In Koch's postulates, what does 'must be isolated and grown in pure culture' imply?

• A. The agent must be present in every case.
• B. The agent must be isolated and grown in pure culture.
• C. The agent must cause disease upon inoculation.
• D. The agent must be recovered from the host.

Answer: (B)

The important idea here is that the suspected microorganism must be obtainable as a pure culture—grown in isolation from other microbes. This means you can separate exactly one organism from a mixture and grow it alone, so you have a known, uncontaminated inoculum. With that pure culture, you can test whether introducing it into a healthy susceptible host reproduces the disease, which helps prove causation. This purification step is about having a single, identifiable organism to study and to use in experiments; it isn’t itself a statement about how often the agent appears in disease or about the outcome after inoculation, which are addressed by the other steps. It also doesn’t entail recovering the agent from the host at this moment—another postulate covers that recovery. The essence is that isolating and growing the organism in a pure culture provides a definitive, controllable way to link a specific microbe to the disease.