Mode of Action of Antibiotics

Introduction

• Antibiotics: Destroy or slow down bacterial growth
  • Bactericidal: Destroys bacteria
  • Bacteriostatic: Slows down bacterial growth

Bacteria Structure Overview

• Cell membrane and cell wall
  • Gram-positive bacteria used as an example
• Cytoplasm
  • Circular DNA
  • RNA synthesis
  • Ribosomes for protein synthesis

Targeting the Cell Membrane

• Disrupt cell membrane function by disrupting the phospholipid bilayer
  • Alters cell membrane structure and makes it more permeable
    • Disrupts osmotic balance
    • Causes leakage of cellular molecules
    • Leads to cell death
• Examples: Polymixins and Polyenes
  • Note: Polyenes target fungal pathogens

Targeting the Cell Wall

• Inhibit cell wall synthesis
  • Bactericidal
  • Human cells unaffected (no peptidoglycan)
• Examples: Penicillin, Cephalosporin, Bacitracin
  • Beta-lactams (Penicillin, Cephalosporin)
  • Glycopeptides (Bacitracin)
• Video on detailed mechanisms planned

Inhibiting DNA/RNA Synthesis

• Prevent DNA/RNA synthesis
  • DNA synthesis: Quinolones, Nalidixic acids
  • RNA synthesis: Rifampicin
• Results in the inhibition of protein synthesis

Inhibiting Protein Synthesis

• Targets ribosomes
  • 50S subunit: Erythromycin, Chloramphenicol
  • 30S subunit: Tetracycline, Streptomycin, Gentamicin
• Results in bacteria being unable to produce proteins
• Usually bacteriostatic (prevents growth)

Inhibiting Folic Acid Metabolism

• PABA (para-aminobenzoic acid) to Folate
  • Essential for adenine and thymine synthesis
• Antibiotics prevent conversion to Folate
  • Example: Sulfonamides, Trimethoprim
  • Specific to bacteria because humans cannot synthesize folic acid

Conclusion

• Overview of antibiotic actions
• Emphasis on selective toxicity to bacteria