Lecture 3: Bacterial Cell Envelope PDF

Lecture 3 Bacterial Cell Envelope By Prof. Dr. Mohammed Ali Ahmed Eid 24/10/2024 Bacterial Cell Envelope Most bacteria have a chemically complex external covering, termed the cell envelope, that lies outside of the cytoplasm. It is composed of two basic layers known as the cell wall, and t...

Lecture 3 Bacterial Cell Envelope By Prof. Dr. Mohammed Ali Ahmed Eid 24/10/2024 Bacterial Cell Envelope Most bacteria have a chemically complex external covering, termed the cell envelope, that lies outside of the cytoplasm. It is composed of two basic layers known as the cell wall, and the cell membrane. The layers of the envelope are stacked one upon another and are often tightly bonded together like. Although each envelope layer performs a distinct function, together they act as a single protective unit. The envelope is extensive and can account for one-tenth to one-half of a cell’s volume.  Bacterial Cell wall Immediately below the glycocalyx lies a second layer, the cell wall. Importance of the bacterial cell wall: 1. It determines the shape of a bacteria. 2. It save the bacterium from bursting or collapsing because of changes in osmotic pressure (osmotic lysis). 3. It might have components that contribute to pathogenicity. 4. The cell wall can protect the bacterial cell from toxic substances 5. It is the site of action of several antibiotics. 6. Cell wall composition helps identify bacterial species.  Christian Gram 1884 Christian Gram (1853-1938) was a Danish (‫)ﻣن اﻟدﻧﻣﺎرك‬ bacteriologist who made a fundamental contribution to microbiology through the development of the Gram staining technique in 1884, which revolutionized bacterial classification. He found that certain bacteria retained crystal violet dye after iodine fixation. This led to classification of bacteria as: – Gram-positive (retain ‫ ﺗﺳﺗﺑﻘﻲ‬the crystal violet stain) – Gram-negative (don't retain the crystal violet stain) This discovery helped advance understanding of bacterial infections, and contributed to development of targeted treatments Gram-positive: Gram-negative: Thick peptidoglycan layer (20-80 nm) Thin peptidoglycan layer (2-7 nm) (90%) (10%) No outer membrane Has outer membrane Contains teichoic acids Contains lipopolysaccharides (LPS) Single lipid membrane Double lipid membrane More susceptible to penicillin More resistant to antibiotics Sensitive to lysozyme Protected by outer membrane Peptidoglycan structure Sugar derivatives N-acetylglucosamine (NAG) N-acetylmuramic acid (NAM); NAG-lactic acid. Tetra peptide chain L-alanine D-glutamic acid meso-diaminopimelic acid D-alanine. Peptidoglycan structure Peptidoglycan Cross-Links Gram-positive bacteria: Uses interpeptide bridge (pentaglycine bridge; 5 glycine molecules) Gram-negative bacteria: Direct cross-linking No interpeptide bridge needed Peptidoglycan Teichoic acids structure Teichoic acids Polymers of glycerol or ribitol joined by phosphate Ribitol Amino acids such as D-alanine or sugars like glucose are attached to the glycerol and ribitol groups It is connected covalently with N- acetylmuramic (NAM) acid or to plasma membrane lipids (Lipoteichoic acid). Glycerol Peptidoglycan structure Gram positive cell wall Gram negative cell wall Outer Membrane Proteins: Porins: Other Proteins: Lipoproteins (Braun's lipoprotein)  Gram Negative cell wall Lipopolysaccharide (LPS) Structure Lipid A: Glucosamine disaccharide backbone 6-7 fatty acid chains Phosphate groups at positions 1 and 4’ Core Polysaccharide: Inner core: KDO (3-deoxy-D-manno-octulosonic acid) Heptose Outer core: Glucose Galactose N-acetylglucosamine O-antigen: Repeating oligosaccharide units Variable length (0-40 repeats) Strain-specific structure Function of outer membrane 1. Serve as a protective barrier 2. It prevents or slows the entry of antibiotics (Ex. Beta lactam class like penicillin and cephalosporin , and other toxic substances. 3. It contains porin proteins: Create channels for passive diffusion, and control permeability of outer membrane 4. It prevents the loss of constituents like periplasmic enzymes  Periplasmic Space: This is the region between the inner (cytoplasmic) membrane and the outer membrane in Gram-negative bacteria. It contains a gel-like matrix called the periplasm, Periplasm contains several proteins that allows for various Periplasmic biochemical processes, such as: 1. It contains hydrolytic enzymes that break down nutrients. space and 2. It contains transport proteins for nutrients and ions from the periplasm outer membrane into the cell. 3. It contains enzymes involved in cell wall and peptidoglycan biosynthesis. 4. It contains enzymes that can degrade harmful substances before they enter the cytoplasm such as β-lactamases, which can degrade antibiotics like penicillin. 5. It contains chaperones and enzymes that fold proteins, ensuring that secreted proteins are properly folded and functioning.