Which of the following correctly lists the four steps of Koch's postulates in the order they are normally applied?

• A. 1. The pathogen must be isolated and grown in pure culture. 2. The suspected pathogen must be present in all cases of the disease. 3. The same pathogen must be re-isolated from the experimentally infected host. 4. The cultured pathogen must cause disease when introduced into a healthy host.
• B. 1. The suspected pathogen must be present in all cases of the disease and absent from healthy organisms. 2. The pathogen must be isolated and grown in pure culture. 3. The cultured pathogen must cause disease when introduced into a healthy host. 4. The same pathogen must be re-isolated from the experimentally infected host.
• C. 1. The suspected pathogen must be present in healthy organisms. 2. The pathogen must be isolated and grown in mixed culture. 3. The cultured pathogen must not cause disease in a healthy host. 4. The same pathogen must be re-isolated from any host.
• D. 1. The same pathogen must be re-isolated from the experimentally infected host. 2. The suspected pathogen must be present in all cases of the disease. 3. The pathogen must be isolated and grown in pure culture. 4. The cultured pathogen must cause disease when introduced into a healthy host.

Answer: (B)

Verified step by step guidance

1. Step 1: Identify the first step of Koch's postulates, which is to confirm that the suspected pathogen is present in all cases of the disease and absent from healthy organisms. This establishes a correlation between the pathogen and the disease.
2. Step 2: The second step is to isolate the suspected pathogen from the diseased host and grow it in pure culture. This ensures that the pathogen can be studied independently of other microorganisms.
3. Step 3: Next, introduce the cultured pathogen into a healthy, susceptible host to see if it causes the same disease. This step tests the causative role of the pathogen in producing the disease symptoms.
4. Step 4: Finally, re-isolate the same pathogen from the experimentally infected host. This confirms that the disease was caused by the same microorganism originally isolated.
5. By following these steps in order, Koch's postulates provide a systematic method to link a specific microorganism to a specific disease.