Bacterial virulence factors

Adhesion & Biofilms - Sticky Business

• Adhesion: The crucial first step in pathogenesis, mediated by bacterial adhesins.

  • Pili (Fimbriae): Hair-like protein appendages for attachment.
    • E. coli in UTIs (Type 1 pili).
    • N. gonorrhoeae (Type IV pili).
  • Afimbrial Adhesins: Surface proteins, e.g., M protein of Streptococcus pyogenes.

• Biofilms: Structured communities of bacteria encased in a self-produced polymeric matrix.

  • Adhere strongly to surfaces (e.g., catheters, prosthetic joints, teeth).
  • Provide significant protection from antibiotics and host immune responses.
  • Key examples: Pseudomonas aeruginosa (CF lungs), Staphylococcus epidermidis (devices).

⭐ Biofilms on prosthetic devices are a primary cause of chronic, treatment-resistant infections. S. epidermidis is a classic example.

Immune Evasion - Hide & Seek Champs

Bacteria employ clever tactics to survive host defenses. Key strategies involve hiding from or inactivating immune components like phagocytes and antibodies.

• Capsules (The Invisibility Cloak)

  • A polysaccharide layer that prevents opsonization (by C3b) and phagocytosis.
  • 📌 Mnemonic: Some Killers Have Pretty Nice Big Capsules
    • S. pneumoniae, K. pneumoniae, H. influenzae, P. aeruginosa, N. meningitidis, B. anthracis, C. neoformans

• Antigenic Variation

  • Altering surface proteins to evade recognition by antibodies.
  • Classic example: Neisseria gonorrhoeae pilus protein variation.

• Intracellular Survival

  • Bacteria live inside host cells to hide from the immune system.
  • Listeria & Salmonella escape the phagosome or prevent its fusion with the lysosome.

⭐ Listeria monocytogenes uses host cell actin to create "actin rockets," propelling it from one cell directly into another, avoiding extracellular immune surveillance entirely.

Toxins - Toxic Shock & Awe

• Endotoxin (LPS): Component of the outer membrane of Gram-negative bacteria, released upon cell lysis.

  • Mechanism: Lipid A moiety binds TLR4 on macrophages, triggering massive release of cytokines (IL-1, IL-6, TNF-α).
  • Causes fever, hypotension, DIC, and septic shock.

• Exotoxins: Secreted proteins from Gram (+) and Gram (-) bacteria. Highly potent and specific.

⭐ Superantigens (e.g., S. aureus TSST-1) cross-link MHC-II and T-cell receptors, causing massive polyclonal T-cell activation and a cytokine storm, leading to toxic shock syndrome.

📌 Mnemonic: EXotoxins are EXcreted.

Secretion Systems - Special Delivery

Bacterial protein complexes that inject virulence factors (effectors, toxins) directly into host cells, acting like molecular syringes.

• Type III (Injectisome): Syringe-like apparatus in many Gram-negative bacteria.
  • Examples: Salmonella, Shigella, E. coli, Pseudomonas.
• Type IV: Versatile system that can transfer proteins and DNA.
  • Examples: H. pylori, Legionella pneumophila.

⭐ Helicobacter pylori uses a Type IV secretion system to inject CagA, an oncoprotein linked to the development of gastric cancer.

High‑Yield Points - ⚡ Biggest Takeaways

• Adhesion via pili/fimbriae is the crucial first step for colonization.
• Capsules prevent phagocytosis; classic examples are S. pneumoniae, H. influenzae, and N. meningitidis.
• Endotoxin (LPS) in Gram-negatives binds TLR4, triggering massive cytokine release and septic shock.
• Exotoxins are highly potent secreted proteins, often with an A-B subunit structure, that target specific cellular processes.
• Antigenic variation allows pathogens to evade the host immune response by altering surface antigens.
• Biofilms create a protective matrix, conferring antibiotic resistance.