dormant spore and recognize the muramic-δ-lactam modification present in the cortex. Germination-specific lytic enzymes across Bacillaceae and Clostridiaceae share this specificity determinant, which ensures that the spore cortex is hydrolyzed while the vegetative cell wall remains unharmed. Bacillus species tend to possess two redundant enzymes, SleB and CwlJ, capable of sufficient cortex degradation, while the clostridia have only one, SleC. Additional enzymes are often present that cannot initiate the cortex degradation process, but which can increase the rate of release of small fragments into the medium. Between the two families, the enzymes also differ in the enzymatic activities they possess and the mechanisms acting to restrict their activation until germination has been initiated.

The bio-physics of condensation of divalent cations into the bacterial wall has implications for growth of Gram-positive bacteria

BACKGROUND

The anionic-polyelectrolyte nature of the wall of Gram-positive bacteria has long been suspected to be involved in homeostasis of essential cations and bacterial growth. A better understanding of the coupling between the biophysics and the biology of the wall is essential to understand some key features at play in ion-homeostasis in this living system.

METHODS

We consider the wall as a polyelectrolyte gel and balance the long-range electrostatic repulsion within this structure against the penalty entropy required to condense cations around wall polyelectrolytes. The resulting equations define how cations interact physically with the wall and the characteristic time required for a cation to leave the wall and enter into the bacterium to enable its usage for bacterial metabolism and growth.

RESULTS

The model was challenged against experimental data regarding growth of Gram-positive bacteria in the presence of varying concentration of divalent ions. The model explains qualitatively and quantitatively how divalent cations interact with the wall as well as how the biophysical properties of the wall impact on bacterial growth (in particular the initiation of bacterial growth).

CONCLUSION

The interplay between polymer biophysics and the biology of Gram positive bacteria is defined for the first time as a new set of variables that contribute to the kinetics of bacterial growth.

GENERAL SIGNIFICANCE

Providing an understanding of how bacteria capture essential metal cations in way that does not follow usual binding laws has implications when considering the control of such organisms and their ability to survive and grow in extreme environments.

Bacillus spore wet heat resistance and evidence for the role of an expanded osmoregulatory spore cortex

Previous work reported that decoated Bacillus cereus spores incubated in 4 mol l(-1) CaCl2 are killed at lower temperatures than spores in water. This wet heat sensitization was suggested to support a role for an osmoregulatory peptidoglycan cortex in spore cores' low water content, and their wet heat resistance. Current work has replicated this finding with spores of B. cereus, Bacillus megaterium and Bacillus subtilis. However, this work found that decoated spores apparently killed at 80°C in 4 mol l(-1) CaCl2 : (i) were recovered on plates containing lysozyme; (ii) lost no dipicolinic acid (DPA) and their inner membrane remained impermeable; (iii) released no DPA upon stimulation with nutrient germinants and could not complete germination; and (iv) released DPA relatively normally upon stimulation with dodecylamine. These results indicate that decoated spores treated with 80°C- 4 mol l(-1) CaCl2 are not dead, but some protein(s) essential for spore germination, most likely germinant receptors, are inactivated by this treatment. Thus, the original finding does not support a role for an osmoregulatory cortex in spore wet heat resistance.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacillus spores' low core water content is a major factor in their wet heat resistance. One suggested mechanism for achieving low spore core water content is osmoregulated expansion of spores' peptidoglycan cortex. Evidence for this mechanism includes a report that decoated Bacillus cereus spores incubated in 4 mol l(-1) CaCl2 exhibit drastically reduced heat resistance. The current work shows that this heat sensitization of decoated spores of three Bacillus species is most likely due to inactivation of some crucial spore germination protein(s), since while treated spores appear dead, their apparent low viability is rescued by triggering spore germination with lysozyme.

Reduction of the peptidoglycan crosslinking causes a decrease in stiffness of the Staphylococcus aureus cell envelope

We have used atomic-force microscopy (AFM) to probe the effect of peptidoglycan crosslinking reduction on the elasticity of the Staphylococcus aureus cell wall, which is of particular interest as a target for antimicrobial chemotherapy. Penicillin-binding protein 4 (PBP4) is a nonessential transpeptidase, required for the high levels of peptidoglycan crosslinking characteristic of S. aureus. Importantly, this protein is essential for β-lactam resistance in community-acquired, methicillin-resistant S. aureus (MRSA) strains but not in hospital-acquired MRSA strains. Using AFM in a new mode for recording force/distance curves, we observed that the absence of PBP4, and the concomitant reduction of the peptidoglycan crosslinking, resulted in a reduction in stiffness of the S. aureus cell wall. Importantly, the reduction in cell wall stiffness in the absence of PBP4 was observed both in community-acquired and hospital-acquired MRSA strains, indicating that high levels of peptidoglycan crosslinking modulate the overall structure and mechanical properties of the S. aureus cell envelope in both types of clinically relevant strains. Additionally, we were able to show that the applied method enables the separation of cell wall properties and turgor pressure.

Spore formation in Bacillus subtilis

Although prokaryotes ordinarily undergo binary fission to produce two identical daughter cells, some are able to undergo alternative developmental pathways that produce daughter cells of distinct cell morphology and fate. One such example is a developmental programme called sporulation in the bacterium Bacillus subtilis, which occurs under conditions of environmental stress. Sporulation has long been used as a model system to help elucidate basic processes of developmental biology including transcription regulation, intercellular signalling, membrane remodelling, protein localization and cell fate determination. This review highlights some of the recent work that has been done to further understand prokaryotic cell differentiation during sporulation and its potential applications.

Hydrogen/deuterium exchange mass spectrometry and site-directed disulfide cross-linking suggest an important dynamic interface between the two lysostaphin domains

Lysostaphin is a peptidoglycan hydrolase secreted by Staphylococcus simulans. It can specifically lyse Staphylococcus aureus and is being tested as a novel antibacterial agent. The protein contains an N-terminal catalytic domain and a C-terminal cell wall targeting domain. Although the two domains from homologous enzymes were structurally determined, the structural organization of lysostaphin domains remains unknown. We used hydrogen/deuterium exchange mass spectrometry (H/DX-MS) and site-directed disulfide cross-linking to probe the interface between the lysostaphin catalytic and targeting domains. H/DX-MS-mediated comparison of peptides from full-length lysostaphin and the separated domains identified four peptides of lower solvent accessibility in the full-length protein. Cross-linking analysis using cysteine pair substitutions within those peptides showed that two pairs of cysteines can form disulfide bonds, supporting the domain association role of the targeted peptides. The cross-linked mutant exhibited a binding capacity to S. aureus that was similar to that of the wild-type protein but reduced bacteriolytic activity probably because of restraint in conformation. The diminished activity was further reduced with increasing NaCl concentrations that can cause contractions of bacterial peptidoglycan. The lytic activity, however, could be fully recovered by reducing the disulfide bonds. These results suggest that lysostaphin may require dynamic association of the two domains for coordinating substrate binding and target cleavage on the elastic peptidoglycan. Our study will help develop site-specific PEGylated lysostaphin to treat systemic S. aureus infections.

Protein transport across the cell wall of monoderm Gram-positive bacteria

In monoderm (single-membrane) Gram-positive bacteria, the majority of secreted proteins are first translocated across the cytoplasmic membrane into the inner wall zone. For a subset of these proteins, final destination is within the cell envelope as either membrane-anchored or cell wall-anchored proteins, whereas another subset of proteins is destined to be transported across the cell wall into the extracellular milieu. Although the cell wall is a porous structure, there is evidence that, for some proteins, transport is a regulated process. This review aims at describing what is known about the mechanisms that regulate the transport of proteins across the cell wall of monoderm Gram-positive bacteria.

Effect of ph and salt gradient on the autolysis of Lactococcus lactis strains

The aim of this work was to assess in-vitro the effect of pH and salt concentration on the rate of autolysis in L. lactis strains. Regardless autolysis variation among L. lactis strains, statistical analysis showed evidence of increase of autolysis in L. lactis under low salt concentration and acidic conditions.

One-step conversion of to its using salts for GC-MSdetection of bacterial endospores

Methyl sulfate (MeSO4-) salts were explored as thermochemolysis-methylation (TCM) reagents for gas chromatographic (GC) analysis of dipicolinic acid (DPA) as its dimethyl ester (Me2DPA) from bacterial endospores. The reaction was carried out under non-pyrolytic conditions by inserting a small coiled wire filament coated with the sample and reagents directly inside a GC injection port at 290 °C. Above 10 : 1 methyl donor/DPA ratios, alkali metal salts of MeSO4- effected 80-90% conversion of DPA to Me2DPA, which was 10-20 times more active than the same amount of tetramethylammonium hydroxide (TMA-OH) at this temperature. A quaternary salt mixture consisting of 1 : 3 : 1 : 3 TMA+/Na+/OH-/MeSO4- methylated spore DPA with an average conversion of 86% (mean conversion by TMA-OH under the same conditions was 4%). Therefore, the sensitivity for detection of bacterial endospores was increased over 20-fold compared to that observed with the more commonly employed TMA-OH methylating reagent. The limit of detection by this method was 9 × 104 total spores. Mechanisms describing the observed behavior are proposed and discussed. This is the first use of MeSO4- as a TCM reagent for GC.

Macromolecules that prefer their membranes curvy

Understanding the mechanisms that underlie the organization of bacterial cells has become a significant challenge in the field of bacterial cytology. Of specific interest are early macromolecular sorting events that establish cellular non-uniformity and provide chemical landmarks for later localization events. In this review, we will examine specific examples of lipids and proteins that appear to exploit differences in membrane curvature to drive their localization to particular regions of a bacterial cell. We will also discuss the physical limits of curvature-mediated localization within bacteria, and the use of modelling to infer biophysical properties of curvature-sensing macromolecules.

Reduced lysis upon growth of Lactococcus lactis on galactose is a consequence of decreased binding of the autolysin AcmA

When Lactococcus lactis subsp. lactis IL1403 or L. lactis subsp. cremoris MG1363 is grown in a medium with galactose as the carbon source, the culture lyses to a lesser extent in stationary phase than when the bacteria are grown in a medium containing glucose. Expression of AcmA, the major autolysin of L. lactis, is not influenced by the carbon source. Binding studies with a fusion protein consisting of the MSA2 protein of Plasmodium falciparum and the C-terminal peptidoglycan-binding domain of AcmA revealed that cell walls of cells from both subspecies grown on galactose bind less AcmA than cell walls of cells grown on glucose. Cells grown on glucose or galactose and treated with trichloroacetic acid prior to AcmA binding bind similar amounts of AcmA. Analysis of the composition of the lipoteichoic acids (LTAs) of L. lactis IL1403 cells grown on glucose or galactose showed that the LTA composition is influenced by the carbon source: cells grown on galactose contain LTA with less galactose than cells grown on glucose. In conclusion, growth of L. lactis on galactose changes the LTA composition in the cell wall in such a way that less AcmA is able to bind to the peptidoglycan, resulting in a decrease in autolysis.

Roles of DacB and spm proteins in clostridium perfringens spore resistance to moist heat, chemicals, and UV radiation

Clostridium perfringens food poisoning is caused mainly by enterotoxigenic type A isolates that typically possess high spore heat resistance. Previous studies have shown that alpha/beta-type small, acid-soluble proteins (SASP) play a major role in the resistance of Bacillus subtilis and C. perfringens spores to moist heat, UV radiation, and some chemicals. Additional major factors in B. subtilis spore resistance are the spore's core water content and cortex peptidoglycan (PG) structure, with the latter properties modulated by the spm and dacB gene products and the sporulation temperature. In the current work, we have shown that the spm and dacB genes are expressed only during C. perfringens sporulation and have examined the effects of spm and dacB mutations and sporulation temperature on spore core water content and spore resistance to moist heat, UV radiation, and a number of chemicals. The results of these analyses indicate that for C. perfringens SM101 (i) core water content and, probably, cortex PG structure have little if any role in spore resistance to UV and formaldehyde, presumably because these spores' DNA is saturated with alpha/beta-type SASP; (ii) spore resistance to moist heat and nitrous acid is determined to a large extent by core water content and, probably, cortex structure; (iii) core water content and cortex PG cross-linking play little or no role in spore resistance to hydrogen peroxide; (iv) spore core water content decreases with higher sporulation temperatures, resulting in spores that are more resistant to moist heat; and (v) factors in addition to SpmAB, DacB, and sporulation temperature play roles in determining spore core water content and thus, spore resistance to moist heat.

Surface viscoelasticity of individual gram-negative bacterial cells measured using atomic force microscopy

The cell envelope of gram-negative bacteria is responsible for many important biological functions: it plays a structural role, it accommodates the selective transfer of material across the cell wall, it undergoes changes made necessary by growth and division, and it transfers information about the environment into the cell. Thus, an accurate quantification of cell mechanical properties is required not only to understand physiological processes but also to help elucidate the relationship between cell surface structure and function. We have used a novel, atomic force microscopy (AFM)-based approach to probe the mechanical properties of single bacterial cells by applying a constant compressive force to the cell under fluid conditions while measuring the time-dependent displacement (creep) of the AFM tip due to the viscoelastic properties of the cell. For these experiments, we chose a representative gram-negative bacterium, Pseudomonas aeruginosa PAO1, and we used regular V-shaped AFM cantilevers with pyramid-shaped and colloidal tips. We find that the cell response is well described by a three-element mechanical model which describes an effective cell spring constant, k(1), and an effective time constant, tau, for the creep deformation. Adding glutaraldehyde, an agent that increases the covalent bonding of the cell surface, produced a significant increase in k(1) together with a significant decrease in tau. This work represents a new attempt toward the understanding of the nanomechanical properties of single bacteria while they are under fluid conditions, which could be of practical value for elucidating, for instance, the biomechanical effects of drugs (such as antibiotics) on pathogens.

Omiganan interaction with bacterial membranes and cell wall models. Assigning a biological role to saturation

Omiganan pentahydrochloride (ILRWPWWPWRRK-NH(2).5Cl) is an antimicrobial peptide currently in phase III clinical trials. This study aims to unravel the mechanism of action of this drug at the membrane level and address the eventual protective role of peptidoglycan in cell walls. The interaction of omiganan pentahydrochloride with bacterial and mammalian membrane models - large unilamellar vesicles of different POPC:POPG proportions - was characterized by UV-Vis fluorescence spectroscopy. The molar ratio partition constants obtained for the two anionic bacterial membrane models were very high ((18.9+/-1.3)x10(3) and (43.5+/-8.7)x10(3)) and about one order of magnitude greater than for the neutral mammalian models ((3.7+/-0.4)x10(3) for 100% POPC bilayers). At low lipid:peptide ratios there were significant deviations from the usual hyperbolic-like partition behavior of peptide vesicle titration curves, especially for the most anionic systems. Membrane saturation can account for such observations and mathematical models were derived to further characterize the peptide-lipid interaction under those conditions; a possible relation between saturation and MIC was deduced; this was supported by differential quenching studies of peptide internalization. Interaction with the bacterial cell wall was assessed using Staphylococcus aureus peptidoglycan extracts as a model. A strong partition towards the peptidoglycan mesh was observed, but not as large as for the membrane models.

Autolysis of Lactococcus lactis is increased upon D-alanine depletion of peptidoglycan and lipoteichoic acids

Mutations in the genes encoding enzymes responsible for the incorporation of D-Ala into the cell wall of Lactococcus lactis affect autolysis. An L. lactis alanine racemase (alr) mutant is strictly dependent on an external supply of D-Ala to be able to synthesize peptidoglycan and to incorporate D-Ala in the lipoteichoic acids (LTA). The mutant lyses rapidly when D-Ala is removed at mid-exponential growth. AcmA, the major lactococcal autolysin, is partially involved in the increased lysis since an alr acmA double mutant still lyses, albeit to a lesser extent. To investigate the role of D-Ala on LTA in the increased cell lysis, a dltD mutant of L. lactis was investigated, since this mutant is only affected in the D-alanylation of LTA and not the synthesis of peptidoglycan. Mutation of dltD results in increased lysis, showing that D-alanylation of LTA also influences autolysis. Since a dltD acmA double mutant does not lyse, the lysis of the dltD mutant is totally AcmA dependent. Zymographic analysis shows that no degradation of AcmA takes place in the dltD mutant, whereas AcmA is degraded by the extracellular protease HtrA in the wild-type strain. In L. lactis, LTA has been shown to be involved in controlled (directed) binding of AcmA. LTA lacking D-Ala has been reported in other bacterial species to have an improved capacity for autolysin binding. Mutation of dltD in L. lactis, however, does not affect peptidoglycan binding of AcmA; neither the amount of AcmA binding to the cells nor the binding to specific loci is altered. In conclusion, D-Ala depletion of the cell wall causes lysis by two distinct mechanisms. First, it results in an altered peptidoglycan that is more susceptible to lysis by AcmA and also by other factors, e.g., one or more of the other (putative) cell wall hydrolases expressed by L. lactis. Second, reduced amounts of D-Ala on LTA result in decreased degradation of AcmA by HtrA, which results in increased lytic activity.

A continuum of anionic charge: structures and functions of D-alanyl-teichoic acids in gram-positive bacteria

Teichoic acids (TAs) are major wall and membrane components of most gram-positive bacteria. With few exceptions, they are polymers of glycerol-phosphate or ribitol-phosphate to which are attached glycosyl and D-alanyl ester residues. Wall TA is attached to peptidoglycan via a linkage unit, whereas lipoteichoic acid is attached to glycolipid intercalated in the membrane. Together with peptidoglycan, these polymers make up a polyanionic matrix that functions in (i) cation homeostasis; (ii) trafficking of ions, nutrients, proteins, and antibiotics; (iii) regulation of autolysins; and (iv) presentation of envelope proteins. The esterification of TAs with D-alanyl esters provides a means of modulating the net anionic charge, determining the cationic binding capacity, and displaying cations in the wall. This review addresses the structures and functions of D-alanyl-TAs, the D-alanylation system encoded by the dlt operon, and the roles of TAs in cell growth. The importance of dlt in the physiology of many organisms is illustrated by the variety of mutant phenotypes. In addition, advances in our understanding of D-alanyl ester function in virulence and host-mediated responses have been made possible through targeted mutagenesis of dlt. Studies of the mechanism of D-alanylation have identified two potential targets of antibacterial action and provided possible screening reactions for designing novel agents targeted to D-alanyl-TA synthesis.

Biofilm acid/base physiology and gene expression in oral bacteria

Environmental pH is one the major factors affecting the composition, biological activities, and pathogenic potential of the biofilms colonizing supragingival surfaces. In periodontal diseases, small changes in pH from the metabolism of amino acids and urea may influence the activity of proteolytic enzymes of host and bacterial origin. Still, there is a significant void in the understanding of pH-dependent gene expression in bacteria, in general, and this is of course a more acute problem when one considers there is virtually no information about gene expression in response to pH in biofilms. The development of new methods and applications of some of the techniques detailed above should help to ameliorate this situation and to generate much-needed data about the role of pH in biofilm composition, stability, and activity.

Biosynthesis of the glycolipid anchor in lipoteichoic acid of Staphylococcus aureus RN4220: role of YpfP, the diglucosyldiacylglycerol synthase

In Staphylococcus aureus RN4220, lipoteichoic acid (LTA) is anchored in the membrane by a diglucosyldiacylglycerol moiety. The gene (ypfP) which encodes diglucosyldiacylglycerol synthase was recently cloned from Bacillus subtilis and expressed in Escherichia coli (P. Jorasch, F. P. Wolter, U. Zahringer, and E. Heinz, Mol. Microbiol. 29:419-430, 1998). To define the role of ypfP in this strain of S. aureus, a fragment of ypfP truncated from both ends was cloned into the thermosensitive replicon pVE6007 and used to inactivate ypfP. Chloramphenicol-resistant (ypfP::cat) clones did not synthesize the glycolipids monoglucosyldiacylglycerol and diglucosyldiacylglycerol. Thus, YpfP would appear to be the only diglucosyldiacylglycerol synthase in S. aureus providing glycolipid for LTA assembly. In LTA from the mutant, the glycolipid anchor is replaced by diacylglycerol. Although the doubling time of the mutant was identical to that of the wild type in Luria-Bertani (LB) medium, growth of the mutant in LB medium containing 1% glycine was not observed. This inhibition was antagonized by either L- or D-alanine. Moreover, viability of the mutant at 37 degrees C in 0.05 M phosphate (pH 7.2)-saline for 12 h was reduced to <0.1%. Addition of 0.1% D-glucose to the phosphate-saline ensured viability under these conditions. The autolysis of the ypfP::cat mutant in the presence of 0.05% Triton X-100 was 1.8-fold faster than that of the parental strain. Electron microscopy of the mutant revealed not only a small increase in cell size but also the presence of pleomorphic cells. Each of these phenotypes may be correlated with either (or both) a deficiency of free glycolipid in the membrane or the replacement of the usual glycolipid anchor of LTA with diacylglycerol.

Transposon-mediated insertional mutagenesis of the D-alanyl-lipoteichoic acid (dlt) operon raises methicillin resistance in Staphylococcus aureus

Two independent mutants of methicillin-resistant Staphylococcus aureus (MRSA), KAN96H1 and KAN96H2, were isolated by insertional mutagenesis with conjugative transposon Tn918. In both, the minimal inhibitory concentration (MIC) of methicillin was increased to 128 compared to 16 mg/L for the parental strain KAN96. By transduction experiments, we verified that the insertion of Tn918 conferred higher methicillin resistance on KAN96H1, but not on KAN96H2. In KAN96H1, the integration site of Tn918 was located in the 6.1-kb HindIII fragment of the KAN96 chromosomal DNA. We identified a novel D-alanyl-lipoteichoic acid (dlt) operon of S. aureus in this fragment. The amino acid sequences of four open reading frames of this operon were highly homologous to those of the dlt operon genes of Bacillus subtilis. The nucleotide sequence of the staphylococcal dlt operon is under the accession number D86240 in the DDBJ/GenBank/EMBL databases. In KAN96H1, Tn918 was inserted in the 5'-terminal region of the putative dltB gene which encoded a hypothetical membrane transporter. dlt transcripts of 4.7 kb were detected in KAN96, but were truncated to 2.3 kb in KAN96H1. No corresponding transcripts were observed in KAN96H2. Our results clearly demonstrated that defects in functions of the putative dlt operon resulted in increased methicillin resistance in MRSA.

Structural analysis of Bacillus subtilis spore peptidoglycan during sporulation

A major structural element of bacterial endospores is a peptidoglycan (PG) wall. This wall is produced between the two opposed membranes surrounding the developing forespore and is composed of two layers. The inner layer is the germ cell wall, which appears to have a structure similar to that of the vegetative cell wall and which serves as the initial cell wall following spore germination. The outer layer, the cortex, has a modified structure, is required for maintenance of spore dehydration, and is degraded during spore germination. Theories suggest that the spore PG may also play a mechanical role in the attainment of spore dehydration. Inherent in one of these models is the production of a gradient of cross-linking across the span of the spore PG. We report analyses of the structure of PG found within immature, developing Bacillus subtilis forespores. The germ cell wall PG is synthesized first, followed by the cortex PG. The germ cell wall is relatively highly cross-linked. The degree of PG cross-linking drops rapidly during synthesis of the first layers of cortex PG and then increases two- to eightfold across the span of the outer 70% of the cortex. Analyses of forespore PG synthesis in mutant strains reveal that some strains that lack this gradient of cross-linking are able to achieve normal spore core dehydration. We conclude that spore PG with cross-linking within a broad range is able to maintain, and possibly to participate in, spore core dehydration. Our data indicate that the degree of spore PG cross-linking may have a more direct impact on the rate of spore germination and outgrowth.

Biosynthesis of lipoteichoic acid in Lactobacillus rhamnosus: role of DltD in D-alanylation

The dlt operon (dltA to dltD) of Lactobacillus rhamnosus 7469 encodes four proteins responsible for the esterification of lipoteichoic acid (LTA) by D-alanine. These esters play an important role in controlling the net anionic charge of the poly (GroP) moiety of LTA. dltA and dltC encode the D-alanine-D-alanyl carrier protein ligase (Dcl) and D-alanyl carrier protein (Dcp), respectively. Whereas the functions of DltA and DltC are defined, the functions of DltB and DltD are unknown. To define the role of DltD, the gene was cloned and sequenced and a mutant was constructed by insertional mutagenesis of dltD from Lactobacillus casei 102S. Permeabilized cells of a dltD::erm mutant lacked the ability to incorporate D-alanine into LTA. This defect was complemented by the expression of DltD from pNZ123/dlt. In in vitro assays, DltD bound Dcp for ligation with D-alanine by Dcl in the presence of ATP. In contrast, the homologue of Dcp, the Escherichia coli acyl carrier protein (ACP), involved in fatty acid biosynthesis, was not bound to DltD and thus was not ligated with D-alanine. DltD also catalyzed the hydrolysis of the mischarged D-alanyl-ACP. The hydrophobic N-terminal sequence of DltD was required for anchoring the protein in the membrane. It is hypothesized that this membrane-associated DltD facilitates the binding of Dcp and Dcl for ligation of Dcp with D-alanine and that the resulting D-alanyl-Dcp is translocated to the primary site of D-alanylation.

Muramic lactam in peptidoglycan of Bacillus subtilis spores is required for spore outgrowth but not for spore dehydration or heat resistance

Bacterial endospores derive much of their longevity and resistance properties from the relative dehydration of their protoplasts. The spore cortex, a peptidoglycan structure surrounding the protoplasm, maintains, and is postulated to have a role in attaining, protoplast dehydration. A structural modification unique to the spore cortex is the removal of all or part of the peptide side chains from the majority of the muramic acid residues and the conversion of 50% of the muramic acid to muramic lactam. A mutation in the cwlD gene of Bacillus subtilis, predicted to encode a muramoyl-L-alanine amidase, results in the production of spores containing no muramic lactam. These spores have normally dehydrated protoplasts but are unable to complete the germination/ outgrowth process to produce viable cells. Addition of germinants resulted in the triggering of germination with loss of spore refractility and the release of dipicolinic acid but no degradation of cortex peptidoglycan. Germination in the presence of lysozyme allowed the cwlD spores to produce viable cells and showed that they have normal heat resistance properties. These results (i) suggest that a mechanical activity of the cortex peptidoglycan is not required for the generation of protoplast dehydration but rather that it simply serves as a static structure to maintain dehydration, (ii) demonstrate that degradation of cortex peptidoglycan is not required for spore solute release or partial spore core rehydration during germination, (iii) indicate that muramic lactam is a major specificity determinant of germination lytic enzymes, and (iv) suggest the mechanism by which the spore cortex is degraded during germination while the germ cell wall is left intact.

Analysis of the peptidoglycan structure of Bacillus subtilis endospores

Peptidoglycan was prepared from purified Bacillus subtilis spores of wild-type and several mutant strains. Digestion with muramidase resulted in cleavage of the glycosidic bonds adjacent to muramic acid replaced by peptide or alanine side chains but not the bonds adjacent to muramic lactam. Reduction of the resulting muropeptides allowed their separation by reversed-phase high-pressure liquid chromatography. The structures of 20 muropeptides were determined by amino acid and amino sugar analysis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. In wild-type spores, 50% of the muramic acid had been converted to the lactam and 75% of these lactam residues were spaced regularly at every second muramic acid position in the glycan chains. Single L-alanine side chains were found on 25% of the muramic acid residues. The remaining 25% of the muramic acid had tetrapeptide or tripeptide side chains, and 11% of the diaminopimelic acid in these side chains was involved in peptide cross-links. Analysis of spore peptidoglycan produced by a number of mutants lacking proteins involved in cell wall metabolism revealed structural changes. The most significant changes were in the spores of a dacB mutant which lacks the sporulation-specific penicillin-binding protein 5*. In these spores, only 46% of the muramic acid was in the lactam form, 12% had L-alanine side chains, and 42% had peptide side chains containing diaminopimelic acid, 29% of which was involved in cross-links.

The D-Alanyl carrier protein in Lactobacillus casei: cloning, sequencing, and expression of dltC

The incorporation of D-alanine into membrane-associated D-alanyl-lipoteichoic acid in Lactobacillus casei requires the 56-kDa D-alanine-D-alanyl carrier protein ligase (Dcl) and the 8.9-kDa D-alanyl carrier protein (Dcp). To identify and isolate the gene encoding Dcp, we have cloned and sequenced a 4.3-kb chromosomal fragment that contains dcl (dltA). In addition to this gene, the fragment contains three other genes, dltB, d1tC, and a partial dltD gene. dltC (246 nucleotides) was subcloned from this region and expressed in Escherichia coli. The product was identified as apo-Dcp lacking the N-terminal methionine (8,787.9 Da). The in vitro conversion of the recombinant apo-Dcp to holo-Dcp by recombinant E. coli holo-ACP synthase provided Dcp which accepts activated D-alanine in the reaction catalyzed by Bcl. The recombinant D-alanyl-Dcp was functionally identical to native D-alanyl-Dcp in the incorporation of D-alanine into lipoteichoic acid. L. casei Dcp is 46% identical to the putative product of dltC in the Bacillus subtilis dlt operon (M. Perego, P. Glaser, A. Minutello, M. A. Strauch, K. Leopold, and W. Fischer, J. Biol. Chem. 270:15598-15606, 1995), and therefore, this gene also encodes Dcp. Comparisons of the primary sequences and predicted secondary structures of the L. casei and B. subtilis Dcps with that of the E. coli acyl carrier protein (ACP) were undertaken together with homology modeling to identify the functional determinants of the donor and acceptor specificities of Dcp. In the region of the phospho-pantetheine attachment site, significant similarity between Dcps and ACPs was observed. This similarity may account for the relaxed acceptor specificity of the Dcps and ACPs in the ligation Of D-alanine catalyzed by Dcl. In contrast, two Dcp consensus sequences, KXXVLDXLA and DXVKXNXD, share little identity with the rest of the ACP family and, thus, may determine the donor specificity of D-alanyl-Dcp in the D-alanylation of membrane-associated D-alanyl-lipoteichoic acid.

The Bacillus subtilis dacB gene, encoding penicillin-binding protein 5*, is part of a three-gene operon required for proper spore cortex synthesis and spore core dehydration

Studies of gene expression using fusions to lacZ demonstrated that the Bacillus subtilis dacB gene, encoding penicillin-binding protein 5*, is in an operon with two downstream genes, spmA and spmB. Mutations affecting any one of these three genes resulted in the production of spores with reduced heat resistance. The cortex peptidoglycan in dacB mutant spores had more peptide side chains, a higher degree of peptide cross-linking, and possibly less muramic acid lactam than that of wild-type spores. These cortex structure parameters were normal in spmA and spmB mutant spores, but these spores did not attain normal spore core dehydration. This defect in spore core dehydration was exaggerated by the additional loss of dacB expression. However, loss of dacB alone did not alter the spore core water content. Spores produced by spmA and spmB mutants germinated faster than did those of the wild type. Spores produced by dacB mutants germinated normally but were delayed in spore outgrowth. Electron microscopy revealed a drastically altered appearance of the cortex in dacB mutants and a minor alteration in an spmA mutant. Measurements of electron micrographs indicate that the ratio of the spore protoplast volume to the sporoplast (protoplast-plus-cortex) volume was increased in dacB and spmA mutants. These results are consistent with spore core water content being the major determinant of spore heat resistance. The idea that loosely cross-linked, flexible cortex peptidoglycan has a mechanical activity involved in achieving spore core dehydration is not consistent with normal core dehydration in spores lacking only dacB.

Role of the D-alanyl carrier protein in the biosynthesis of D-alanyl-lipoteichoic acid

D-Alanyl-lipoteichoic acid (D-alanyl-LTA) is a widespread macroamphiphile which plays a vital role in the growth and development of gram-positive organisms. The biosynthesis of this polymer requires the enzymic activation of D-alanine for its transfer to the membrane-associated LTA (mLTA). A small, heat-stable, and acidic protein that is required for this transfer was purified to greater than 98% homogeneity from Lactobacillus casei ATCC 7469. This protein, previously named the D-alanine-membrane acceptor ligase (V. M. Reusch, Jr., and F. C. Neuhaus, J. Biol. Chem. 246:6136-6143, 1971), functions as the D-alanyl carrier protein (Dcp). The amino acid composition, beta-alanine content, and N-terminal sequence of this protein are similar to those of the acyl carrier proteins (ACPs) of fatty acid biosynthesis. The isolation of Dcp and its derivative, D-alanyl approximately Dcp, has allowed the characterization of two novel reactions in the pathway for D-alanyl-mLTA biosynthesis: (i) the ligation of Dcp with D-alanine and (ii) the transfer of D-alanine from D-alanyl approximately Dcp to a membrane acceptor. It has not been established whether the membrane acceptor is mLTA or another intermediate in the pathway for D-alanyl-mLTA biosynthesis. Since the D-alanine-activating enzyme (EC 6.1.1.13) catalyzes the ligation reaction, this enzyme functions as the D-alanine-Dcp ligase (Dcl). Dcl also ligated the ACPs from Escherichia coli, Vibrio harveyi, and Saccharopolyspora erythraea with D-alanine. In contrast to the relaxed specificity of Dcl in the ligation reaction, the transfer of D-alanine to the membrane acceptor was highly specific for Dcp and did not occur with other ACPs. This transfer was observed by using only D-[14C]alanyl approximately Dcp and purified L. casei membranes. Thus, D-alanyl approximately Dcp is an essential intermediate in the transfer of D-alanine from Dcl to the membrane acceptor. The formation of D-alanine esters of mLTA provides a mechanism for modulating the net anionic charge in the cell wall.

Elastic, flexible peptidoglycan and bacterial cell wall properties

The peptidoglycan sacculus serves as a mechanical framework for the cell walls of most eubacteria and largely determines cell shape. The notion that the structure is a rigid shell is contradicted by findings that peptidoglycan can expand and contract. Thus, the sacculus functions as an elastic, flexible, polyionic, amphoteric, restraining network.

The cortical peptidoglycan from spores of Bacillus megaterium and Bacillus subtilis is not highly cross-linked

Determination by amino acid analyses of the percentage of diaminopimelic acid in the spore cortex of Bacillus megaterium and Bacillus subtilis which is involved in interpeptide cross-links gave values of 31 to 37%. This finding supports the idea that the cortex volume could undergo significant changes in response to changes in pH or ionic strength and could thus play an active role in reducing the water content of the spore protoplast during sporulation.

Elasticity of the sacculus of Escherichia coli

Preparations of purified peptidoglycan of Escherichia coli (i.e., sacculi) were studied by low-angle laser light scattering. Control experiments and theoretical calculations based on the Rayleigh-Gans theory showed that the mean sacculus surface area could be accurately inferred from measurements with our apparatus by using computer routines developed previously. Large changes in the mean saccular surface area resulted from alterations in the stress caused by varying the net charge on the sacculi. The net charge was affected by altering the suspending medium pH, causing carboxyl and amino groups in the peptidoglycan to gain or lose protons, or by acetylation or succinylation of the amino groups. A preponderance of either plus or minus charges caused an expansion of the mean sacculus surface area. The largest increase in area probably represents the elastic limit of the peptidoglycan and was 300% above the area of isoionic sacculi. This degree of expansion is consistent with possible conformations of the intact peptidoglycan structure without necessitating rupture of the wall fabric. Our findings concerning saccular elasticity provide support for the surface stress theory. It provides a mechanism so that bacteria can grow and divide while maintaining turgor pressure, without the necessity of having and using proteins to do the mechanical work.

Mechanical properties of Bacillus subtilis cell walls: effects of ions and lysozyme

Bacterial threads of Bacillus subtilis have been immersed in, and redrawn from, water of various pH values, in solutions of (NH4)2SO4 and NaCl of various concentrations, and in lysozyme solutions. The changes in the tensile strength, elastic modulus, and other mechanical properties of the bacterial cell wall due to these treatments were obtained. The data show that change in pH has little effect but that as the salt concentration is increased, the cell walls become more ductile. A high salt concentration (1 M NaCl) can reduce the modulus by a factor of 26 to 13.5 MPa at 81% relative humidity and the strength by a factor of only 2.5. Despite attacking the septal-wall region of the cellular filaments, lysozyme has no effect on the mechanical properties. There is no significant change in the stress relaxation behavior due to any of the treatments. The dependence of mechanical properties on the salt concentration is discussed in terms of the polyelectrolyte nature of cell walls. The evidence presented in this and the accompanying paper (J. J. Thwaites and U.C. Surana, J. Bacteriol., 173:197-203, 1991) supports the idea that the peptidoglycan in bacterial cell wall is an entanglement network with a large degree of molecular flexibility, with some order but no regular structure.

Mechanical properties of Bacillus subtilis cell walls: effects of removing residual culture medium

Experiments are described in which the tensile strength, the initial (Youngs') modulus, and other mechanical properties of the bacterial cell wall were obtained as functions of relative humidity (RH) in the range of 20 to 95%. These properties were deduced from tensile tests on bacterial thread, a fiber consisting of many highly aligned cells of Bacillus subtilis, from which residual culture medium had been removed by immersion in water. Reasons are given to support the idea that the mechanical properties of bacterial thread relate directly to those of the cylinder wall and that they are not influenced by septa, cytoplasm, or the thread assembly. The data show that the cell wall, like many other heteropolymers, is visco-elastic. When dry, it behaves like a glassy polymer with a tensile strength of about 300 MPa and a modulus of about 13 GPa. When wet, its behavior is more like a rubbery polymer with a tensile strength of about 13 MPa and a modulus of about 30 MPa. Thus, the cell wall is stronger than previously reported. Walls of this strength would be able to bear a turgor pressure of 2.6 MPa (about 26 atm). The dynamic behavior suggests a wide range of relaxation times. The way in which mechanical behavior depends strongly on humidity is discussed in terms of possible hydrogen bond density and the ordering of water molecules. Cell walls in threads containing residual culture medium TB are, except at low RH, 10 times more flexible and about 4 times less strong. All of their mechanical properties appear to vary with change in RH in a manner similar to those of walls from which the culture medium has been washed, but with a downshift of about 18% RH.

Mechanical properties of peptidoglycan as determined from bacterial thread

Experiments are described in which the tensile strength, the extensibility and the initial Young's modulus of bacterial cell wall have been determined as functions of relative humidity in the range 11-98%. Data on stress relaxation and recovery are also given. Standard fibre-measuring technique has been used on 'bacterial thread', made from a cell-separation-suppressed mutant of Bacillus subtilis. The data show that peptidoglycan, the load bearing polymer in the cell wall, behaves very much like other viscoelastic polymers. Its mechanical behaviour when dry is that of a glassy polymer with tensile strength about 300 MPa and modulus about 20 GPa. When wet, it is weaker and much less stiff with tensile strength about 3 M Pa and modulus 10 M Pa. The relaxation data indicate a wide spectrum of relaxation times. The results are discussed in terms of the structure of peptidoglycan and its orientation in the bacterial cell wall. The way in which mechanical behaviour depends strongly on humidity is compared with that of other biopolymers in terms of possible hydrogen-bond density and the ordering of water molecules. The possibility of a well-defined glass transition is briefly examined.

Cell wall mechanical properties as measured with bacterial thread made from Bacillus subtilis

Engineering approaches used in the study of textile fibers have been applied to the measurement of mechanical properties of bacterial cell walls by using the Bacillus subtilis bacterial thread system. Improved methods have been developed for the production of thread and for measuring its mechanical properties. The best specimens of thread produced from cultures of strain FJ7 grown in TB medium at 20 degrees C varied in diameter by a factor of 1.09 over a 30-mm thread length. The stress-strain behavior of cell walls was determined over the range of relative humidities between 11 and 98%. Measurements of over 125 specimens indicated that cell wall behaved like other viscoelastic polymers, both natural and man-made, exhibiting relaxation under constant elongation and recovery upon load removal. This kinetic behavior and also the cell wall strength depended greatly on humidity. The recovery from extension observed after loading even up to a substantial fraction of the breaking load indicated that the properties measured were those of cell wall material rather than of behavior of the thread assemblage. Control experiments showed that neither drying of thread nor the length of time it remained dry before testing influenced the mechanical properties of the cell walls. Specimens drawn from TB medium and then washed in water and redrawn were found to be stiffer and stronger than controls not washed. However, tensile properties were not changed by exposure of cells to lysozyme before thread production. This suggests that glycan backbones are not arranged along the length of the cell cylinder. The strength of the cell wall in vivo was estimated by extrapolation to 100% relative humidity to be about 3 N/mm2. Walls of this strength would be able to bear a turgor pressure of 6 atm (ca. 607.8 kPa), but if the increase in strength of water-washed threads was appropriate, the figure could be 24 atm (ca. 2,431.2 kPa).

A model for the three-dimensional structure of peptidoglycan in staphylococci

Although the monomeric units of peptidoglycan in Staphylococcus aureus and other staphylococci are well known, the complete structure of the peptidoglycan has not been elucidated. The peptidoglycan monomeric unit may be divided into three parts: (1) glycan chain piece, consisting of N-acetylglucosaminyl-N-acetylmuramic acid; (2) connecting peptide extending from L-alanine to the alpha-amino group of L-lysine; (3) peptide chain piece, consisting of D-alanine, the remainder of L-lysine not included in the connecting peptide, and pentaglycine (S. aureus) or mixed glycine and serine residues (other staphylococci) attached to the epsilon amino group of lysine. The deformation of cross wall into hemisphere in the course of cell division, the distensibility of peptidoglycan, and the appearance of circular (? spiral) lines in the cross wall and on the surface of the newly-formed hemisphere are clues to the structure of peptidoglycan. In the proposed model, cross wall is formed as a linear spiral with 20 turns extending in a plane from periphery to center of the cell. During cell division, the cross wall is bisected. The cross wall spiral becomes a spiral forming the peripheral wall of a new hemisphere. The width of the spiral on the cell surface is maintained by rigid glycan chains and by covalent bonds linking turns of the spiral. The length of the spiral is about 30 times the diameter of the cell. Flexible polypeptide sheets consisting of parallel polypeptide chains run along the length of the spiral. Individual polypeptides contain an average of ten peptide chain pieces. The glycan chain is a helix with two disaccharide residues per turn; consequently consecutive connecting peptides project in opposite directions and are perpendicular both to the glycan chain and to the peptide chain. In cross wall, hydrogen bonding between polypeptide chains enables the polypeptide sheet to transmit changes in tension. The deformation of cross wall into peripheral wall requires doubling of the external surface area of the peptidoglycan. A change in the angle of the glycan chain with respect to the peptide chain results in an increase of the distance between peptide chains, causing the doubling of surface area. Implications of the model include explanations for the initiation of cell division and for the existence of osmotically growth-dependent staphylococci.