Bacterial Toxins: Exotoxins and Endotoxins

Bacterial Toxins

• Some microorganisms, especially bacteria, disrupt host cell/tissue function by:
  • Directly damaging cells/tissues.
  • Changing cellular functions.
• Toxin production isn't universal among microorganisms; for some bacteria, it's the primary determinant of pathogenicity.
  • Loss of toxin production equates to loss of disease-causing ability.
• Viruses don't produce toxins; they damage cells via other mechanisms.

Types of Toxins

• Two main types:
  • Exotoxins
  • Endotoxins
• Important Note: These terms are distinct from "exogenous" and "endogenous" disease.
  • Exotoxins and endotoxins relate to the toxin's origin relative to the bacterial cell.

Exotoxins

• Secreted by bacteria into their environment.
  • Can travel to distant sites within the host.
  • Can be released into food, soil, or water.

Endotoxins

• Integral part of bacterial cell structure.
  • Typically found in the cell membrane (outer cell membrane of Gram-negative bacteria).
  • Released when the bacterial cell is damaged or dies.

Exotoxins: Detailed Characteristics

• Produced by a limited number of bacteria.
• Usually proteins.
• Heat-labile: Heating renders them non-toxic.
• Secreted or released from bacterial cells.
• Exhibit specific effects on host cells.
• Generally highly toxic.
• Stimulate antibody production in the host.
• Can be modified into non-poisonous forms (toxoids) for immunization purposes.
• Some bacteria produce single exotoxins, others multiple.
• Some exotoxins are extremely potent, e.g., botulinum toxin.
  • Botulinum toxin (Clostridium botulinum): 10^{-7} grams (one hundred millionth of a gram) can kill an adult male.
  • One gram could kill nearly half of Australia's population.
  • 300 grams (approximately one cup) could kill the entire world population.
• Most bacteria produce toxins while actively growing.
• Clostridium botulinum and Staphylococcus aureus can secrete toxins into food, leading to ingestion by humans.

Categories of Exotoxins

• Neurotoxins: Affect the nervous system.
• Enterotoxins: Affect the gut.
• Toxemia: Affect the blood system.

Toxins That Break Down Cells

• Some exotoxins cause host cell lysis (rupture).
  • Damage the host cell membrane structure.
  • Form donut-shaped structures that insert into the membrane, creating pores or channels.
  • Pore formation allows water to enter, causing the cell to burst.
• Hemolysins: Toxins that damage red blood cells.
  • Example: Streptococcus pyogenes grown on blood agar.
    • Alpha type and Gamma type: does not cause rupture.
    • Beta type: causes hemolysis which makes the agar clear.
      • Beta-hemolytic Streptococcus causes strep throat.
      • Produces a clear zone around the bacterial colony on blood agar, aiding in identification.

Toxins That Interfere with Cellular Functions

• Clostridium diphtheriae toxin:
  • Binds to receptors on the host cell membrane and enters via endocytosis.
  • Blocks protein synthesis by inhibiting a specific enzyme.
  • Results in cell death.
  • Causes diphtheria:
    • Presents with a sore throat.
    • Formation of a pseudomembrane consisting of dead cells infected with the toxin.
    • In children, this membrane can obstruct the airway, leading to a high fatality rate.
    • Overall fatality rate in humans is 5-10%, with most deaths occurring in children.
    • Historical context: Diphtheria outbreak in Adelaide in the 1850s, with accounts of children's deaths due to airway obstruction.