Bacterial Cell Walls and Layers

Bacterial Cell Walls and Layers

Cell Wall Basics

  • The cell wall is unique to almost all bacteria, providing shape and support.
  • It's a semi-rigid structure that defines the shape of bacteria (coccus, bacilli).
  • Differential staining of cell walls aids in bacterial identification under a microscope.
  • The cell wall prevents osmotic shock.
  • Peptidoglycan, a polymer of amino acids and sugars, is the major component of the cell wall.
  • The integrity of bacteria relies on the cell wall.
  • Antibiotics target cell walls, damaging or preventing their formation, leading to bacterial death.
  • Peptidoglycan contains N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which cross-link to form the cell wall structure.

Gram-Positive vs. Gram-Negative Bacteria

  • Bacteria are broadly classified into Gram-positive and Gram-negative categories based on cell wall structure.
  • Gram-positive bacteria have a thick peptidoglycan layer above their cell membrane.
  • Gram-negative bacteria possess a more complex structure with a thin peptidoglycan layer and an outer membrane.
  • Cell membranes are similar in both types, but the cell wall structure differs significantly.

Gram-Positive Cell Wall

  • Simpler structure with a thick peptidoglycan layer interspersed with teichoic acids.
  • Teichoic acids provide rigidity and antigenic specificity, acting as labels for the immune system to recognize specific bacteria.

Gram-Negative Cell Wall

  • More complex structure including:
    • Cell membrane.
    • Thin peptidoglycan layer.
    • Outer layer composed of phospholipids, lipoproteins, porin proteins, and lipopolysaccharides (LPS).

Lipopolysaccharides (LPS)

  • Found in Gram-negative bacteria.
  • Removed by alcohol or acetone.
  • LPS is an endotoxin that causes fever.
  • Endotoxins are part of the bacteria, while exotoxins are secreted by bacteria into their environment.
  • LPS consists of lipid A and a polysaccharide.
  • Functions:
    • Often toxic, causing septicaemia.
    • Pyrogenic, inducing fever.
    • Stimulates immunity.
    • Heat-resistant.
    • Affects colony appearance on agar plates (smooth/mucoid vs. rough/dry).

Colony Morphology and LPS

  • Colonies are visible masses of millions of bacteria.
  • Mucoid colonies have a smooth, glossy appearance due to a large capsule.
  • Dry colonies have a rougher appearance with less capsule material.

Acid-Fast Bacteria

  • A class of bacteria that are neither Gram-positive nor Gram-negative.
  • Mycobacterium tuberculosis, the causative agent of tuberculosis, is an example.
  • Possess a cell wall similar to Gram-positive bacteria but with a thick, waxy layer preventing Gram staining.

Summary of Cell Walls

  • Cell walls determine bacterial shape.
  • Provide strength to prevent osmotic shock and rupture.
  • Found in 20-40% of bacteria.
  • Unique to bacteria, making them a target for antibiotics.
  • Bacteria affecting humans are divided into Gram-negative, Gram-positive, and acid-fast categories.

Key Structural Differences

  • Gram-positive: Simple cell wall with a thick peptidoglycan layer and teichoic acids.
  • Gram-negative: Complex cell wall with a thin peptidoglycan layer and a lipopolysaccharide outer layer.