Overview
This lecture explains the concepts of Infectious Dose-50 (ID-50) and Lethal Dose-50 (LD-50), their significance in microbiology, and provides real-world examples to illustrate variability among organisms and toxins.
Infectious Dose-50 (ID-50)
- ID-50 represents the number of organisms needed to infect 50% of exposed individuals.
- Lower ID-50 values indicate a more easily transmittable or virulent microbe.
- ID-50 values vary by organism, for example:
- Salmonella Typhi: ~1,000 organisms (causes typhoid fever, ~10% mortality rate).
- E. coli O157:H7: as low as 4–100 organisms (highly infectious).
- Enterotoxigenic E. coli: 10 million–10 billion organisms required.
- Vibrio cholerae (cholera): ~1 million organisms.
- Portal of entry affects ID-50, e.g. Bacillus anthracis:
- Cutaneous: 10–50 endospores.
- Inhalation: 10,000–20,000 endospores.
- Ingestion: 250,000–1,000,000 endospores.
- ID-50 values are median values and can vary based on host health.
Lethal Dose-50 (LD-50)
- LD-50 is the dose required to kill 50% of exposed individuals.
- Often used to describe toxin potency, not just pathogens.
- Examples of LD-50 for various toxins (in nanograms per kilogram body weight):
- Botulinum toxin: 0.03 ng/kg (extremely potent).
- Shiga toxin: 250 ng/kg.
- Staphylococcal enterotoxin: 1,350 ng/kg.
- LD-50 does not indicate infectiousness, but rather lethality.
Key Terms & Definitions
- ID-50 — Infectious Dose-50; number of organisms causing infection in 50% of hosts.
- LD-50 — Lethal Dose-50; amount of agent causing death in 50% of hosts.
- Virulent — Highly infectious or able to cause disease easily.
- Portal of entry — The route by which a microbe enters the host body.
Action Items / Next Steps
- Review examples of ID-50 and LD-50 for various pathogens and toxins.
- Note the significance of portal of entry and host factors on infectious and lethal doses.