Bacterial Cell Walls and Layers Summary

Bacterial Cell Walls and Layers

Cell Wall Basics

  • Unique to almost all bacteria; not all have it, but it's mostly unique to bacteria.
  • Semi-rigid structure that determines the shape of the bacteria (coccus, bacilli).
  • Takes up different stains, aiding in identification.
  • Provides support and prevents osmotic shock.
  • Composed of peptidoglycan (amino acids and sugars).
  • Essential for bacterial integrity and a target for antibiotics.
  • Contains sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which cross-link to form structure.

Gram-Positive vs. Gram-Negative Bacteria

  • Gram-positive: Thick peptidoglycan layer above the cell membrane.
  • Gram-negative: Thin peptidoglycan layer with an outer membrane of lipopolysaccharide, porins, and proteins.
  • Cell membranes are similar in both.

Gram-Positive Cell Wall

  • Simpler peptidoglycan outer cell wall with teichoic acids.
  • Teichoic acids provide rigidity and antigenic specificity (act as labels for recognition by the immune system).

Gram-Negative Cell Wall

  • More complex structure.
  • Has a thinner peptidoglycan layer.
  • Features an extra layer of phospholipids, lipoproteins, porin proteins, and lipopolysaccharides.
  • Lipopolysaccharides can be washed off with alcohol or acetone.
  • Lipopolysaccharide acts as endotoxin that causes fever. Endotoxins are part of the bacteria, while exotoxins are excreted by bacteria.
  • Lipopolysaccharide is composed of lipid type A and a polysaccharide.

Lipopolysaccharide Functions

  • Often toxic and can cause fever (pyrogen).
  • Stimulates immunity.
  • Resistant to heat.
  • Colonies can appear mucoid (smooth, glossy) or dry based on lipopolysaccharide composition.

Acid-Fast Bacteria

  • Neither gram-negative nor gram-positive.
  • Mycobacterium tuberculosis is an example.
  • Have a thick waxy layer that prevents gram staining.

Summary

  • Cell walls determine bacteria shape and prevent osmotic shock.
  • Unique to bacteria; antibiotics target cell wall formation.
  • Bacteria are categorized into gram-negative, gram-positive, and acid-fast based on cell wall structure.
  • Gram-positive: Simple cell wall with peptidoglycan and teichoic acids.
  • Gram-negative: More complex with a thin peptidoglycan layer and a lipopolysaccharide layer.