Koch's postulates concerning the causative agent of a disease:

A.) include the concept that the organism must be obtained in pure culture

B.) are dependent upon isolating the DNA of the organism

C.) depends upon finding the Gram reaction

D.) have not been valid since the early 1900's

The answer is A, include the concept that the organism must be obtained in pure culture

More specifically, Koch's postulates are a series of criteria which, if met, are a strong indication that a particular microorganism is the causitive agent of a particular disease. This microorganism must

1) be found in organisms suffering from the disease
2) be isolated from the tissues of the diseased organism and grown in pure culture,
3) cause disease when inoculated into a healthy yet susceptible organism and
4) be reisolated from the tissues of the experimentally inoculated diseased host and found to be identical to the originally isolated microorganism.