Lecture Notes: Differences Between Gram-Positive and Gram-Negative Bacteria

Introduction

• Overview of gram-positive and gram-negative bacteria.
• Both have a cell membrane similar to human cells called a phospholipid bilayer.

Cell Membrane

• Human cell membranes contain sterols (cholesterol) which bacteria lack.
  • This affects the rigidity of bacterial membranes.

Cell Wall

• Gram-Positive Bacteria:

  • Thick cell wall.
  • Made of sugars (glycans) and proteins (peptides).
  • Cell wall maintains structure to prevent bursting due to osmotic pressure.
  • Target for antibiotics like penicillins and cephalosporins.
  • Lysozymes in human tears, saliva, and mucus break down these sugars, defending against bacteria.

• Gram-Negative Bacteria:

  • Thin cell wall.
  • Additional outer phospholipid bilayer.
  • Contains Lipoprotein layer between cell wall and outer membrane.
  • Outer phospholipid bilayer has Lipopolysaccharides (LPS).
    • Lipid A component: an endotoxin causing endotoxic shock.
    • Polysaccharide component: immunogenic but not pathogenic.

Endotoxins

• Lipid A:
  • Released during bacterial replication or antibiotic action.
  • Causes endotoxic shock by activating the immune system to open blood vessels.

Immune Response

• Polysaccharides on LPS are recognized by the immune system for a targeted response.
• Lipid A is pathogenic and identical across gram-negative bacteria but not immunogenic.

Structural Differences

• Gram-Positive:
  • Simple structure: phospholipid bilayer and thick cell wall.
• Gram-Negative:
  • Complex structure: inner membrane, thin cell wall, Lipoprotein layer, outer membrane with LPS.
  • Porins in the outer membrane allow certain substances to pass.

Antibiotic Penetration

• Gram-negative bacteria are harder to penetrate due to dual bilayers and pores.
• Penicillin G (first class) could not penetrate gram-negative due to size but was effective on gram-positive.

Naming and Discovery

• Named after Danish physician Christian Gram.
  • Gram-positive: cell wall stains.
  • Gram-negative: cell wall does not stain.

Conclusion

• Understanding these differences is crucial for developing antibiotics and targeting bacterial infections.
• Exploiting structural differences aids in treatment strategies.