Infection, Pathogenicity, and Virulence – Bacterial Toxins

🦠 Core Concepts

Pathogenicity

• The ability of an organism to cause disease by harming the host.

Virulence

• The degree of pathogenicity, indicating how severe the disease caused by a pathogen can be.

Virulence Factors

• Molecules, structures, or systems that enable microorganisms to cause disease.

Bacterial Toxins

• A type of virulence factor; these are substances produced by bacteria that are harmful to host cells.

🧬 Bacterial Pathogenesis

Entry and Movement

• Bacteria use motility mechanisms (e.g., flagella) to reach entry sites like wounds.

Purpose of Host Damage

• Facilitates invasion by weakening barriers or immune responses.

• Accesses nutrients through lysed host cells.

• Reduces competition by eliminating other microbes.

• Aids dissemination via symptoms like diarrhoea or coughing.

• Sometimes, damage is accidental.

🧪 Host Response vs. Disease

• Highly virulent pathogens: Consistently cause disease.

• Less virulent pathogens: Cause disease in weakened hosts or when present in large numbers.

🧫 Extracellular Enzymes

Hyaluronidase and Collagenase

• Break down connective tissues, aiding bacterial spread, increasing the severity of infections and allowing for greater invasion of host tissues.

Coagulase and Kinase

• Coagulase induces clot formation, shielding bacteria.

• Kinase dissolves clots, allowing dissemination.

☠ Bacterial Toxins

Endotoxins

• Found in Gram-negative bacteria; released upon bacterial death.

• Composed of lipopolysaccharides (LPS).

• Trigger strong immune responses, potentially leading to septic shock.

Exotoxins

• Secreted by both Gram-positive and Gram-negative bacteria.

• Highly potent and specific; can act far from the infection site.

• Examples include toxins causing diphtheria, botulism, and tetanus.

🔬 Types of Toxins

Cytotoxins

• Target and destroy host cells.

• Example: Staphylococcus aureus α-toxin forms pores in cell membranes, leading to cell lysis.

Superantigens

• Bypass normal antigen presentation, activating a large proportion of T-cells.

• Cause massive cytokine release, leading to toxic shock syndrome.

AB Toxins

• Consist of two parts: A (active) and B (binding).

• The B subunit binds to host cells, allowing the A subunit to enter and disrupt cellular functions.

• Often interfere with protein synthesis or signal transduction.

🧬 Specific Toxins

Cholera Toxin

• Produced by: Vibrio cholerae.

• Mechanism:

  • Binds to GM1 ganglioside receptors on intestinal cells.

  • Activates adenylate cyclase, increasing cAMP levels.

  • Leads to chloride and water efflux, causing severe diarrhoea.

Botulinum Toxin

• Produced by: Clostridium botulinum.

• Mechanism:

  • Blocks acetylcholine release at neuromuscular junctions.

  • Results in flaccid paralysis.

• Applications:

  • Used medically to treat muscle spasms and cosmetically to reduce wrinkles.

Tetanus Toxin

• Produced by: Clostridium tetani.

• Mechanism:

  • Inhibits release of inhibitory neurotransmitters (e.g., glycine).

  • Causes continuous muscle contraction (spastic paralysis).

• Prevention:

  • Vaccination with tetanus toxoid.

💉 Toxoids and Vaccination

• Toxoids: Inactivated toxins that retain their antigenicity.

• Usage:

  • Employed in vaccines (e.g., Diphtheria, Tetanus, Pertussis) to elicit immune responses without causing disease.