3.1 BACTERIAL CELL WALL

Describe bacterial cell wall

Bacterial Cell Wall

Basic Function and Composition+ Difference between Gram+ and Gram- + Protection from Immune Responses+ Toxic Responses 

Basic Function and Composition: The cell wall provides structural support and protection to the bacterial cell. It is primarily made up of a polymer called peptidoglycan (also known as murein).

Difference between Gram (+) and Gram (-) Cell Walls

  • Gram-positive Bacteria:

    • Peptidoglycan + Teichoic and Lipoteichoic Acids+ Absence of Outer Membrane 

    • Peptidoglycan: Gram-positive bacteria have a thick layer of peptidoglycan. This thick layer traps the crystal violet stain during the Gram staining process, giving the bacteria a purple appearance under a microscope.

    • Teichoic and Lipoteichoic Acids: Present in the cell wall, they play roles in cell viability and are involved in bacterial virulence.

    • Absence of Outer Membrane: Gram-positive bacteria lack the outer lipid membrane that is present in Gram-negative bacteria.

  • Gram-negative Bacteria:

    • Peptidoglycan+ Outer membrane&Lipopolysaccharides+ Gram Staining 

    • Peptidoglycan: These bacteria have a thin layer of peptidoglycan located between the inner cytoplasmic membrane and an outer lipid membrane.

    • Outer Membrane & Lipopolysaccharides (LPS): The outer membrane contains LPS, which is made up of lipid A, a core polysaccharide, and O-antigen. Lipid A acts as an endotoxin and is a potent immune activator. The O-antigen provides a barrier, limiting the accessibility of substances like the complement system.

    • Gram Staining: The thin peptidoglycan layer doesn't retain the crystal violet stain during the Gram staining process. Instead, the cells are counterstained with safranin, giving them a pink or red appearance under a microscope.

Protection from Immune Responses:

  • Gram-positive Bacteria:

    • Thick peptidoglycan layer 

    • The thick peptidoglycan layer provides a barrier against environmental threats and host immune defenses.

  • Gram-negative Bacteria:

    • O antigens in LPS+ Outer membrane

    • The O-antigens in the LPS limit the access of the complement membrane attack complex to the membrane.

    • The outer membrane serves as a protective barrier against larger molecules and antibacterial agents.

  • Capsule: Found in both Gram-positive and Gram-negative bacteria, it provides protection against immune defenses like antibodies, complement system, and phagocytosis.

  • Protein Inhibitors: Some bacteria produce proteins that interfere with specific immune functions. For instance, Staphylococcus protein A binds the Fc portion of IgG antibodies, and Streptococcus M protein is antiphagocytic, meaning it prevents phagocytosis by immune cells.

Toxic Responses:

LPS+ Cell wall components+ Flagellin 

  • LPS: The lipid A portion of LPS in Gram-negative bacteria acts as an endotoxin. It's a powerful activator of the toll-like receptor (TLR) system and other immune receptors.

  • Cell Wall Components: Components like teichoic acid and peptidoglycan are weaker activators of TLRs compared to lipid A.

  • Flagellin: This is the protein component of bacterial flagella and can act as an immune activator by binding to specific pathogen pattern receptors.

MIND MAP: Bacterial Cell Wall

Basic Function and Composition

  • Provides structural support and protection to the bacterial cell

  • Primarily made up of peptidoglycan (murein) polymer

Difference between Gram (+) and Gram (-) Cell Walls

Gram-positive Bacteria

  • Peptidoglycan + Teichoic and Lipoteichoic Acids + Absence of Outer Membrane

  • Thick layer of peptidoglycan

  • Traps crystal violet stain during Gram staining

  • Purple appearance under microscope

  • Teichoic and Lipoteichoic Acids present in cell wall

  • Roles in cell viability and bacterial virulence

  • Lack outer lipid membrane

Gram-negative Bacteria

  • Peptidoglycan + Outer membrane & Lipopolysaccharides (LPS) + Gram Staining

  • Thin layer of peptidoglycan between inner cytoplasmic membrane and outer lipid membrane

  • Outer membrane contains LPS

  • LPS composed of lipid A, core polysaccharide, and O-antigen

  • Lipid A acts as endotoxin and immune activator

  • O-antigen provides barrier, limiting accessibility of substances like complement system

  • Thin peptidoglycan layer doesn't retain crystal violet stain

  • Counterstained with safranin

  • Pink or red appearance under microscope

Mind Map: Protection from Immune Responses

Central Idea: Protection mechanisms employed by bacteria to evade or counteract host immune responses

Gram-positive Bacteria:

  • Thick peptidoglycan layer

    • Provides a barrier against environmental threats and host immune defenses

Gram-negative Bacteria:

  • O antigens in LPS

    • Limit the access of the complement membrane attack complex to the membrane

  • Outer membrane

    • Serves as a protective barrier against larger molecules and antibacterial agents

  • Capsule

    • Provides protection against immune defenses like antibodies, complement system, and phagocytosis

  • Protein Inhibitors

    • Some bacteria produce proteins that interfere with specific immune functions

      • Staphylococcus protein A

        • Binds the Fc portion of IgG antibodies

      • Streptococcus M protein

        • Antiphagocytic, prevents phagocytosis by immune cells

Toxic Responses:

  • LPS

    • Acts as an endotoxin, activating toll-like receptor (TLR) system and other immune receptors

  • Cell Wall Components

    • Weaker activators of TLRs compared to lipid A

  • Flagellin

    • Protein component of bacterial flagella

    • Can act as an immune activator by binding to specific pathogen pattern receptors