Guide The Bacterial Cell Wall

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In almost all cells this structure is a phospholipid bilayer that surrounds and contains the cytoplasm. In addition to lipid components biological membranes are composed of proteins; the proteins are what make each membrane unique. Despite its obvious importance, membranes and their associated functions remained poorly understood until the s. Before this, many viewed the membrane as a semipermeable bag. The bacterial cell envelope, i. Unlike cells of higher organisms, the bacterium is faced with an unpredictable, dilute and often hostile environment. To survive, bacteria have evolved a sophisticated and complex cell envelope that protects them, but allows selective passage of nutrients from the outside and waste products from the inside.

The following discussion concerns the organization, composition, and the functions of the various layers and compartments that make up this remarkable cellular structure. It is easily appreciated that a living system cannot do what it does without the ability to establish separate compartments in which components are segregated.

Specialized functions occur within different compartments because the types of molecules within the compartment can be restricted. However, membranes do not simply serve to segregate different types of molecules. They also function as surfaces on which reactions can occur. Recent advances in microscopy, which are discussed in other articles on this subject, have revealed strikingly nonrandom localization of envelope components.

Here, we will highlight recent advances in our understanding of how these extracellular organelles are assembled. More than years ago Christian Gram developed a staining procedure that allowed him to classify nearly all bacteria into two large groups, and this eponymous stain is still in widespread use. One group of bacteria retain Christian's stain, Gram-positive, and the other do not, Gram-negative. The basis for the Gram stain lies in fundamental structural differences in the cell envelope of these two groups of bacteria.

For our discussion of the Gram-negative bacterial cell envelope we will use Escherichia coli , an extensively-studied organism that has served as a model for understanding a number of fundamental biological processes. In comparing Gram-negative and Gram-positive cell envelopes we will use Staphylococcus aureus as a reference point but will highlight specific differences between it and Bacillus subtilis. Care should be taken in generalizing from examples drawn from particular microorganisms.

This need not be a pressing issue for other Gram-negative bacteria, and their envelopes may differ in species- and environmentally specific ways. Nonetheless, the ability to use the Gram stain to categorize bacteria suggests that the basic organizational principles we present are conserved. In addition, many bacteria express an outermost coat, the S-layer, which is composed of a single protein that totally encases the organism. S-layers and capsules, which are coats composed of polysaccharides, are beyond the scope of this review.

After more than a decade of controversy, techniques of electron microscopy were improved to the point in which they finally revealed a clearly layered structure of the Gram-negative cell envelope Fig. There are three principal layers in the envelope; the outer membrane OM , the peptidoglycan cell wall, and the cytoplasmic or inner membrane IM.

Bacteria: Cell Walls | Microbiology

The two concentric membrane layers delimit an aqueous cellular compartment that Peter Mitchell first termed the periplasm. During a similar time frame biochemical methods were developed to isolate and characterize the distinct set of proteins found in the periplasm Heppel, , and to characterize the composition of both the inner and outer membranes Miura and Mizushima, ; Osborn et al.

Studies since then have only reinforced their basic conclusions. Transenvelope machines in the Gram-negative cell envelope. The flagellar basal body hook structure connects the motor to the flagella DePamphilis and Adler, Distances shown provide a reasonable estimate of the size of the cellular compartments shown.

PE, periplasm; CYT, cytoplasm. Starting from the outside and proceeding inward the first layer encountered is the OM. The OM is a distinguishing feature of Gram-negative bacteria; Gram-positive bacteria lack this organelle. Like other biological membranes, the OM is a lipid bilayer, but importantly, it is not a phospholipid bilayer. The OM does contain phospholipids; they are confined to the inner leaflet of this membrane. LPS is an infamous molecule because it is responsible for the endotoxic shock associated with the septicemia caused by Gram-negative organisms Raetz and Whitfield The human innate immune system is sensitized to this molecule because it is a sure indicator of infection.

Lipoproteins contain lipid moieties that are attached to an amino-terminal cysteine residue Sankaran and Wu It is generally thought that these lipid moieties embed lipoproteins in the inner leaflet of the OM.

In other words, these proteins are not thought to be transmembrane proteins. There are about OM lipoproteins in E. Not surprisingly, some of these OMPs, such as the porins, OmpF, and OmpC, function to allow the passive diffusion of small molecules such as mono- and disaccharides and amino acids across the OM. When induced by the presence of maltose or phosphate starvation, respectively, these proteins are very abundant as well.

1. Overview

OmpA is another abundant OMP. It is monomeric, and it is unusual in that it can exist in two different conformations Arora et al. A minor form of the protein, with an unknown number of transmembrane strands, can function as a porin, but the major, nonporin form has only eight transmembrane strands, and the periplasmic domain of this form performs a largely structural role see later discussion.

The OM is essential for the survival of E. For example, there is a phospholipase PldA Snijder et al. The active site of all of these enzymes is located in the outer leaflet, or it faces the exterior of the cell OmpT. Mutants lacking any of these enzymes exhibit no striking phenotypes. The only known function of the OM is to serve as a protective barrier, and it is not immediately obvious why this organelle is essential.

But what a barrier it is. Salmonella , another enteric bacterium, can live at the site of bile salt production in the gall bladder Sinnott and Teall, , and it is generally true that Gram-negative bacteria are more resistant to antibiotics than are their Gram-positive cousins. Indeed, some Gram-negative bacteria, such as Pseudomonas , are notorious in this regard. LPS plays a critical role in the barrier function of the OM. It is a glucosamine disaccharide with six or seven acyl chains, a polysaccharide core, and an extended polysaccharide chain that is called the O-antigen Raetz and Whitfield The acyl chains are largely saturated, and this facilitates tight packing.

The nonfluid continuum formed by the LPS molecules is a very effective barrier for hydrophobic molecules. This coupled with the fact that the porins limit diffusion of hydrophilic molecules larger than about Daltons, make the OM a very effective yet, selective permeability barrier Nikaido Bacteria do not lyse when put into distilled water because they have a rigid exoskeleton. Peptidoglycan is made up of repeating units of the disaccharide N-acetyl glucosamine-N-actyl muramic acid, which are cross-linked by pentapeptide side chains Vollmer et al.

The peptidoglycan sacculus is one very large polymer that can be isolated and viewed in a light microscope. Because of its rigidity, it determines cell shape.

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The enterics are rod shaped, but cell shapes can vary. For example, vibrios and caulobacters are comma shaped. Recent results suggest that the glycan chains run perpendicular to the long axis of a rod shaped cell, i. Agents such as enzymes or antibiotics that damage the peptidoglycan cause cell lysis owing to the turgor pressure of the cytoplasm. Lysis can be prevented in media of high osmolarity. However, without the peptidoglycan, cells lose their characteristic shape. The resulting cells are called spheroplasts. However, special methods can be used to produce L forms, which are spherical in shape and can be propagated on high osmolarity media Joseleau-Petit et al.

The OM is basically stapled to the underlying peptidoglycan by a lipoprotein called Lpp, murein lipoprotein, or Braun's lipoprotein Braun, The lipids attached to the amino terminus of this small protein 58 amino acids embed it in the OM. Lpp is the most abundant protein in E.

In addition, proteins such as OmpA bind peptidoglycan noncovalently.

Bacterial cell wall composition and the influence of antibiotics by cell-wall and whole-cell NMR

The OM and IM delimit an aqueous cellular compartment called the periplasm. The periplasm is densely packed with proteins and it is more viscous than the cytoplasm Mullineax et al. Cellular compartmentalization allows Gram-negative bacteria to sequester potentially harmful degradative enzymes such as RNAse or alkaline phosphatase.

Because of this, the periplasm has been called an evolutionary precursor of the lysosomes of eukaryotic cells De Duve and Wattiaux, Other proteins that inhabit this compartment include the periplasmic binding proteins, which function in sugar and amino acid transport and chemotaxis, and chaperone-like molecules that function in envelope biogenesis Ehrmann, ; see later discussion. One of the hallmarks of eukaryotic cells is the presence of intracellular organelles. These organelles are defined by limiting membranes, and these organelles perform a number of essential cellular processes.

The mitochondria produce energy, the smooth endoplasmic reticulum ER synthesize lipids, protein secretion occurs in the rough ER, and the cytoplasmic membrane contains the receptors that sense the environment and the transport systems for nutrients and waste products. Bacteria lack intracellular organelles, and consequently, all of the membrane-associated functions of all of the eukaryotic organelles are performed in the IM. Many of the membrane proteins that function in energy production, lipid biosynthesis, protein secretion, and transport are conserved in bacteria, but their cellular location is different.

In bacteria, these proteins are located in the IM. The IM is a phospholipid bilayer. Other minor lipids include polyisoprenoid carriers C55 , which function in the translocation of activated sugar intermediates that are required for envelope biogenesis Raetz and Dowhan Certain types of surface appendages such as flagella DePamphilis and Adler ; Macnab , which are required for bacteria motility; Type III secretion systems Kubori et al. The structures of some of these machines are known at sufficient resolution to provide meaningful insight into the size of the various cellular compartments in E.

However, it should be noted that experimental measurements of the volume of the periplasm, for example, vary widely Stock et al. All of the components of the Gram-negative cell envelope are synthesized either in the cytoplasm or at the inner surface of the IM. Accordingly, all of these components must be translocated from the cytoplasm or flipped across the IM. Periplasmic components must be released from the IM, peptidoglycan components must be released and polymerized, and OM components must be transported across the aqueous, viscous periplasm and assembled into an asymmetric lipid bilayer.

All of this construction takes place outside of the cell in a potentially hostile environment that lacks an obvious energy source. It seems clear that there is no ATP out there for example. In this section we will summarize what is currently known about the assembly of the major envelope components; proteins, including lipoproteins, LPS, and phospholipids. All proteins, of course, are synthesized in the cytoplasm.

Proteins destined for the periplasm or the OM are made initially in precursor form with a signal sequence at the amino terminus. The signal sequence targets them for translocation from the cytoplasm Driessen and Nouwen The signal sequence and this heterotrimeric membrane protein complex are conserved throughout biology Rapoport Periplasmic and OM proteins are generally translocated in post-translational fashion, i.

Proteins must be secreted in linear fashion from the amino to the carboxy terminus like spaghetti through a hole; SecYEG cannot handle folded molecules. The cytoplasmic SecB chaperone maintains these secreted proteins in unfolded form until they can be secreted Randall and Hardy During the secretion process the signal sequence is proteolytically removed by Signal Peptidase I Paetzel et al. Other components of the Sec translocon, such SecD, SecF, and YajC, perform important but nonessential function s during translocation, perhaps facilitating release of secreted proteins into the periplasm.

Once released, periplasmic proteins are home, but it seems likely that chaperones function to prevent misfolding and aggregation. For example, the periplasmic protein MalS, which contains disulfide bonds, requires the periplasmic disulfide oxidase DsbA for proper folding. Periplasmic chaperones function to protect OMPs during their transit through the periplasm. Three such proteins have been well characterized and shown have general chaperone activity: Mutants lacking either one of these pathways are viable, but cells cannot tolerate loss of both Rizzitello et al.

By definition then, these chaperone pathways are redundant. However, this redundancy does not reflect equal roles in OMP assembly.

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  • It may be that many minor OMPs show no pathway preference. It is also possible that other periplasmic proteins have chaperone function that is important for the assembly of a subset of OMPs. The structure of a large fraction of the BamA periplasmic domain has been determined Kim et al. Each of the four visible POTRA domains has a nearly identical fold, despite the fact that the amino acid sequence identity between them is very low.

    There are homologs of BamA in both mitochondria and chloroplasts Moslavac et al. BamD is the only essential lipoprotein in the Bam complex Malinverni et al. The remaining three lipoproteins are not essential, and they are conserved to varying degrees. The cellular machineries required for OM biogenesis. The signal sequence directs these precursors to the Sec machinery for translocation from the cytoplasm. For OM lipoproteins, after the signal sequence is removed and lipids are attached to the amino-terminal cysteine residue, the Lol machinery delivers them to the OM.

    Lipoproteins are made initially with an amino-terminal signal sequence as well, and they too are translocated by the Sec machinery. However, the signal sequence is removed by a different signal peptidase, signal peptidase II Paetzel et al. Signal sequence processing of lipoproteins requires the formation of a thioether diglyceride at the cysteine residue, which will become the amino terminus of the mature lipoprotein.

    Once the signal sequence is removed, an additional fatty acyl chain is added to the cysteine amino group Sankaran and Wu These lipid moieties tether the newly formed lipoprotein to the outer leaflet of the IM. Some lipoproteins remain in the IM, and their biogenesis is complete after signal sequence processing and lipid addition. However, most of the lipoproteins in E. The Lol system, which transports lipoproteins to the OM has been well characterized Fig. The most common Lol avoidance signal is an aspartate residue at position two of the mature lipoprotein.

    There is a second protein translocation system in the IM called Tat that translocates folded proteins Sargent et al. Other bacteria, such as thermophiles, use the Tat system extensively; presumable because it is easier to fold proteins in the cytoplasm than it is in the hostile environments they live in. In terms of components, the Tat system is remarkable simple; three components. TatB and TatC function to target proteins for translocation by TatA, but how this system recognizes that the substrate is folded, and how it accomplishes the translocation reaction are not yet understood.

    Proteins destined for the IM are handled by the Sec machinery as well. Presumably, posttranslational translocation of these hydrophobic substrates would be inefficient and perhaps dangerous, owing to their great potential for aggregation. Prokaryotic SRP is much simpler than its eukaryotic counterpart. The first trans-membrane segment functions as a signal sequence to initiate translocation of the sequences that follow it. These transmembrane sequences tend to be longer and more hydrophobic than typical signal sequences, and this serves as the basis for SRP recognition Hegde and Bernstein This signal sequence is not cleaved; it remains attached serving now as a typical transmembrane helix.

    The second transmembrane helix functions to stop the translocation reaction and this helix exits the SecYEG translocator laterally where it remains in the IM Van den Berg et al. The third transmembrane helix functions again as an uncleaved signal sequence. These alternating start and stop translocation signals stitch IM proteins into the membrane in stepwise fashion. Small IM proteins, especially those with small periplasmic domains, can be inserted into the membrane by a second IM translocase called YidC. YidC family members can be found in mitochondria and chloroplasts.

    Like their mitochondrial homologs, YidC plays an important role in the assembly of energy-transducing membrane proteins such as subunit c of ATPase. LPS, including the core polysaccharide, and the O-antigen are both synthesized on the inner leaflet of the IM. O-antigen is synthesized on a polyisoprenoid carrier, which then flips it to the outer leaflet. Note that the common laboratory strain E. In the last several years a combination of genetics, biochemistry, and bioinformatics was employed to identify seven essential proteins that are required to transport LPS from the outer leaflet of the OM to the cell surface Fig.

    LptA is made with a cleavable signal sequence and resides in the periplasm. An alternative model proposes that all seven proteins together form a transenvelope machine that transports LPS directly from the IM to the cell surface in analogy with efflux pumps. What is clear is that if any of the seven proteins are removed, LPS accumulates in the outer leaflet of the IM Sperandeo et al.

    MsbA can flip these molecules to the outer leaflet of the IM Doerrler et al. How phospholipids reach the OM is not known. This is true even for lipids like cholesterol which are not naturally found in bacteria. This could suggest sites of IM-OM fusion, or hemi-fusion, that allow intermembrane phospholipid trafficking by diffusion, a hypothesis made by Manfred Bayer Bayer long ago that has remained highly controversial. The Gram-positive cell envelope differs in several key ways from its Gram-negative counterpart Fig. First and foremost, the outer membrane is absent.

    The outer membrane plays a major role in protecting Gram-negative organisms from the environment by excluding toxic molecules and providing an additional stabilizing layer around the cell. Because the outer membrane indirectly helps stabilize the inner membrane, the peptidoglycan mesh surrounding Gram-negative cells is relatively thin. Gram-positive bacteria often live in harsh environments just as E. To withstand the turgor pressure exerted on the plasma membrane, Gram-positive microorganisms are surrounded by layers of peptidoglycan many times thicker than is found in E.

    Threading through these layers of peptidoglycan are long anionic polymers, called teichoic acids, which are composed largely of glycerol phosphate, glucosyl phosphate, or ribitol phosphate repeats. One class of these polymers, the wall teichoic acids, are covalently attached to peptidoglycan; another class, the lipoteichoic acids, are anchored to the head groups of membrane lipids Neuhaus In addition to the TAs, the surfaces of Gram-positive microorganisms are decorated with a variety of proteins, some of which are analogous to proteins found in the periplasm of Gram-negative organisms Dramsi et al.

    Because there is no outer membrane in Gram-positive organisms to contain extracellular proteins, all these proteins feature elements that retain them in or near the membrane. Some contain membrane-spanning helices and some are attached to lipid anchors inserted in the membrane. Others are covalently attached to or associated tightly with peptidoglycan Scott and Barnett Still others bind to teichoic acids. The major structural elements of Gram-positive cell walls, excluding capsules, will be described below.

    Depiction of Gram-positive and Gram-negative cell envelopes: The chemical structure of peptidoglycan in Gram-positive organisms is similar to that in Gram-negatives in that it is composed of a disaccharide-peptide repeat coupled through glycosidic bonds to form linear glycan strands, which are crosslinked into a meshlike framework through the peptide stems attached to the disaccharide repeat.

    The major difference between Gram-positive and Gram-negative peptidoglycan involves the thickness of the layers surrounding the plasma membrane. Whereas Gram-negative peptidoglycan is only a few nanometers thick, representing one to a few layers, Gram-positive peptidoglycan is 30— nm thick and contains many layers. There are many differences among Gram-positive organisms with respect to the details of peptidoglycan structure, but perhaps the most notable difference relates to the peptide crosslinks between glycan strands Vollmer ; Vollmer et al.

    Bacteria: Cell Walls

    This pentaglycine branch is assembled by a set of nonribosomal peptidyl transferases known as FemA, B, and X Ton-That et al. Staphylococci can tolerate, albeit with difficulty, the loss of FemA or B, but not of FemX, which attaches the first glycine unit to the stem peptide Hegde and Shrader ; Hubscher et al. Many Gram-positive organisms contain branched stem peptides, but B. Branched stem peptides in S. Chief among these roles, they serve as attachment sites for covalently-associated proteins discussed in more detail later. They have also been implicated in resistance to beta lactam antibiotics Chambers Beta lactams inactivate transpeptidases that catalyze the peptide crosslinking step of peptidoglycan synthesis by reacting with the active site nucleophile of transpeptidases.

    Transpeptidases that couple branched stem peptides are mechanistically similar to those that couple unbranched stem peptides; however, their substrate specificity is sufficiently different that they only recognize unbranched stem peptides, and some of them are resistant to beta lactams Rohrer and Berger-Bachi ; Pratt ; Sauvage et al. For example, methicillin-resistant S.

    Many other Gram-positive organisms are also thought to harbor low affinity PBPs that preferentially recognize and couple branched stem peptides. It is thus speculated that the evolution of branched peptides in the peptidoglycan biosynthetic pathway may be an adaptation that enables escape from beta lactams.

    As described in the following section, however, branched stem peptides also play other important roles. Breaches in the epithelium occasionally result in invasive S. The ability to adhere to host tissue is a crucial first step in effective colonization by S. These factors include teichoic acids, which are discussed in the following section, as well as surface proteins that recognize components of host extracellular matrix such as fibronectin, fibrinogen, and elastin Clarke and Foster Some of these surface proteins, called adhesins, are attached via noncovalent ionic interactions to peptidoglycan or teichoic acids, but many are attached covalently to stem peptides within the peptidoglycan layers Dramsi et al.

    If we wash away everything else DNA, proteins, membranes, etc. Peptidoglycan assembly is the target of many of our best antibiotics - penicillins block cross-linking of sugar chains by binding to enzyme catalysts, resulting in the death of bacteria. Recent advances in AFM technology, allowing increased force sensitivity through the use of smaller probes, let us visualise the material in sufficient detail to resolve individual chains of sugars in the wall. Seeing these after about a decade of looking at peptidoglycan with various AFMs was a pretty good day on the microscope!

    We found that the chains were oriented in roughly the same direction around the circumference of the rod-shaped cells, and that this orientation was maintained at the cell poles which are roughly hemispherical. We often hypothesise or assume that different shaped bacteria require different peptidoglycan polysaccharide chain organisation, and we finally had a tool to investigate this. We decided to artificially change the shape of a rod-shaped species, E. Our AFM images of pure peptidoglycan from such cells revealed that they had lost their orientational order - the polysaccharide chains were essentially oriented at random.

    This was an exciting result as it forged a new connection between the nanometre scale organisation of the peptidoglycan polymer and the much larger micrometre scale of cell shape. It seems likely that circumferentially oriented chains are required for rod shape, and we can now being to explore this.

    A long term goal of our research is to experimentally determine how the structural organisation of peptidoglycan results in its mechanical properties how stretchy it is, or how much force is needed to break it and thus the mechanical properties of the cell. This should enable a better understanding of how the material performs its functions of resisting force and maintaining cell shape.

    We also now have an assay for changes to peptidoglycan structure on a polysaccharide chain level, so we can ask how a particular mutation, chemical inhibitor or antibiotic alters this for the first time. This will provide new insights into structural changes caused by antibiotics like penicillin and how these might contribute to bacterial death. And while this research might not lead to new therapies in the short term, fundamental insights like these are vital for a long-term sustainable response to the problems of antimicrobial resistance.

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