What does the fourth Koch postulate require?

• A. Must cause disease when inoculated into an animal.
• B. Agent must be present in every case.
• C. Must be recovered from the animal and identified.
• D. Must be isolated and grown in pure culture.

Answer: (C)

After you induce the disease in a susceptible host, the same agent must be recovered from that animal and identified. This step ties the disease directly to the same microorganism, confirming that the illness seen in the experimental host is caused by that specific agent and not by a different microbe or a contaminant. The requirement to cause disease in an animal is a separate step, the agent being present in every case relates to the initial observation of the disease in hosts, and isolation in pure culture is the second step. Modern practice sometimes uses molecular methods when cultivation isn’t feasible, but the idea remains: re-isolate and identify the same organism to complete the causal link.