Understanding Koch's Postulates

Koch's postulates are a set of criteria used to establish a causal relationship between a specific microbe and a specific disease. They were developed by Robert Koch.

Koch's Four Postulates

• Postulate 1: The microorganism must be present in every case of the disease and absent from healthy individuals.
• Postulate 2: The microorganism must be isolated from the diseased host and grown in pure culture.
• Postulate 3: The cultured microorganism must cause the same disease when introduced into a healthy, susceptible host.
• Postulate 4: The microorganism must be re-isolated from the experimentally infected host and shown to be the original, identical microorganism.

Analyzing the Options

Let's examine each option based on Koch's postulates:

• Option 1: "The same organism must be present in every case of the disease." This aligns with the first part of Postulate 1.
• Option 2: "The organism must be isolated from the diseased host and grown in culture." This is exactly Postulate 2.
• Option 3: "The isolate must cause the disease when incubated." This corresponds to Postulate 3, where the cultured organism causes disease in a new host.
• Option 4: "The organism may not be reisolated from the incubated diseased host." This statement directly contradicts Postulate 4, which requires the organism *to be* reisolated.

Conclusion

The statement that contradicts Koch's postulates is that the organism may not be reisolated. The fourth postulate explicitly states the organism *must* be reisolated from the host it caused disease in.