Bacterial Toxins Summary

Bacterial Toxins (Section 4A)

Introduction

• Some bacteria interfere with host cells or tissues by:
  • Directly damaging cells or tissues.
  • Changing the way cells work.
• A minority of microorganisms produce toxins.
• For some bacteria, toxin production is the primary determinant of pathogenicity (ability to cause disease). If the bacteria loses the ability to produce toxins, it will lose its ability to cause disease.
• Viruses do not produce toxins; they cause cell damage through other mechanisms.

Types of Toxins

• Two main types: exotoxins and endotoxins.
• Important Note: These terms are different from "exogenous" and "endogenous" (referring to the origin of disease).

Exotoxins

• Secreted into the environment where the bacteria are growing.
• Can travel to distant sites (tissues) or be released into food, soil, or water.

Endotoxins

• Part of the bacterial cell structure, typically the cell membrane or outer cell membrane of Gram-negative bacteria.
• Not actively secreted but released when the bacterial cell is damaged or dies.

Exotoxins vs. Endotoxins

• Exotoxins:
  • Produced by a few bacteria.
  • Usually proteins.
  • Heat-labile (destroyed by heat).
  • Secreted or released from the bacterial cell.
  • Specific effects on host cells.
  • Very toxic.
  • Stimulate the host to produce antibodies.
  • Can be modified into toxoids for immunization.
• Some bacteria produce a single exotoxin, while others produce many.
• Potency: 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 almost half the population of Australia.
  • Three hundred grams could kill the entire world population.
• Most bacteria produce toxins while actively growing, but some (e.g., Clostridium botulinum, Staphylococcus aureus) can secrete toxins into food, leading to ingestion of the toxin.

Categories of Exotoxins

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

Toxins That Break Down Cells

• Some toxins cause host cell lysis (rupture).
• Damage the host cell membrane structure.
• Form a donut shape, inserting themselves into the cell membrane and creating a pore or channel.
• Water enters the cell through the pore, causing it to burst (lyse).
• Toxins that damage red blood cells are called hemolysins.
• Example: Streptococcus pyogenes on blood agar:
  • Alpha type: No significant change.
  • Beta type: Clear zone around the bacterial colony due to red blood cell rupture.
    • Beta-hemolytic streptococcus is responsible for strep throat.
  • Gamma type: No significant change.

Toxins That Interfere with Cellular Functions

• Clostridium diphtheriae toxin:
  • Binds to a receptor on the host cell membrane and enters by endocytosis.
  • Blocks protein synthesis by stopping a particular enzyme from working.
  • The cell dies.
  • Causes diphtheria, presenting with a sore throat and a pseudomembrane (dead cells infected with the toxin) in the throat and nasal cavities.
  • Higher fatality rate in children due to airway obstruction.
  • Overall, 5-10% case fatality rate in humans, mostly in children.
  • Historical example: Outbreak in Adelaide in the 1850s.
    • Symptoms: Inflamed throat, white spots that increase until they choke up the windpipe.