Bacterial Cell Walls and Layers

Bacterial Cell Walls and Layers

Cell Wall Basics

  • Unique to almost all bacteria; not all bacteria have cell walls, but most do.
  • Semi-rigid structure that determines the shape of the bacteria (e.g., coccus, bacilli).
  • Cell walls take up different stains, which aids in bacterial identification under a microscope.
  • Provide support to prevent osmotic shock.
  • Composed primarily of peptidoglycan, a polymer of amino acids and sugars, which is a major component.
  • Essential for bacterial integrity.
  • Targeted by antibiotics to damage or prevent cell wall formation, leading to bacterial death.
  • Contain sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
  • NAG and NAM cross-link to form the cell wall structure.

Gram-Positive vs. Gram-Negative Bacteria

  • Bacteria are classified into gram-positive and gram-negative categories based on cell wall structure.
  • Gram-positive:
    • Thick peptidoglycan layer above the cell membrane.
    • Contains teichoic acids within the peptidoglycan layer, providing rigidity and antigenic specificity.
  • Gram-negative:
    • Thin peptidoglycan layer.
    • Outer membrane made of lipopolysaccharide (LPS), porins, and proteins.

Gram-Positive Cell Wall

  • Simpler structure with a peptidoglycan outer cell wall and teichoic acids.
  • Teichoic acids are unique to gram-positive bacteria.
    • Provide rigidity.
    • Give antigenic specificity (act as labels for recognition by the immune system).

Gram-Negative Cell Wall

  • More complex structure.
  • Cell membrane, thin peptidoglycan layer, and an outer layer of phospholipids, lipoproteins, porin proteins, and polysaccharides.
  • Lipopolysaccharides (LPS) are found in gram-negative bacteria and can be washed off with alcohol or acetone.
  • LPS is an endotoxin that causes fever.
  • Toxins:
    • Endotoxins: Part of the bacteria (e.g., lipopolysaccharide).
    • Exotoxins: Secreted by bacteria into their environment.
  • Lipopolysaccharide Composition:
    • Lipid A
    • Polysaccharide

Lipopolysaccharide (LPS) Functions

  • Toxic and can cause septicaemia.
  • Pyrogenic, causing fever.
  • Stimulates immunity.
  • Resistant to heat.
  • Colony appearance on agar plates varies based on LPS composition.
    • Mucoid colonies: Smooth, glossy appearance due to ample capsule.
    • Dry colonies: Rougher appearance due to less capsule.

Acid-Fast Bacteria

  • A class of bacteria that are neither gram-negative nor gram-positive.
  • Mycobacterium tuberculosis, which causes tuberculosis, is an example.
  • Possess a cell wall similar to gram-positive but with a thick, waxy layer that prevents gram staining.

Summary

  • Cell walls determine the shape of bacteria.
  • Provide strength to prevent osmotic shock and rupture.
  • Found in almost all bacteria and are unique to bacteria.
  • Antibiotics target cell walls to prevent proper formation.
  • Bacteria are divided into gram-negative, gram-positive, and acid-fast categories based on cell wall structure.
  • Gram-positive: Simple cell wall with a peptidoglycan layer and teichoic acids.
  • Gram-negative: More complex cell wall with a thin peptidoglycan layer and a lipopolysaccharide layer.