Module 2 Home

Biological Lab

Social Science Issue

Process of Science

Web Exercise

How do scientists decide what is the cause of a disease?	Go to Quiz
In Module 2 - Process of Science, you learned about hypotheses and how they drive scientific discovery. Here you will learn how hypotheses concerning the identity of pathogens are tested

Part I: Koch's Postulates Imagine that you are a doctor examing a sick individual, or a plant pathologist examining a looking at a diseased plant. In order to prescribe the best treatment, you need to know what is causing the disease symptoms. Isolate or identify the organism causing the disease using one of more tests such as: culturing the organism from diseased tissue, immunological tests, or PCR. If an agent is identified that has been previously associated with the symptoms, treatment can proceed But sometimes a bacterium, virus, or fungus will be found that has not been previously associated with the symptoms you see. Question: How do you tell if this organism is really the cause of the disease? Answer: Apply Koch's Postulates! What are Koch's Postulates? Koch's Postulates are four rules that must be met before a parasite of pathogen can be considered the cause of a disease. (they are named for Robert Koch, who published them in 1890). These rules are: The organism must be found in all plants or animals suffering from the disease, but not in healthy ones. The organism must be isolated from a diseased plant or animal and grown in pure culture. The cultured organism should cause disease when introduced back into a healthy plant or animal. The organism must be reisolated from the experimentally infected plant or animal. Sometimes Koch's Postulates are difficult to fulfill, especially if the conditions for growing an organism in culture have not been figured out. In these cases, the link between the possible agent and the disease is not as strong. An example experiment for using Koch's Postulates to demonstrate pathogenicity can be found here Part II: QUIZ What did you learn during this module?  Find out with this online Quiz!