Detailed Study Notes on Antimicrobial Drugs and Antibiotics

Introduction to Antimicrobial Drugs

  • Definition: Drugs that inhibit or kill microorganisms.
  • Historical context: Safe and effective treatments developed primarily since the early 1900s.
  • Antimicrobials are divided into subcategories, including antibiotics.

Antibiotics

  • Definition: Antibiotics are derived from living organisms, primarily bacteria and fungi.
  • Key principle: Selective toxicity — ability to kill bacteria without harming human cells.
    • Example of non-selective: Killing with hydrogen cyanide would kill both bacteria and human cells.

Early Discovery of Antibiotics

  • Paul Ehrlich

    • Early 1900s researcher.
    • Developed Salvarsan for treating syphilis and other infections caused by treponemes, including Treponema pallidum (syphilis) and Trypanosoma cruzi (Chagas disease).
    • Salvarsan was recognized for its effectiveness, though it was toxic and painful to administer.

  • Chagas Disease

    • Caused by Trypanosoma cruzi, leading to severe prolonged health issues.
    • Symptoms resemble severe flu.

  • Syphilis Stages

    • Primary syphilis: Characterized by a single sore (chancre).
    • Secondary syphilis: Can present rashes and sores which resolve autonomously.
    • Tertiary syphilis: Severe consequences including major organ damage and potential death.

  • Alexander Fleming (1928)

    • Discovery of penicillin—an antibiotic derived from Penicillium fungi.
    • Discovery was accidental; Nobel Prize awarded for subsequent developments in the late 1940s after purification.
    • Penicillin effectively kills many pathogenic bacteria with minimal harm to human cells.

  • Sulfa Drugs

    • Invented by Gerhard Domagk in the 1930s as the first widely used antibacterial agents, predating penicillin.
    • Mechanism: Inhibit bacterial growth allowing the human immune system to combat the pathogens.

Classification of Antimicrobials

  • Natural vs. Synthetic vs. Semisynthetic

    • Natural: Directly derived from living organisms.
    • Semisynthetic: Chemically modified natural antibiotics to improve effectiveness and pharmacokinetics.
    • Synthetic: Completely created in laboratories without direct biological source.

  • Common sources of modern antibiotics include:

    • Fungi: A few but significant in their efficacy.
    • Bacteria: Mainly derived from soil microorganisms like Streptomyces, a major producer of antibiotics (e.g., erythromycin, neomycin).

Selective Toxicity in Antimicrobials

  • Mechanisms of Targeting:
    • Easiest targets are organisms substantially different from human cells, primarily bacteria.
    • More challenging for fungi, protozoans (unicellular eukaryotes), helminths (worms), and viruses due to structural and functional similarities with human cells.

Mechanisms of Action Against Bacteria

Inhibition of Cell Wall Synthesis

  • Beta-lactam Antibiotics (e.g., Penicillins, Cephalosporins)

    • Act by targeting the peptidoglycan layer of bacterial cell walls, which is a component not found in human cells.
    • Mechanism: Inhibit transpeptidation activity by binding to penicillin-binding proteins (PBPs), preventing cross-linking of peptidoglycan strands.
    • Result: Weakening of cell wall leading to bacterial lysis under osmotic pressure.

  • Osmotic pressure: Higher in bacteria than human cells, hence drastic effects when antibiotics are applied.

Effects on Protein Synthesis

  • Examples of Antibiotics:
    • Aminoglycosides (e.g., Streptomycin)
    • Bind to 30S ribosomal subunit causing misreading of mRNA.
    • Tetracyclines
    • Bind to 30S preventing tRNA from attaching, disrupting protein synthesis.
    • Macrolides (e.g., Erythromycin)
    • Bind to 50S ribosomal subunit inhibiting peptide chain elongation.

Inhibition of Nucleic Acid Synthesis

Quinolones
  • Antibiotics that primarily target DNA gyrase (Topoisomerase II) and Topoisomerase IV, preventing DNA replication.
  • Fluoroquinolones: Enhanced potency via fluorine substitutions.
Rifampin
  • Targets RNA polymerase in bacteria, inhibiting RNA synthesis without affecting eukaryotic RNA polymerase significantly.

Challenges in Drug Development

  • Toxicity and Resistance
    • Issues arise with naturally occurring antibiotics as bacteria may become resistant (e.g., MRSA). Synthetics are more resilient against resistance mechanisms.
  • Development of antivirals targeting viruses is extremely complex due to their reliance on host cellular machinery for replication.

Conclusion

  • Understanding the mechanisms of action and the nature of antimicrobial drugs is crucial for ongoing research and development of effective treatments, particularly in combating resistance in bacterial pathogens.