Question

Which of the following statements are part of Koch’s Postulates? Check all that apply.

The disease-causing microorganism must be present in at least half of the cases of disease.

The microorganism can be isolated and grown in a pure culture.

The microorganism from the pure culture will cause the disease in at least 80% of healthy organisms.

A re-isolated microorganism may evolve into a different species than the original.

Answer 1

Only the second statement is part of Koch’s postulates:

- The microorganism can be isolated and grown in a pure culture. — Correct (Koch’s 2nd postulate).

Why the others are wrong:
- Must be present in at least half of cases — Incorrect. Koch required the pathogen be present in all cases of the disease (and absent from healthy individuals).
- Will cause disease in at least 80% of healthy organisms — Incorrect. Koch required that the isolated organism cause disease when introduced into a susceptible healthy host (no “80%” threshold).
- A re-isolated microorganism may evolve into a different species — Incorrect. Koch’s 4th postulate requires re-isolation of the same microorganism (i.e., identical to the original).