Cell Wall PDF

**[Cell Wall]** - The cell wall is the layer, usually fairly rigid, that lies just outside the plasma membrane. - Except for the mycoplasmas and some Archaea, most bacteria have strong walls that give them shape and protect them from osmosis; wall shape and strength is primarily du...

**[Cell Wall]** - The cell wall is the layer, usually fairly rigid, that lies just outside the plasma membrane. - Except for the mycoplasmas and some Archaea, most bacteria have strong walls that give them shape and protect them from osmosis; wall shape and strength is primarily due to peptidoglycan. - The cell walls of many pathogens have components that contribute to their pathogenicity (the property of causing disease). - The wall can protect a cell from toxic substances and is the site of action of several antibiotics. - After Christian Gram developed the Gram stain in 1884, it soon became evident that bacteria could be divided into two major groups based on their response to the Gram-stain procedure, Gram-positive bacteria stained purple, whereas gram-negative bacteria were colored pink or red by the technique. - The gram-positive cell wall consists of a single 20 to 80 nm thick homogeneous peptidoglycan or murein layer lying outside the plasma membrane. - In contrast, the gram-negative cell wall is quite complex. It has a 2 to 7 nm peptidoglycan layer surrounded by a 7 to 8 nm thick outer membrane. - Because of the thicker peptidoglycan layer, the walls of gram-positive cells are stronger than those of gram-negative bacteria. - Microbiologists often call all the structures from the plasma membrane outward the envelope or cell envelope. This includes the wall and structures like capsules when present. - A space seen between the plasma membrane and the outer membrane in electron micrographs of gram-negative bacteria, and sometimes a similar but smaller gap may be observed between the plasma membrane and wall in gram-positive bacteria. This space is called the **periplasmic space.** - The periplasmic space may be filled with a loose network of peptidoglycan. - Possibly it is more a gel than a fluid-filled space. - The substance that occupies the periplasmic space is the **periplasm**. - Gram-positive cells may have periplasm even if they lack a discrete, obvious periplasmic space. - Size estimates of the periplasmic space in gram-negative bacteria range from 1 nm to as great as 71 nm. - When cell walls are disrupted carefully or removed without disturbing the underlying plasma membrane, periplasmic enzymes and other proteins are released and may be easily studied. - The periplasmic space of gram-negative bacteria contains many proteins that participate in nutrient acquisition--- for example, hydrolytic enzymes attacking nucleic acids and phosphorylated molecules, and binding proteins involved in transport of materials into the cell. - The periplasmic space also contains enzymes involved in peptidoglycan synthesis and the modification of toxic compounds that could harm the cell. **[Exoenzymes ]** - Gram-positive bacteria may not have a visible periplasmic space and do not appear to have as many periplasmic proteins; rather, they secrete several enzymes that ordinarily would be periplasmic in gram-negative bacteria. Such secreted enzymes are often called exoenzymes. - Some enzymes remain in the periplasm and are attached to the plasma membrane. - The Archaea differ from other prokaryotes in many respects. - Although they may be either gram positive or gram negative, their cell walls are distinctive in structure and chemical composition. - The walls lack peptidoglycan and are composed of proteins, glycoproteins, or polysaccharides. Composition of Bacterial Cell Wall **[Peptidoglycan Structure]** - Peptidoglycan or murein is an enormous polymer composed of many identical subunits. - The presence of D-amino acids protects against attack by most peptidases (involved in the degradation of off-function proteins in lysosomes, cytosol, plasma membranes, or in extracellular space). - The peptidoglycan subunit present in most gram-negative bacteria and many gram-positive ones is shown in figure below,  Micro: 4 and 5 Bacteria Flashcards \| Quizlet - The backbone of this polymer is composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic (NAM) acid residues. A tetra-peptide chain of four alternating amino acids- L-alanine attached to NAM at position1, D-glutamic acid at position2, L-lysine (or meso-diaminopimelic acid in gram-negative cell wall) at position3, and D-alanine at position4. - NAM and NAG are amino sugars that has **β-1, 4-glycosidic bond** between them. - Chains of linked peptidoglycan subunits are joined by crosslinks between the peptides. - Often the carboxyl group of the terminal D-alanine is connected directly to the amino group of diaminopimelic acid, but a **peptide interbridge/cross bridge** may be used instead. - **Peptide interbridge/cross bridge** is usually made up of 5 glycine residues. - Most gram-negative cell wall peptidoglycan lacks the peptide interbridge. - In gram-positive cells, these cross bridges can occur between tetra-peptides of adjacent peptidoglycan chains or between tetra-peptides of the same chain, so that a rigid multi-layered sheet of peptidoglycan is formed. - This cross-linking results in an enormous peptidoglycan sac that is actually one dense, interconnected network. - These sacs have been isolated from gram-positive bacteria and are strong enough to retain their shape and integrity, yet they are elastic and somewhat stretchable, unlike cellulose. They also must be porous, as molecules can penetrate them. - Penicillin interferes with cell wall synthesis by preventing the formation of these cross bridges, resulting in the formation of a defective cell wall. - Penicillin has no effect on the existing cross bridges in peptidoglycan. - Lysosome is an enzyme that breaks the glycosidic bond between the NAM and NAG giving rise to the formation of spheroplasts. - **Autolysin is an enzyme present in the bacteria that breaks the existing cell wall so that new cell wall can be formed.** **[Gram-Positive Cell Walls]** **[Features]** - Eg: *Streptococcus pneumonia*, *Sarcina*, *Staphylococcus epidermidis*, *Bacillus anthracis.* - Multiple layers of peptidoglycan (almost 40 sheets) comprise up to 90% of cell wall. - Electron micrographs show the peptidoglycan of gram-positive cells to be 20-80 nm thick. - Components of Gram-positive cell wall: -Peptidoglycan -Teichoic acid -Teichouranic acid -Polysaccharide. - Gram-positive cells stain purple due to retention of the crystal violet dye during the Gram stain procedure. - If peptidoglycan is removed away from the cell, gram positive cells lose their cell wall and become protoplasts, while the gram negative cells become spheroplasts. - Peptidoglycan or murein is an enormous polymer composed of many identical subunits. - The presence of D-amino acids protects against attack by most peptidases (involved in the degradation of off-function proteins in lysosomes, cytosol, plasma membranes, or in extracellular space). - The peptidoglycan subunit present in most gram-negative bacteria and many gram-positive ones is shown in figure below,  Micro: 4 and 5 Bacteria Flashcards \| Quizlet - The backbone of this polymer is composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic (NAM) acid residues. A tetra-peptide chain of four alternating amino acids- L-alanine attached to NAM at position1, D-glutamic acid at position2, L-lysine at position3, and D-alanine at position4. - NAM and NAG are amino sugars that has **β-1, 4-glycosidic bond** between them. - Chains of linked peptidoglycan subunits are joined by crosslinks between the peptides. - Often the carboxyl group of the terminal D-alanine is connected directly to the amino group of diaminopimelic acid, but a **peptide interbridge/cross bridge** may be used instead. - **Peptide interbridge/cross bridge** is usually made up of 5 glycine residues. - Most gram-negative cell wall peptidoglycan lacks the peptide interbridge. - In gram-positive cells, these cross bridges can occur between tetra-peptides of adjacent peptidoglycan chains or between tetra-peptides of the same chain, so that a rigid multi-layered sheet of peptidoglycan is formed. - This cross-linking results in an enormous peptidoglycan sac that is actually one dense, interconnected network. - These sacs have been isolated from gram-positive bacteria and are strong enough to retain their shape and integrity, yet they are elastic and somewhat stretchable, unlike cellulose. They also must be porous, as molecules can penetrate them. - Penicillin interferes with cell wall synthesis by preventing the formation of these cross bridges, resulting in the formation of a defective cell wall. - Penicillin has no effect on the existing cross bridges in peptidoglycan. - Lysosome is an enzyme that breaks the glycosidic bond between the NAM and NAG giving rise to the formation of spheroplasts. - **Autolysin is an enzyme present in the bacteria that breaks the existing cell wall so that new cell wall can be formed.** - Water soluble - Polymers containing glycerol or ribitol residues joined together by phosphodiester bond. - There are two types of teichoic acids: -Wall teichoic acids -Membrane teichoic acids - **Wall Teichoic acids** are present in the wall and linked to peptidoglycan. - **Membrane Teichoic acids (aka Lipoteichoic acids)** are linked to the membrane glycolipids. - The teichoic acid constitutes the major surfaces antigen of the gram positive cells. - Most of the teichoic acids are present between the cytoplasmic membrane and peptidoglycan layer. - The teichoic acid may be composed of 30 or more repeating units of glycerol or ribitol. - Sometimes sugars like glucose, galactose or N-acetylglucosamine or rhamnose may be joined to glycerol or ribitol unit. - Most teichoic acids contain large amount of D-alanine. - **The composition of teichoic acids vary from species to species.** - **The teichoic acids bind to magnesium ions and supply it to the cells.** **[Teichouronic acid]** - Polymers similar to teichoic acids but are made up of repeating units of sugar acids like N-acetyl mannosuronic acid or D-glucosuronic acid. (Diagram) **[Polysaccharides]** - Polysaccharides such as arabinose, glactose, etc. may be present in the cell wall of gram-positive cells. - Group A Streptococci also has M protein in their cell wall. - Protein A of *Staphylococcus aureus*. (Diagram) **[Gram-Negative Cell Walls]** - Eg: *Nisseria meningitides*, *Vibrio cholera*, *E. coli* **[Features]** - The gram-negative cell walls are much more complex than gram-positive walls. - The thin peptidoglycan layer next to the plasma membrane may constitute not more than 5 to 20% of the wall weight. - Teichoic acids are not present in gram-negative bacteria. - The components of gram-negative cell wall include: -Peptidoglycan -Lipoprotein -Outer membrane -Lipopolysaccharide (LPSs) **[Peptidoglycan]** - Peptidoglycan or murein is an enormous polymer composed of many identical subunits. - The presence of D-amino acids protects against attack by most peptidases (involved in the degradation of off-function proteins in lysosomes, cytosol, plasma membranes, or in extracellular space). - The peptidoglycan subunit present in most gram-negative bacteria and many gram-positive ones is shown in figure below,  2.3: The Peptidoglycan Cell Wall - Biology LibreTexts - The backbone of this polymer is composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic (NAM) acid residues. A tetra-peptide chain of four alternating amino acids- L-alanine attached to NAM at position1, D-glutamic acid at position2, L-lysine (or meso-diaminopimelic acid in gram-negative cell wall) at position3, and D-alanine at position4. - NAM and NAG are amino sugars that has **β-1, 4-glycosidic bond** between them. - Chains of linked peptidoglycan subunits are joined by crosslinks between the peptides. - Often the carboxyl group of the terminal D-alanine is connected directly to the amino group of diaminopimelic acid, but a **peptide interbridge/cross bridge** may be used instead. - **Peptide interbridge/cross bridge** is usually made up of 5 glycine residues. - Most gram-negative cell wall peptidoglycan lacks the peptide interbridge. - In gram-positive cells, these cross bridges can occur between tetra-peptides of adjacent peptidoglycan chains or between tetra-peptides of the same chain, so that a rigid multi-layered sheet of peptidoglycan is formed. - This cross-linking results in an enormous peptidoglycan sac that is actually one dense, interconnected network. - These sacs have been isolated from gram-positive bacteria and are strong enough to retain their shape and integrity, yet they are elastic and somewhat stretchable, unlike cellulose. They also must be porous, as molecules can penetrate them. - Penicillin interferes with cell wall synthesis by preventing the formation of these cross bridges, resulting in the formation of a defective cell wall. - Penicillin has no effect on the existing cross bridges in peptidoglycan. - Lysosome is an enzyme that breaks the glycosidic bond between the NAM and NAG giving rise to the formation of spheroplasts. **[Lipoprotein]** - Contains a lipid content and protein content. - Also called Braun's Lipoprotein. - Connect peptidoglycan to the outer membrane. - The protein component of the lipoprotein has amino acids that are linked to the diaminopimelic acid of the tetra-peptide chain of the peptidoglycan and the lipid component is inserted into the outer membrane. - The lipoprotein stabilizes the outer membrane and anchors it to the peptidoglycan. **[Outer membrane]** - A phospholipid bilayer in which the phospholipids of the outer layer are replaced by lipopolysaccharides. - Like a cell membrane, the outer membrane is a fluid mosaic layer with many proteins embedded in it. - The outer membrane serves as an impermeable barrier that prevents the entry of large molecules such as antibiotics, detergents and dyes; but it allows the entry of smaller molecules such as purine, pyramidines, disaccharides and amino acids. - Molecules pass through diffusion channels formed by special proteins called [porins]. - The molecules enter through these porins based on their electric charges and size. - Other non-porin protein such as enzymes and carrier proteins are present in the outer membrane. - The outer membrane prevents the leakage of essential enzymes that are present in the [periplasmic space ( space between the cytoplasmic membrane and outer membrane)] - Lipopolysaccharides and porins that are synthesized within the cell can be transported to the outer membrane through region of adhesion called [Bayer's junction] (they act as export sites). **[Lipopolysaccharides (LPS)]** - Located in the outer layer of the outer membrane. - Consists of three parts: **-**Lipid A- a complex lipid **-**Core polysaccharide or polysaccharide core **-**A terminal series of repeat units **[Lipid A]** - Region that is firmly embedded in the membrane - It consists of a chain of glucosamine disaccharide units to which a number of long chain fatty acids are connected. - It is toxic in nature and is called endotoxin because it is firmly bound to cell wall and released only when cell is lysed. - It acts as poison causing fever, diarrhea, destruction of RBC and potentially fatal shock. **[Core polysaccharide]** - Located at the membrane surface and consists of about ten sugars, many of which are unusual (2-keto, 3-deoxyoctanate, heptose). - It is similar in all gram negative species. - These sugars are charged and contain phosphates and hence, contribute to the negative charge on the bacterial surface **[Terminal series of repeat units or 'O' antigens]** - They are polysaccharides that extend as whiskers from the surface. - Each species contains unique terminal repeat units. - It consist of variable number of repeating oligosaccharides with three to five sugars like glucose, mannose, rhamnose and also some unique sugars - They are responsible for the serological properties of LPS and also serve as attachment sites for bacteriophage. - The gram negative cell wall is protected from anti-peptidoglycan substances like penicillin and lysozyme as they cannot penetrate the outer membrane. - Their cell wall differs from the regular bacterial cell wall. - Instead of mureins they have pseudomureins/pseudo peptidoglycan. - They don't have NAM, instead N-acetytalosaminuronic acid (NAT) is present. - NAG and NAT have 1, 3-**β glycosidic linkages.**  - Insensitive to lysosome. - They also have glycoprotein and protein polysaccharides. - They have an S-layer. - They have L-amino acid instead of D-amino acid. - The cell wall helps Archaea live in hot, acidic and saline environments without damaging the cell membrane. (Diagram) **[Envelope of Acid Fast Bacteria]** - They have peptidoglycan, lipids and an arabinogalactan polysaccharide. - About half the lipids are mycolic acids. - When mycolic acids are attached to arabinogalactan, it forms a water insoluble complex known as [Wax D]. - The large lipid content is responsible for their peculiar staining properties and hydrophobic nature of the cell wall which may prevent nutrient entry into the cell. This perhaps explains the slow growth rate of most *Mycobacteria*.  **[Principle of Gram Staining]** - According to the explanations give to the Gram stain reaction results, it seems that the difference between the gram positive and gram negative bacteria is due to the physical nature of the cell wall. - If the cell wall removed from the gram positive bacteria, it becomes gram negative. - The peptidoglycan itself is not stained; instead it seems to act as a permeability barrier preventing loss of crystal violet. - During the procedure, the bacteria is first stained with crystal violet and next treated with iodine to promote dye retention. - Gram-positive bacteria then are decolorized with ethanol, the alcohol is thought to shrink the pores of the thick peptidoglycan. - Thus the dye-iodine complex is retained during the short decolorization step and the bacteria remain purple. - In contrast, gram-negative peptidoglycan is very thin, not as highly cross-linked, and has larger pores. - Alcohol treatment also may extract enough lipids from the gram-negative wall to increase its porosity further. - For these reasons, alcohol more readily removes the purple crystal violet-iodine complex from gram-negative bacteria. **[Significance of Cell Wall]** - Maintains the cell shape, any cell that loses the cell wall, loses its shape as well. - Protects bacteria from osmotic lysis. - Acts as a barrier protects cell contents from external environment. - Determines reactivity to gram stain, cells become gram negative if they lose cell wall. - Attachment site for flagella. - Site of action for certain anti-microbial agents (E.g.: Penicillin, Cephalosporin) - Confer specific antigenicity to stain/species that can be exploited to detect and identify an isolate.