Selective Toxicity

Overview

Selective toxicity is the foundational principle behind antimicrobial therapy—it refers to the ability of a drug to target harmful microorganisms without damaging host cells. This concept enables the safe treatment of infections by exploiting structural and functional differences between pathogens and human cells. Understanding how selective toxicity works is crucial for prescribing effective antimicrobials while minimising side effects and resistance.

Definition

Selective toxicity is the ability of a drug to harm a pathogen without harming the host. It is essential for the effectiveness and safety of antimicrobial agents.

Microbial Cell Biology & Implications

Host Cells

Human cells are Eukaryotes
OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Bacteria

  • Prokaryotes
    • Lack nucleus and organelles → Highly different from human cells
    • Enables strong selective toxicity for antibacterials
  • Gram-positive bacteria:
    • Thick peptidoglycan cell wall
  • Gram-negative bacteria:
    • Thin peptidoglycan, outer lipopolysaccharide (LPS) layer
    • Harder to penetrate with antibiotics
Bacteria
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Fungi & Parasites

  • Eukaryotes
    • Similar to human cells → Harder to target without host toxicity
    • Explains higher side effect profiles of antifungals and antiparasitics

Viruses

  • Obligate intracellular pathogens
    • Rely on host-cell machinery for replication
    • Antivirals often affect host processes → Limited selective toxicity
Viruses
User:YK Times, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons

Principle of Antimicrobial Therapy

Origins

  • Many antimicrobials are derived from natural organisms
    • Penicillin discovered from Penicillium mold

Selective Toxicity in Action

  • Targets features unique to pathogens (e.g. bacterial cell walls, viral enzymes)
  • Reduces risk to host tissue

Spectrum of Activity

  • Bactericidal: Kills bacteria
    • E.g. Penicillin
  • Bacteriostatic: Inhibits bacterial growth
    • E.g. Tetracycline
    • Preferred in sepsis, to avoid endotoxin release
  • Synergistic combinations:
    • E.g. Aminoglycosides + β-lactams
      • β-lactam damages wall → enhances aminoglycoside entry

Empirical vs Directed Therapy

Broad-Spectrum (Empirical)

  • Covers Gram-positive + Gram-negative
  • Used when pathogen is unknown

Narrow-Spectrum (Directed)

Antimicrobial Resistance

  • Bacteria evolve rapidly → High mutation rate
  • Can share resistance genes (via plasmids, conjugation)
  • Selective pressure from antibiotics → Promotes survival of resistant strains

“Restraint in antimicrobial use is the best way to ensure long-term efficacy.”

Common Sites of Selective Toxicity

Sites for Selective Toxicity

Summary

Selective toxicity underpins antimicrobial therapy by allowing drugs to specifically target microbial structures without damaging host tissues. Understanding microbial differences helps tailor effective, evidence-based treatments while minimising side effects and resistance. For a broader context, see our Pharmacology & Toxicology Overview page.

Shopping Cart
Scroll to Top