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Understanding Antimicrobial Drugs and Resistance

Mar 26, 2025

Chapter 20: Antimicrobial Drugs

Key Topics Covered

  • History and Concepts of Chemotherapy
  • Selective Toxicity
  • Antibiotics and Antimicrobial Drugs
  • Antibiotic Resistance
  • Types and Functions of Antimicrobial Drugs
  • Antibiotic Misuse and Safety
  • Future of Chemotherapeutic Agents

Key Concepts

History of Chemotherapy

  • Selective Toxicity: Toxic to the pathogen but not the host.
  • Chemotherapy: Use of chemicals to treat diseases.
  • Antibiotics: Substances produced by microbes inhibiting other microbes (e.g., penicillin).
  • Antimicrobial Drugs: Synthetic substances interfering with microbial growth.

Antibiotic Resistance

  • Discovered by Alexander Fleming.
  • Clinical trials of penicillin began in 1940.
  • MRSA: Example of antibiotic-resistant bacteria.

Types of Antibiotics

  • Narrow Spectrum: Affect specific microbial types.
  • Broad Spectrum: Affect a wide range of bacteria.
  • Superinfection: Overgrowth of antibiotic-resistant microbiota.

Bactericidal vs. Bacteriostatic

  • Bactericidal: Kills microbes directly.
  • Bacteriostatic: Inhibits microbial growth.

Mechanisms of Action

Inhibiting Cell Wall Synthesis

  • Drugs like penicillin prevent peptidoglycan formation.

Inhibiting Protein Synthesis

  • Streptomycin: Changes the 30s subunit.
  • Chloramphenicol: Affects the 50s subunit.
  • Tetracycline: Interferes with tRNA binding.

Injuring Plasma Membrane

  • Polymyxin B: Causes cytoplasmic content leakage.

Inhibiting Nucleic Acid Synthesis

  • Blocks DNA/RNA replication and transcription.

Inhibiting Essential Metabolite Synthesis

  • Sulfonamides: Block folic acid synthesis by competing with PABA.

Specific Drugs and Their Targets

  • Penicillins: Block cell wall synthesis using beta-lactam rings.
  • Cephalosporins: Similar to penicillins but with different beta-lactam rings.
  • Vancomycin: Last-resort for MRSA.
  • Isoniazid and Ethambutol: Target mycolic acid in mycobacteria.
  • Macrolides (e.g., Erythromycin): Target gram-positive bacteria.

Antibiotic Testing Methods

  • Kirby-Bauer Test: Determines antibiotic effectiveness via zone of inhibition.
  • E-Test: Determines MIC (Minimal Inhibitory Concentration).
    • Uses broth dilution to find MBC (Minimal Bactericidal Concentration).

Antibiotic Resistance Mechanisms

  • Persister Cells: Survive antibiotic exposure.
  • Superbugs: Resistant to multiple antibiotics.
  • Horizontal Gene Transfer: Spreads resistance among bacteria.

Combating Resistance

  • Avoid misuse: Complete prescribed courses; don't use for viral infections.
  • Synergism: Combined effect of drugs is greater than individual.

Future of Chemotherapeutic Agents

  • Targeting virulence factors.
  • Developing new drugs and research on phage therapy.

Important Terms to Know

  • Selective Toxicity, Chemotherapy, Antibiotics, Antimicrobial Drugs.
  • Bactericidal vs. Bacteriostatic.
  • Narrow vs. Broad Spectrum.
  • Superinfection, Kirby-Bauer Test, MIC, MBC.
  • Synergism vs. Antagonism.

These notes summarize key points from the lecture on antimicrobial drugs, focusing on drug mechanisms, resistance, and testing methods necessary for understanding and application in medical settings.

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