How does thickness of peptidoglycan affect

An interesting case deserves to be mentioned. The nature of the enzyme catalyzing the unusual transpeptidation reaction between d -Ala acyl donor and the N-terminal l -Ala acyl acceptor is unknown. This gives rise to the appearance of 3—3 cross-links, which were originally discovered in Mycobacteria Wietzerbin et al. Their formation is catalyzed by penicillin-insensitive l , d -transpeptidases Mainardi et al. As for the main peptide chain, the interpeptide bridge can be further modified after its assembly.

In Thermus thermophilus , where the amino acid at position 3 is l -Orn and the bridge consists of a diglycyl residue between positions 3 and 4, a significant proportion of glycyl residues not engaged in the bridge with the donor peptide stem are acylated with phenylacetic acid Quintela et al.

Besides the diversity in the nature of cross-linkage, there is a considerable variation in the degree of cross-linkage, which varies from ca. Translated in terms of muropeptide content, these figures mean that in E.

Considering that the variations in peptidoglycan structure have taxonomic implications, Schleifer and Kandler established a tri-digital system of classification of peptidoglycans.

The first digit, a Roman capital letter, represents the mode of cross-linkage A and B for the 3—4 and 2—4 cross-linkages, respectively. The second digit, a number, refers to the type of interpeptide bridge, or lack of it, involved in the cross-linkage. The third digit, a Greek letter, indicates the amino acid found at position 3 of the peptide stem. As a consequence, the examples of peptidoglycan depicted in Fig.

The fine structure of the bacterial sacculi is reflected in the detailed muropeptide composition of peptidoglycan as determined by means of high-resolution techniques.

Information on the abundance and peculiarities of families of muropeptides with specific structural functions is crucial to understand the architecture and physiology of the sacculus itself. Systematic studies in the model bacterium E. Aging brings with it a progressive variation of the indicated parameters in a process that apparently requires about one mass doubling time to complete.

Peptidoglycan fine structure is also subjected to global variations when the state of growth changes Pisabarro et al. The transition of E. Of course, the inverse transition also takes place when cells resume active growth from a resting condition.

Although data are quite limited, recovery of the muropeptide composition characteristic for actively growing cells might involve active modification of total peptidoglycan in addition to the expected variation due to mixing of old resting phase and new growth phase peptidoglycans de la Rosa, ; Pisabarro et al. A rather surprising ability of E. Studies conducted under conditions limiting supply of precursors showed that E. Cells with reduced peptidoglycan content were nevertheless more sensitive to penicillin and other damaging agents.

A comprehensive survey of peptidoglycan fine structure in different bacterial species is simply nonexistent at present. Only a few Gram-positive bacteria have lent themselves to analysis by HPLC and in most cases their composition could be only partially solved Garcia-Bustos et al. A large variability in fine structure is evident, as expected from their heterogeneity in chemical composition and cross-linking. Even among the more homogeneous Gram-negative organisms, large differences in fine structure have been clearly shown in spite of the limited number of well-studied organisms Folkening et al.

Therefore, it seems that there is no optimal or standard value for parameters as cross-linkage or glycan strand length, but rather each species selects the values or range of values appropriate for its particular life conditions. Variations in peptidoglycan fine structure have also been associated with bacterial pathogenesis in a number of cases. The nature of the profits bacteria derived from these adaptations is still unknown, but is likely relevant for their survival.

When present at a high concentration in the growth medium, analogues of peptidoglycan amino acids can be incorporated into the macromolecule and modify its composition. The most-known example is that of glycine, which can replace alanine at position 1, 4 or 5 in several bacterial species Hammes et al. The fact that several A 2 pm analogues are able to complement A 2 pm auxotrophy in E. Interestingly, the presence of hydroxylysine, which is often considered to be a natural constituent of the peptidoglycan of certain species, is in most cases the result of particular growth conditions, namely lysine deprivation and hydroxylysine supplementation see e.

Peptidoglycan composition varies in mutants or genetically engineered cells with respect to wild type. This is well documented in E. A dapF mutant lacking A 2 pm epimerase was shown to contain a huge pool of ll -A 2 pm that was incorporated into peptidoglycan Mengin-Lecreulx et al.

Recenty, the peptidoglycan of E. The replacement of meso -A 2 pm at position 3 by an analogue resulted in a decrease of the proportion of dimer. For cells overexpressing the S. There kinds of experiments were also applied to genes coding for Fem transferases. The heterospecific expression of the femhB , femA and femB genes of S.

Similar results were obtained when the bppA1 gene of Enterococcus faecalis was expressed in Enterococcus faecium Magnet et al. Peptidoglycan composition and structure can also evolve under the selective pressure of antibiotics.

This aspect is developed in the present issue by Mainardi Peptidoglycan sacculi are bag-shaped molecules with unique biophysical properties. On the one hand, sacculi have the strength to withstand the cell's turgor of up to 25 atmospheres present in Gram-positive bacteria. On the other hand, the sacculi are not rigid walls but are flexible, allowing reversible expansion under pressure, and they have relatively wide pores, enabling diffusion of large molecules such as proteins.

Because the peptidoglycan completely surrounds the cytoplasmic membrane, the sacculus has a similar size and shape as the bacterial cells from which it was isolated. Upon staining with a heavy metal, the thin sacculi from Gram-negative bacteria appear in electron microscopy EM pictures as flat, empty cell envelopes Fig.

The thickness of peptidoglycan has been determined from electron micrographs of thin sections of E. However, the results should be considered with caution when chemical fixation and dehydration are used in combination with certain staining techniques. Depending on the procedure applied, the peptidoglycan layer has a thickness between 1.

The main concerns with these methods are 1 that the dehydration and fixation might change the thickness of the peptidoglycan and 2 that the measurements determine the thickness of the line formed by the contrasting metal, which is not necessarily identical to the thickness of the peptidoglycan layer de Petris, ; Wientjes et al.

The introduction of cryo-transmission electron microscopy cryo-TEM of frozen hydrated sections proved to be a major improvement because this technique omits the chemical fixation and staining procedures.

With this method, the peptidoglycan can be seen as a thin line beneath the outer membrane in thin sections of Gram-negative bacteria. These data are in agreement with measurements of the thickness of isolated sacculi from E. Small-angle neutron scattering of hydrated, isolated sacculi without bound lipoprotein yielded a thickness of 2.

By atomic force microscopy AFM , a thickness of 3. AFM showed, too, that sacculi from P. EM of purified sacculi. Scale bars represent 0. Note the folds at the polar regions as the cylindrical structure collapses onto a flat surface. In all species analyzed, there is a zone of low density presumably lacking polymeric wall structure next to the plasma membrane.

The thickness of the OWZ most likely varies with the species, growth phase of the cells and growth conditions, and was determined to be in the range of 15—30 nm in S. Interestingly, the septal cell wall region in S. AFM allowed visualization of concentric rings surrounding a central depression on a new hemisphere after daughter cell separation, as well as a network of fibers and large holes with diameters of 50—60 nm at the older regions of the cell surface Touhami et al.

Upon mechanical removal of patches of outer wall, AFM produced images of regularly arranged, about 26 nm thick strands running perpendicular to the long axis of rod-shaped cells of Lactobacillus helveticus.

Although the nature of these structures is unknown, it has been proposed that they are made of bundles of twisted peptidoglycan strands Firtel et al. Low-angle laser light scattering was used to determine the change of the mean surface of E. Osmotic challenge of growing E.

Similarly, living cells as well as isolated cell walls of Bacillus megaterium contract when they are transferred from water to salt solution Marquis, When the cytoplasmic membrane of filamentous grown E. Apparently, the peptidoglycan is under dynamic stress in the living cell due to the cell's turgor and there is a limit to which the peptidoglycan can be maximally stretched.

Interestingly, isolated E. The elastic modulus is lower for material with greater elasticity. This is consistent with the observation that the changes in the volume of osmotically shocked E. It was suggested that the anisotropy in elasticity is the consequence of the predominant alignment of the more flexible peptides in the direction of the long axis of the cell and of the more rigid glycan strands perpendicular to the direction of the long axis.

Such a network has been modelled, and the theoretical calculations of the elastic moduli are in good agreement with the measured values Boulbitch et al. Fluorescence-labelled dextrans of different sizes have been used to determine the diameters of holes in the peptidoglycan network in E. Interestingly, the pores were of similar average sizes in peptidoglycans from Gram-negative and Gram-positive species and they were relatively homogenous in size: the mean radius of the pores was 2. From these values it was calculated that globular, uncharged proteins with molecular weights of up 22—24 kDa should be able to penetrate the isolated, relaxed peptidoglycan.

Globular proteins of up to 50 kDa or more should be able to diffuse through stretched peptidoglycan layer in the cell. Indeed, disruption of the outer membrane of E. Perhaps this value is determined by the molecular sieving properties of the stretched peptidoglycan layer Vazquez-Laslop et al. To understand how a biological structure grows, a detailed knowledge of how the individual components are organized and arrayed with respect to each other is of prime relevance.

Unravelling the molecular architecture of the bacterial sacculus has been a constant aspiration for many microbiologists, but it is proving to be a frustrating topic. In particular, the architecture of the cell wall of Gram-positive bacteria is far from being understood. Gram-positive species not only have a thick, multi-layered peptidoglycan but other major surface polymers linked to it Vollmer, Many species also have capsular polysaccharides which are often covalently linked to the peptidoglycan.

In addition, there are many surface proteins either linked covalently to peptidoglycan or bound noncovalently to cell wall polymers. To decipher the architecture of this three-dimensional assembly of different polymeric components and its enlargement during bacterial growth will be a major challenge for the future.

With respect to the architecture of the sacculus of Gram-negative bacteria, E. Therefore, the structural aspects of sacculi from this organism will be concentrated on.

The idea of the following comments is not so much to criticize existing models, but to point out aspects of cell wall biology and biochemistry which are overlooked, but must be accounted for, by present day models to help improve future developments. As commented above the sacculus is a covalently closed structure built up from glycan strands that are cross-linked to each other through peptide bridges.

These basic properties, defined long ago, naturally lead to the concept of the sacculus as static, regular and planar net-like polymeric macromolecule, a concept which can still be traced down to present-day textbooks.

However, this somewhat simplistic vision seems to be far from reality and the bacterial sacculus is proving itself to be a particularly intractable subject for structural studies. Application of even the more powerful tools in structural analysis, as X-ray diffraction, EM, AFM, low angle neutron scattering, and others have provided only limited information Formanek et al.

From a structural point of view, the basic problem is to define how individual glycan strands are arranged relative to each other and to the cell axes. The interactions among neighbouring glycan strands are in turn conditioned by three parameters; thickness, cross-linkage and length distribution of the glycan strands. These three parameters determine the number of chemical bonds per unit of surface area opposing the turgor, and define the basic constrains in model making.

The extreme thinness 3—4 nm assigned to the E. However, later evidence from different fields suggests a more complex situation. Application of AFM Yao et al. Perhaps more convincing than absolute thickness measurements is the fact that sacculi from another typical Gram-negative organism, P. As sacculi from both species are made of identical subunits Quintela et al. The sacculus is made up of glycan strands cross-linked to each other through peptide bridges. As the physico chemical properties of the glycan and peptide moieties are very different, in particular the ability of each to change conformation under stress Barnickel et al.

The peptide stems are of a fixed length, and in principle distributed regularly along the glycan backbone. Therefore the number of possible interstrand connections is also a direct function of glycan chain length GCL.

For a long time the only way to determine GCL was based on quantification of glycan strand terminal muropeptides Schindler et al. Reported results Pisabarro et al. Altmutter, unpublished data. Application of a new method based on enzymatic clipping of peptide stems with human serum amidase followed by HPLC separation of the resulting linear polysaccharides permitted an accurate analysis of the size distribution of glycan strands Harz et al.

However the method still suffers from a key limitation in that only glycan strands between 1 and 30 disaccharide units can be individually resolved. Longer strands cannot be separated from each other but at least the proportion of muropeptides in strands longer than 30 disaccharides can be evaluated, and an average value can be calculated from the proportion of 1,6-anhydroMur N Ac-containing muropeptides.

Information gathered by this method Harz et al. Therefore, sacculi of E. Cross-linking in Gram-negative species, in particular in E. Sacculi from growing, wild type E. That means that on average every third to second disaccharide in a peptidoglycan strand would be cross-linked to another adjacent strand. Similar values seem to be common among Gram-negative species, although data are still limited Quintela, a; , Costa et al. However the function and distribution of A 2 pm-A 2 pm bridges in the sacculus remains unknown.

Tetramers are barely detectable with reported abundances about 0. Following the same argument as above scarce tetramers could still be structurally significant Glauner et al.

Because disaccharide subunits in peptidoglycan strands are rotated with respect to each other due to the influence of the lactyl group in Mur N Ac, consecutive peptide stems point out in different directions Labischinski et al. The periodicity of peptidoglycan conditions the cross-linking between adjacent strands, as only those peptide stems with the correct relative orientation can be proficient substrates for transpeptidation. An immediate consequence of these facts is that adjacent peptidoglycan strands are unlikely to be cross-linked to each other by consecutive muropeptides Koch, a , b.

The more controversial aspect related to the structure of the sacculus is the orientation and distribution of the glycan strands Formanek et al. Most available evidence favoured models postulating glycan strands oriented parallel to the cell surface, and in most cases with the glycan backbones transversal to the cell longitudinal axis.

More recently an alternative model based on glycan strands oriented perpendicular to the surface of the sacculus has been proposed Dmitriev et al. As indicated above, it is not intended here to enter into a discussion of models so far proposed, but rather point out some aspects, in the authors' opinion, overlooked in those models. A good starting point is to refer to a recent comment Young, on the same topic which emphasizes what a weak point for most models is: the requirement for rather restrictive structural and morphological parameters.

As commented in the preceding sections the main structural parameters in the sacculus are subjected to drastic changes on the course of normal growth. This calls for dynamic rather than static models because the sacculus as such is in a continuous state of change. Furthermore, any credible model should have an intrinsic ability to accommodate the size and shape changes in particular in cell diameter, but also more dramatic ones as round and branched shapes that cells can exhibit under specific conditions.

Sacculi are made of glycan strands with a very wide distribution of sizes, with a substantial proportion of total peptidoglycan in strands too short to be connected to nearby strands by more than two cross-links.

Such strands could structurally be assimilated to a long range cross-link, connecting longer and relatively distant strands Costa et al. Even if it is generally assumed, there is no evidence at all that cross-linkage happens regularly alongside the glycan strands.

Indeed, the tendency of glycan strand termini to be highly cross-linked means that cross-linkage in internal positions is lower than the mean value for total peptidoglycan. Therefore, it is likely that long glycan strands might have relatively long uncross-linked stretches. A final comment on the organization of glycan strands comes from the very existence of cross-linked trimers and tetramers. The interesting point about these two families of muropeptides is that they represent connecting hubs for several crossing glycan strands.

An attractive idea is that trimers and tetramers represent linking points of short-to-long glycan strands. A normal cross-link bridging two nearby, long glycan strands could act as the attachment point for a short glycan strand acting as a long range connection to another relatively remote long glycan strand. That is, glycan strand termini seem to have a high tendency to cross-link to a dimeric muropeptide to form a trimer.

Whether this tendency is more marked in short or long glycan strands cannot be decided at present, and of course trimers could show no preference at all relative to glycan strand length. As for the remaining, major fraction of cross-linked trimers and tetramers geometrical arguments require that three, or four, peptidoglycan strands cross over Glauner et al. There are no data available on the angles between multiple glycan strands linked together at these positions.

In the extreme cases, they could run parallel, antiparallel, or perpendicular to each other. Regions of multiple glycan strand crossings could be related with the postulated regions of multilayered peptidoglycan.

Whether or not simple crossing of strands could be enough to explain the neutron diffraction results Labischinski et al. Finally, another aspect models should be able to account for is the ability of E. Under normal growth conditions results suggest that E. It is interesting to note that recent measurements of cell wall thickness in E. If indeed E. Many data on the chemical structure and the biophysics of peptidoglycan have been gathered over the last decades.

However, knowledge of this fascinating molecule is still very limited. For example, the analysis of peptidoglycan composition with high-resolution techniques has been performed so far only for a few bacterial species.

A significantly enlarged data set on peptidoglycan fine structures will be of interest for different research fields including bacterial taxonomy, physiology, and pathogenesis. Moreover, such research can lead to the discovery of peptidoglycans whose structure diverge from the overall structure defined at the beginning of this review. A recent example is represented by T. It is likely that such an unusual motif, which coexists with the conventional l -Lys-containing motif, is at the origin of a particular type of cross-link.

Another example is that of Chlamydiae , for which no muramic acid-containing peptidoglycan was detected to date. It is difficult to imagine that these enzymes do not participate in the synthesis of a specific macromolecule, the structure of which presumably greatly differs from that of usual peptidoglycan.

In the last two decades, the improvement of analytical methods HPLC, MS has allowed to show that, within a particular species, variations in peptidoglycan fine structure occur as a function of aging, medium composition, pathogenesis, or presence of antibiotics. This type of research has implications not only in the field of bacterial physiology, but also in those of innate immunity, pathogenicity, and antibacterial therapy. One major task for the future is to determine the molecular architecture of peptidoglycan in Gram-positive and Gram-negative species, which is not possible with today's techniques.

This includes determination of the orientation of the glycan strands and peptides with respect to the cell's axes and the distribution pattern of particular structures ld -cross-links, dimeric and trimeric peptides, glycan strand ends, attachment sites for other polymers on the surface of the cell wall.

Knowing the architecture of peptidoglycan is a prerequisite for solving the mechanism s of cell wall growth. As outlined in this review, models of peptidoglycan architecture should be based on the length distribution of the glycan strands, the degree of cross-linkage, the thickness of the macromolecule, the number of subunits per cell surface, as well as on biophysical data on the porosity and elasticity of the sacculus.

J Gen Microbiol : — Google Scholar. J Biol Chem : — J Bacteriol : — Microbiology : — Eur J Biochem 95 : — Eur J Biochem : — Beveridge TJ Structures of gram-negative cell walls and their derived membrane vesicles. Google Preview. Structure 12 : — Phys Rev Lett 85 : — Braun V Rehn K Chemical characterization, spatial distribution and function of a lipoprotein murein-lipoprotein of the E.

The specific effect of trypsin on the membrane structure. Eur J Biochem 10 : — The attachment site of the lipoprotein on the murein. Eur J Biochem 13 : — Biochemistry 30 : — Peptidoglycan is a wonderful substance. Now it seems that peptidoglycan can control the site of cell division, in S.

Turner, R. Peptidoglycan architecture can specify division planes in Staphylococcus aureus Nature Communications, 1 3 , DOI: Formation of the glycan chains in the synthesis of bacterial peptidoglycan. Glycobiology, 11 3 PMID: The views expressed are those of the author s and are not necessarily those of Scientific American.

Dr James Byrne has a PhD in Microbiology and works as a science communicator at the Royal Institution of Australia RiAus , Australia's unique national science hub, which showcases the importance of science in everyday life. Follow James Byrne on Twitter. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Discover World-Changing Science.

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Create your free OpenLearn profile. Course content Course content. Understanding antibiotic resistance Start this free course now. Free course Understanding antibiotic resistance. Figure 10 Arrangement of the cell wall in a Gram-positive and b Gram-negative bacteria.