Koch’s postulates are a series of rules developed by the German scientist Robert Koch, used to identify if a disease was caused by a microorganism.
A sample of microorganisms are isolated from the infected subject. These are then grown in pure culture, and the culture administered to a new test subject. If the new test subject also develops the disease, the microorganisms should be grown in pure culture again, and the microbes studied microscopically to ensure they are the same as the original culture isolated from the first test subject.
Koch’s postulates cannot be used in many circumstances: the pathogen cannot secrete a toxin, as it is the toxin that causes the disease and not the pathogen in this case. Pathogens with fastidious/unknown growth requirements are also not able to satisfy Koch’s postulates – this is due to the inability to grow the microorganism in pure culture. If there is no suitable animal model available for the microorganism of interest, it is not possible to satisfy Koch’s postulates. Growing some pathogens in culture can lead to a loss of virulence – also causing the postulates to fail.
However, where Koch’s postulates can be satisfied, evidence of the disease caused by a given microorganism is acquired.