Genetic Basis of Bacterial Virulence

Virulence Factors & Genes - Tiny Terrors' Toolkit

• Virulence Factors: Pathogen-produced molecules enhancing disease capability. 📌 CAPTured (Capsules, Adhesins, Pili/PAIs, Toxins).
  • Types:
    • Toxins: Damage host (e.g., exotoxins, endotoxins).
    • Adhesins: Mediate attachment (e.g., pili, fimbriae).
    • Invasins: Facilitate host cell/tissue penetration.
    • Capsules: Evade phagocytosis.
    • Enzymes: Spread/damage tissues (e.g., coagulase).
• Genetic Basis: Virulence genes located on:
  • Bacterial Chromosome
  • Plasmids
  • Bacteriophages (e.g., toxin production via lysogeny)
  • Transposons
  • Pathogenicity Islands (PAIs)

⭐ Pathogenicity islands (PAIs) are large (10-200 kb) chromosomal or plasmid regions containing multiple virulence genes, often acquired by horizontal gene transfer, and frequently associated with tRNA genes or insertion sequences.

Virulence Gene Regulation - Sneaky Switches

Bacteria control virulence genes via:

• Environmental Signals: Host cues:
  • Temperature (e.g., Yersinia Yops at 37°C)
  • pH, osmolarity, ↓iron (diphtheria toxin)
  • Host signals
• Two-Component Systems (TCS): Signal relay. 📌 Sir Kenneth Responds Rapidly.
  • Sensor Kinase (SK): Detects signal, autophosphorylates.
  • Response Regulator (RR): Phosphorylated by SK, alters gene expression.

• Quorum Sensing (QS): Density-dependent.
  • Autoinducers (AIs): Acyl-homoserine lactones (AHLs - Gram-neg), Autoinducing peptides (AIPs - Gram-pos).
  • ↑Cell density → ↑AIs → Coordinated attack.

  ⭐ Quorum sensing in Pseudomonas aeruginosa (AHLs) controls biofilm and virulence factors (elastase, pyocyanin), a key anti-virulence target.

• Global Regulators: Master switches.
  • Fur (iron), H-NS (temp/osmolarity), cAMP-CRP (glucose).

Genetic Exchange & Virulence - Sharing Secrets

Horizontal Gene Transfer (HGT) enables bacteria to acquire virulence genes, Pathogenicity Islands (PAIs), & antibiotic resistance. 📌 Bacteria Get Virulent Through The Clever Conversions (Transformation, Transduction, Conjugation).

• Transformation:
  • Uptake of naked DNA from environment.
  • Requires cell competence.
• Transduction:
  • Bacteriophage-mediated DNA transfer.
  • Generalized: Any bacterial DNA.
  • Specialized: Specific genes adjacent to prophage site.

  ⭐ Specialized transduction is responsible for the transfer of genes encoding potent exotoxins such as diphtheria toxin (Corynebacterium diphtheriae by β-phage) and cholera toxin (Vibrio cholerae by CTXφ phage).

• Conjugation:
  • Plasmid-mediated direct cell-to-cell DNA transfer.
  • Requires F-pilus (sex pilus).
  • Hfr (High-frequency recombination) strains transfer chromosomal genes.

Key Virulence Mechanisms - Masterminds of Mayhem

• Toxins:
  • Exotoxins (secreted):
    • A-B type (e.g., cholera, diphtheria)
    • Membrane-damaging (e.g., hemolysins)
    • Superantigens (e.g., TSST-1, causes cytokine storm)
  • Endotoxin: LPS (Lipid A of Gram-negatives); fever, shock.
• Adhesion: Pili/fimbriae, afimbrial adhesins for attachment.
• Invasion: Invasins (entry), effector proteins (manipulation).
• Secretion Systems: Deliver effectors.
  • Type III (T3SS): "Molecular syringe".
  • Type IV (T4SS - DNA/protein transfer), Type VI (T6SS - contact-dependent).
• Biofilms: Protective communities; ↑resistance, evasion.
• Iron Acquisition: Siderophores (e.g., enterobactin) scavenge Fe.

⭐ Type III Secretion Systems (T3SS), found in many Gram-negative pathogens like Salmonella, Shigella, and E. coli, act like molecular syringes to inject bacterial effector proteins directly into the host cell cytoplasm, subverting host cellular functions.

High‑Yield Points - ⚡ Biggest Takeaways

• Virulence factors like toxins and adhesins are genetically encoded.
• Virulence genes reside on chromosomes (Pathogenicity Islands - PAIs), plasmids, or bacteriophages.
• PAIs are large, distinct DNA segments acquired by Horizontal Gene Transfer (HGT).
• HGT (transformation, transduction, conjugation) facilitates rapid spread of virulence.
• Plasmids frequently carry genes for toxins (e.g., Anthrax) and antibiotic resistance.
• Lysogenic bacteriophages transfer toxin genes (e.g., Diphtheria, Cholera, Shiga toxins).