L06-BIOS324-Cell-surface-NP

Microcosm of Life

  • Overview of cell surface structures

Learning Objectives

  • Define key terms:

    • Important terminology related to bacterial cell surface

  • Explain differences:

    • Study the structural contrasts between Gram-positive and Gram-negative bacterial cell envelopes

  • Analyze the role of peptidoglycan:

    • Understand how peptidoglycan keeps cells shaped and protects from osmotic stress

  • Evaluate surface structures:

    • Importance of various surface features on bacterial cell functions

  • Design experiments:

    • Create experiments to identify cells based on unique cellular envelope features

Bacterial Cell Structure

  • Components:

    • Outer Membrane: Found in Gram-negative bacteria, absent in some.

    • Peptidoglycan: Provides structural support and is thicker in Gram-positive bacteria.

    • Inner Membrane (Cytoplasmic Membrane): Semi-permeable barrier.

    • Cytoplasm: Contains ribosomes and nucleoid (bacterial DNA)

The Cellular Envelope

  • Capsule: Protective layer, enhancing infection ability.

  • Cell Wall: Rigid structure; varies in thickness between Gram-positive and Gram-negative.

  • Flagellum: Provides motility.

  • Membrane Structure:

    • Comprehends inner membrane, outer membrane, and peptidoglycan layers.

Detailed Structure of Cell Envelope

  • Typical Bacterial Cell:

    • Consists of a capsule, cell wall, inner membrane, and periplasmic space.

    • Key for maintaining integrity and shape:

      • Periplasm: Space between inner and outer membranes in Gram-negative bacteria.

Inner Membrane Structure

  • Composed of:

    • Lipids: Form the phospholipid bilayer.

    • Proteins: Act as transporters and enzymes.

Phospholipid Bilayer

  • Structure:

    • Composed of hydrophilic heads facing outward and hydrophobic tails inward.

    • Important for maintaining cellular integrity and transport.

Unique Lipids in Membranes

  • Bacterial and Archaeal lipids:

    • Ether-linked lipids in Archaea: More stable than ester-linked in Bacteria.

    • Higher branching in Archaea leads to increased membrane stability.

Peptidoglycan Structure

  • Composition:

    • Made of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by peptide cross-bridges.

    • Vital for structural integrity and shapes of bacterial cells.

  • Antibiotic Resistance:

    • Penicillin and vancomycin affect peptidoglycan cross-linking.

Gram Staining Technique

  • Process:

    • Stains cells using crystal violet and differentiating between Gram-positive (purple, retains stain) and Gram-negative (red, does not retain stain).

Teichoic Acids in Gram-Positive Bacteria

  • Function:

    • Negatively charged, help maintain structural integrity and repel detergents.

  • Location:

    • Embedded within the thick peptidoglycan layer.

Unusual Cell Walls in Archaea

  • Pseudopeptidoglycan:

    • Composed of different components (e.g., NAT instead of NAM).

  • Functionality:

    • Similar to bacterial peptidoglycan in function, providing rigidity and osmotic protection.

The Outer Membrane of Gram-negative Bacteria

  • Structure:

    • Contains lipopolysaccharides (LPS), which can provoke strong immune responses when bacteria die.

  • Porins:

    • Channels that permit small molecules to pass into the cell while preventing harmful substances.

Extracellular Polysaccharides (EPS)

  • Types:

    • Capsule: Firmly attached, aiding in virulence.

    • Slime Layer: Loosely associated, providing moisture retention and adhesion.

  • Advantages:

    • Enhance surface adhesion, prevent desiccation, and shield from immune responses.

S-layer

  • Definition:

    • Protective crystalline layer made of proteins or glycoproteins, often found in archaea and some bacteria.

  • Function:

    • Guarding against environmental insults like phages and physical damage.

Transport Mechanisms Across Membranes

  • Passive Diffusion: Non-energy requiring movement through the membrane.

  • Facilitated Diffusion: Requires protein channels for transport without energy.

  • Active Transport: Expends energy to move substances against their concentration gradient.

Summary of Transport Types

  • Porins allow passive diffusion of small molecules.

  • Active transport mechanisms like ABC transporters are essential for nutrients such as sugars.

Learning Objectives Recap

  • Covering terminology, structural differences between Gram-positive and Gram-negative bacteria, role of peptidoglycan, significance of surface structures, and designing differentiative experiments.