Bacterial Cell Walls PDF

The classification system most widely accepted by microbiologists is that of Bergey’s Manual of Systematics of Archaea and Bacteria, a major taxonomic treatment of Bacteria and Archaea that has served microbiologists since 1923 as a compendium of information on all recognized prokaryotic species. A...

The classification system most widely accepted by microbiologists is that of Bergey’s Manual of Systematics of Archaea and Bacteria, a major taxonomic treatment of Bacteria and Archaea that has served microbiologists since 1923 as a compendium of information on all recognized prokaryotic species. A second major source describing the physiology, ecology, phylogeny, enrichment, isolation, and cultivation of Bacteria and Archaea is The Prokaryotes. Bergey’s Manual and The Prokaryotes offer microbiologists both the concepts and the details of the biology of Bacteria and Archaea as we know them today and are the primary resources for microbiologists characterizing newly isolated organisms.  The Cell Wall The cytoplasm of prokaryotic cells maintains a high concentration of dissolved solutes that creates significant osmotic pressure—about 2 atm (203 kPa); this is about the same as the pressure in an automobile tire. This osmotic pressure is sufficient to cause the cell membrane to burst and the cell to die—a process called cell lysis. To withstand this turgor pressure, the cell envelopes of most Bacteria and Archaea have a layer outside the cytoplasmic membrane called the cell wall. Besides protecting against osmotic lysis, cell walls also maintain cell shape and rigidity. The cell envelopes of most Bacteria can be classified as being either gram- positive or gram-negative based on their organization and cell wall structures. The structures of gram-positive and gram-negative cell envelopes differ markedly as viewed in the electron microscope. The cell envelope of a gram-positive cell typically contains a cytoplasmic membrane and a thick cell wall, whereas a gram-negative cell has a cytoplasmic membrane, a thin cell wall, an outer membrane, and a periplasm, which is a compartment between the cytoplasmic and outer membranes.  Cell envelopes of Bacteria (a, b) Schematic diagrams of gram-positive and gram-negative cell envelopes. The photo of Gram- stained bacteria in the center shows cells of Staphylococcus aureus (purple, gram-positive) and Escherichia coli (pink, gram-negative). (c, d) Transmission electron micrographs showing the cell wall of a gram-positive bacterium and a gram- negative bacterium, respectively. (e, f) Scanning electron micrographs of gram-positive and gram-negative bacteria, respectively. Each cell is about 1 mm wide.  The names we use to describe the typical gram- positive and gram-negative cell envelopes are based on their Gram stain reactions. The Gram stain reaction is determined primarily by the thickness of the cell wall rather than the number of layers in the cell envelope, and so Gram stain reaction does not always correlate with cell envelope structure. However, the Gram stain reaction is sufficiently predictive of cell envelope structure in Bacteria that the names of the two most common bacterial cell envelopes—gram positive and gram- negative—are based on their typical reactions to the Gram stain.  Knowledge of cell wall envelope structure and function is important not only for understanding the biology of microbial cells but also for medical reasons. Certain antibiotics, for example, the penicillins and cephalosporins, target bacterial cell wall synthesis, leaving the cell susceptible to osmotic lysis. Since human and other animal cells lack cell walls and are therefore not a target of such antibiotics, these drugs are of obvious benefit for treating bacterial infections. The major component of the bacterial cell wall, and a target of many antibiotics, is a molecule called peptidoglycan.  Bacterial Cell Walls The cell walls found in Bacteria contain a rigid polysaccharide called peptidoglycan that confers structural strength on the cell. Peptidoglycan is found in all Bacteria that contain a cell wall, but it is unique to Bacteria and is not found in Archaea or Eukarya. The sugar backbone of peptidoglycan is composed of alternating repeats of two modified glucose residues called N-acetylglucosamine and N- acetylmuramic acid joined by a b-1,4 linkage. Attached to the latter residue is a short peptide side chain. The amino acid composition of this peptide side chain can vary considerably between bacterial species. In Escherichia coli this peptide contains the amino acids l-alanine, d- alanine, d-glutamic acid, and diaminopimelic acid (DAP), though in other bacteria, l-lysine can be substituted for DAP. The presence of d stereoisomer amino acids, d-alanine and d-glutamic acid, is an unusual feature of peptidoglycan since proteins are always constructed of l-amino acids. These constituents are connected in an ordered way to form the glycan tetrapeptide, and long chains of this basic unit form peptidoglycan.  Structure of the repeating unit in peptidoglycan, the glycan tetrapeptide The structure given is that for the peptidoglycan of Escherichia coli and most other gram-negative Bacteria. Cross-links can be formed between adjacent peptide side chains at residues having free amino and carboxyl groups (circled in blue). For example, cross-links in E. coli most commonly occur between the amino group of diaminopimelic acid on one peptide and the terminal carboxyl group of d-alanine on a different peptide  Strands of peptidoglycan run parallel to each other around the circumference of the cell. The peptide side chains of adjacent peptidoglycan strands are cross-linked together by covalent peptide bonds, and in this way, the peptidoglycan forms one single enormous molecule. In gram-negative bacteria, the crosslinks form primarily between the amino group of DAP on one glycan strand and the carboxyl group of the terminal d-alanine on the adjacent glycan strand. The cell wall in the gram-negative cell envelope is 2–7 nm thick consisting primarily of a single layer of peptidoglycan, though it can be up to three layers thick in some places. The peptidoglycan mesh so formed is flexible and porous, but strong enough to resist turgor pressure and prevent rupture of the cytoplasmic membrane and cell lysis.  Peptidoglycan structure in the cell wall (a) Gram-negative cells that have thin cell walls, such as the cell wall of E. coli, mostly have direct cross-links between peptide side chains. (b) Gram-positive cells that have thick cell walls, such as S. aureus, can also have peptide interbridges that extend between cross-linked peptide side chains. (c) Conformation of peptidoglycan in the gram-negative cell wall. G, N-acetylglucosamine; M, N-acetylmuramic acid. Note how glycosidic bonds confer strength on peptidoglycan around the circumference of the cell whereas peptide bonds confer strength along the axis of the cell.  The typical bacterial gram-positive cell envelope contains a thick peptidoglycan cell wall, which can measure 20 to 35 nm in thickness and is usually much thicker than the wall of gram-negative organisms. As much as 90% of the gram-positive cell envelope can consist of peptidoglycan. Whereas the gram-negative cell wall typically contains only a single layer of peptidoglycan, the gram-positive cell wall can be 15 or more layers thick. The peptidoglycan of the gram-positive cell wall is stabilized three-dimensionally by peptide cross-links, which form between adjacent peptidoglycan strands both horizontally and vertically. In gram-positive bacteria, peptide cross-links often contain a short peptide “interbridge,” the kinds and numbers of amino acids in the interbridge varying between species. In the gram-positive bacterium Staphylococcus aureus, for example, the interbridge often consists of five glycines.  In addition to peptidoglycan, many gram-positive bacteria produce acidic molecules called teichoic acids embedded in their cell wall. Teichoic acids are composed of glycerol phosphate or ribitol phosphate with attached molecules of glucose or d-alanine (or both). Individual alcohol molecules are then connected through their phosphate groups to form long strands, and these are then covalently linked to peptidoglycan. Some teichoic acids are covalently bonded to membrane lipids rather than to peptidoglycan, and these are called lipoteichoic acids. Peptidoglycan can be destroyed by lysozyme, an enzyme that cleaves the glycosidic bond between N-acetylglucosamine and N-acetylmuramic acid. This weakens the peptidoglycan and can cause cell lysis. Lysozyme is present in human secretions including tears, saliva, and other bodily fluids, and functions as a major line of defense against bacterial infection. Many antibiotics, including penicillin, also target peptidoglycan. Whereas lysozyme destroys preexisting peptidoglycan, penicillin blocks the formation of peptide cross-links, which compromises the strength of the peptidoglycan, leading to cell lysis. Phospholipid bilayer membrane  Phospholipid bilayer membrane (a) Transmission electron micrograph of a cell with membrane region shown in detail. (b) General architecture of a bilayer membrane; phospholipids are composed of a hydrophilic head group (blue spheres) with fatty acid tails (yellow lines). The phospholipid head groups of the bilayer are visible in a as parallel dark lines, between which can be found a lighter region comprising the hydrophobic region of the membrane. (c) Structure of the phospholipid phosphatidylethanolamine. Each fatty acid side chain is connected to the head group by an ester bond (boxed with a red dashed line); ester linkages are a characteristic feature of lipids from Bacteria and Eukarya but not those of Archaea. Structure of the cytoplasmic membrane  Structure of the cytoplasmic membrane The inner surface (In) faces the cytoplasm and the outer surface (Out) faces the environment. Phospholipids compose the matrix of the cytoplasmic membrane with proteins embedded (integral) or surface associated (peripheral). The general design of the cytoplasmic membrane is similar in both prokaryotic and eukaryotic cells, although there can be Differences in the chemistry between different species. Note that this membrane is shown in a relaxed shape to better illustrate its inner and outer surfaces; in a living cell, cytoplasmic turgor pressure would cause the membrane to have convex curvature.  Structure of the gram- positive bacterial cell wall (a) Schematic of a gram-positive cell wall showing the internal architecture of the peptidoglycan and its relationship to teichoic acids. Peptide cross-links form between peptidoglycan strands that are adjacent both horizontally and vertically, and peptidoglycan can also form covalent bonds to teichoic acids. (b) Structure of a ribitol teichoic acid. The teichoic acid is a polymer of the repeating ribitol unit shown here. (c) Summary diagram of the gram- positive bacterial cell wall. Lipoteichoic acids tether the cell wall to the cell membrane.  LPS: The Outer Membrane Most of the gram-negative cell envelope is composed of the outer membrane. The outer membrane is a second lipid bilayer found external to the cell wall, but its structure and function differs from that of the cytoplasmic membrane. The outer membrane and cytoplasmic membrane are similar in that they both contain phospholipid and protein, but a major difference is that the outer membrane also contains polysaccharide molecules covalently bound to lipids. Hence, the outer membrane is often called the lipopolysaccharide layer, or simply LPS for short. LPS molecules have several unique functions: They can facilitate surface recognition, they are important virulence factors for some bacterial pathogens, and they contribute to the mechanical strength of the cell. Major difference between the cytoplasmic and outer membranes is that the outer membrane contains porins, which are transmembrane proteins that allow for the nonspecific transport of solutes. Hence, we will see that the outer membrane is far more permeable than is the cytoplasmic membrane.  The gram-negative bacterial cell envelope (a) Arrangement of lipopolysaccharide, lipid A, phospholipid, porins, and Braun lipoprotein in the outer membrane. (b) Transmission electron micrograph of a cell of Escherichia coli showing the cytoplasmic membrane and wall. (c) Molecular model of porin proteins showing their hollow pores that allow solute transport across the outer membrane. The view of the porin is perpendicular to the plane of the membrane. The black dashed circle highlights some of the hydrophilic amino acids that line the inside of the pore.  Structure of bacterial lipopolysaccharide The chemical structures of lipid A and polysaccharides can vary among gram negative Bacteria, but the major components (lipid A– KDO–core–O-specific) are typically invariant. The O-specific polysaccharide is highly variable among species. KDO, ketodeoxyoctonate; Hep, heptose; Glu, glucose; Gal, galactose; GluNac, N-acetylglucosamine; GlcN, glucosamine; P, phosphate. Glucosamine and the lipid A fatty acids are linked through the amine groups of GlcN. The lipid A portion of LPS can be toxic to animals and comprises the endotoxin complex.  An important biological activity of LPS is its toxicity to animals. Common gram-negative pathogens for humans include species of Salmonella, Shigella, and Escherichia, among many others, and some of the gastrointestinal symptoms these pathogens elicit are due to their toxic outer membrane components. Toxicity is specifically linked to the LPS layer, in particular, to lipid A. The term endotoxin refers to this toxic component of LPS. Some endotoxins cause violent symptoms in humans, including gas, diarrhea, and vomiting, and the endotoxins produced by Salmonella and enteropathogenic strains of Escherichia coli transmitted in contaminated foods are classic examples of this.  Archaeal Cell Walls The cell envelopes of Archaea differ in fundamental ways from those of Bacteria. We have already learned that the cytoplasmic membranes of Archaea, while functionally analogous to those of Bacteria, differ in chemical structure. Another major difference is that Archaea lack peptidoglycan. In addition, Archaea typically lack an outer membrane. One consequence of these differences is that the Gram stain reaction is not very useful for predicting the structures of archaeal cell envelopes and so we typically do not use the terms gram-positive and gram-negative to describe cells of Archaea. Most Archaea lack a polysaccharide-containing cell wall and instead have an S-layer, which is a rigid protein shell that functions to prevent osmotic lysis just as does the bacterial cell wall. While some Archaea do have cell walls, these walls have unique chemical structures not found in Bacteria. For example, the cell walls of certain methane-producing Archaea (methanogens) contain a polysaccharide called pseudomurein, which is structurally remarkably similar to peptidoglycan (the term murein is from the Latin word for “wall” and was an old term for peptidoglycan).  The backbone of pseudomurein is formed from alternating repeats of N-acetylglucosamine (also present in peptidoglycan) and N-acetyltalosaminuronic acid; the latter replaces the N-acetylmuramic acid of peptidoglycan. Pseudomurein also differs from peptidoglycan in that the glycosidic bonds between the sugar derivatives are b-1,3 instead of b-1,4, and the amino acids are all of the l stereoisomer. Because in many respects they are so similar, it is likely that peptidoglycan and pseudomurein are variants of a cell wall polysaccharide originally present in the common ancestor of Bacteria and Archaea. However, although they are structurally and functionally very similar, they differ sufficiently that pseudomurein is immune from destruction by both lysozyme and penicillin, molecules that destroy peptidoglycan.  Pseudomurein Structure of pseudomurein, the cell wall polymer of Methanobacterium species. Note the similarities and differences between pseudomurein and peptidoglycan Major lipids of Archaea and the architecture of archaeal membranes  Major lipids of Archaea and the architecture of archaeal membranes (a, b) Archaea can have lipid bilayers composed of phosphoglycerol diether lipids. The hydrophobic portions of archaeal lipids are comprised of isoprenoid chains synthesized from repeated units of isoprene (in dashed red ovals); this contrasts with the lipids of Bacteria and Eukarya, which have fatty acid tails (Figure 2.1). Note that these isoprenoids are bonded to glycerol by an ether linkage (in dashed red box). (b, c) Some Archaea can also have lipid monolayers composed of diphosphoglycerol tetraether lipids or other isoprenoid lipids such as crenarchaeol. The isoprenoid lipids in b are phytanyl (C20), biphytanyl (C40), and crenarchaeol. Isoprene lipids can often contain 5- and 6-carbon rings such as those present in crenarchaeol. The membrane structure in Archaea may form a lipid bilayer or a lipid monolayer (or a mix of both). Major functions of the cytoplasmic membrane Although physically weak, the cytoplasmic membrane controls at least three critically important cellular functions: maintaining selective permeability, anchoring proteins, and conserving energy.  Alternative cell envelope structures Cell envelope structures including cytoplasmic membranes (CM), cell walls (CW), outer membranes (OM), and S-layers (SL) can be found in both bacterial and archaeal species. (a) Vibrio cholerae has a classic gram-negative type bacterial cell envelope. (b) Caulobacter crescentus is a bacterium with a gram-negative envelope and an S-layer. (c) Nitrosopumilus maritimus has a typical archaeal cell envelope containing a CM and an SL. (d) Mycoplasma pneumoniae is a pathogenic bacterium whose cell envelope consists of only a CM. S-layers, while typically composed of a paracrystalline protein or glycoprotein layer can vary considerably in molecular structure (compare the S-layers in b and c).  Cell Surface Structures Many Bacteria and Archaea secrete sticky or slimy materials on their cell surface that consist of either polysaccharide or protein. However, these are distinct from and external to the cell envelope. The terms “capsule” and “slime layer” are used to describe these layers. These outer layers can mediate attachment, they can protect the cell from attack and from environmental stresses, and they can alter the diffusive environment of the cell.  Capsules and Slime Layers The terms capsule and slime layer are used to describe a sticky coat of polysaccharide formed outside of the cell envelope. If the polysaccharide layer is organized in a tight matrix that excludes small particles and is tightly attached to the cell, it is called a capsule. Capsules are readily visible by light microscopy if cells are treated with India ink, which contains particulates that stain the background but cannot penetrate the capsule; capsules can also be seen in the electron microscope. By contrast, if the surface layer is easily deformed and loosely attached, it will not exclude particles and is more difficult to see microscopically. Such a loosely attached polysaccharide coat is called a slime layer, and it is easily detected in colonies of slime-forming species such as the lactic acid bacterium Leuconostoc.  Outer surface layers have several functions. Surface polysaccharides assist in the attachment of microorganisms to solid surfaces. Pathogenic microorganisms that enter the body by specific routes usually do so by first binding to specific surface components of host tissues; this binding is often facilitated by bacterial cell surface polysaccharides. When the opportunity arises, many bacteria will bind to solid surfaces, often forming a thick layer of cells called a biofilm. Extracellular polysaccharides play a key role in the development and maintenance of biofilms as well. Besides attachment, outer surface layers have other functions. These include contributing to the infectivity of a bacterial pathogen and preventing dehydration. For example, the causative agents of the diseases anthrax and bacterial pneumonia—Bacillus anthracis and Streptococcus pneumoniae, respectively—each contain a thick capsule of either protein (B. anthracis) or polysaccharide (S. pneumoniae). Encapsulated cells of these bacteria avoid destruction by the host’s immune system because the immune cells that would otherwise recognize these pathogens as foreign and destroy them are blocked from doing so by the bacterial capsule. In addition to this role in disease, bacterial outer surface layers bind water, and this helps protect the cell from desiccation in periods of dryness.  Bacterial capsules and slime formation (a) A semisolid colony of the bacterium Leuconostoc mesenteroides (lifted up by an inoculating loop) contains a thick dextran (glucose polymer) slime layer formed by the cells. (b) Capsules of Acinetobacter species observed by phase-contrast microscopy after negative staining with India ink. India ink does not penetrate the capsule and so the capsule appears as a light area surrounding the cell, which appears black. (c) Transmission electron micrograph of a thin section of cells of Rhodobacter capsulatus with capsules (arrows) clearly evident; cells are about 0.9 mm wide. (d) Transmission electron micrograph of Rhizobium leguminosarum biovar trifolii stained with ruthenium red to reveal the capsule. The cell is about 0.7 mm wide.  Fimbriae, Pili, and Hami Pili are thin (2–10 nm in diameter) filamentous structures made of protein that extend from the surface of a cell and can have many functions. Short pili that mediate attachment are often called fimbriae. Pili enable bacterial cells to stick to surfaces, including animal tissues, or to form pellicles (thin sheets of cells on a liquid surface) or biofilms on solid surfaces. All gram-negative bacteria produce pili of one sort or another, and many gram-positive bacteria also contain these structures. Pili, by allowing bacteria to attach to other cells, often contribute to the virulence of pathogens. Pili can enable bacteria to adhere to surfaces and this function can allow pathogens to target and invade specific host tissues. However, pili are diverse and they can have several other important functions as well. For example, conjugative pili facilitate genetic exchange by causing cell-to-cell attachment during a process called conjugation. In addition, electrically conductive pili (also known as nanowires, can conduct electrons toward or away from the cell and in so doing play an important role in the energy metabolism of diverse microbes. Lastly, a type of pili called type IV pili not only facilitate adhesion but also support anunusual form of cell movement called twitching motility in certain bacterial species.  Fimbriae. Pili Electron micrograph of a The pilus on an Escherichia coli cell dividing cell of that is undergoing conjugation. Salmonella The cells are about 0.8 mm wide. enterica The visibility of the pilus in this (typhi ), showing electron micrograph has been flagella and improved because it is coated with fimbriae. A viral particles that bind to the pilin single cell is about 0.9 mm protein. wide.  Unique attachment structures in the SM1 group of Archaea : Hami (a) Cells of SM1 Archaea showing the pili-like surface structures called hami. (b) Transmission electron micrograph of isolated hami. A hamus “grappling hook” (labeled “Hook” in the micrograph) is about 60 nm in diameter. (c) A biofilm of SM1 cells showing the network of hami connecting individual cells.  Twitching motility allows cells to move along a solid surface. In twitching motility, pili are extended away from the cell, attach to a surface, and are subsequently retracted, dragging the cell forward. ATP supplies the energy necessary for extension and retraction of the pilus. Type IV pili assist in infectivity by certain pathogens, including the gram-negative bacteria Vibrio cholerae (cholera) and Neisseria gonorrhoeae (gonorrhea) and the gram positive bacterium Streptococcus pyogenes (strep throat and scarlet fever). The twitching motility of these organisms assists them in locating specific sites for attachment to initiate the disease process. Type IV pili are also widespread in the Archaea, functioning in surface adhesion and cell aggregation events that lead to biofilm formation. An unusual group of Archaea, the SM1 group, forms a unique attachment structure called a hamus (plural, hami) that resembles a tiny grappling hook. The SM1 group inhabits anoxic groundwater in Earth’s deep subsurface, and hami function to affix cells to a surface to form a networked biofilm. Hami structurally resemble type IV pili except for their barbed terminus, which functions to attach cells both to surfaces and to each other. The biofilms formed by SM1 Archaea are likely an ecological strategy that allows these microbes to more efficiently trap the scarce nutrients present in their deep subsurface habitat.