Ask students: How do scientists establish cause? (Answer: Experiments.) So what was missing from our investigation earlier that would make it an experiment? (Answer: A control group.)
Tell students (using slides) they have the following materials to work with to design their experiment:
1.) As many healthy cows as they need
2.) Liquid culture of B. anthracis in test tubes
3.) Liquid culture of P. ruminicola in test tubes
Ideally, students will devise an experiment of infecting a group of cows with B. anthracis (the test group) and another group with P. ruminicola (the control, presumed safe). If B. anthracis is the cause of the disease, the test group will die and the control survives. If B. anthracis is benign, all cows will live. If giving cows large quantities of bacteria in general is harmful, cows in both groups will die.
Note that this lesson is based on methods used in historical medicine, in which it was assumed to be acceptable to run experiments on animals. Medical science has moved increasingly away from using animals in experimentation, especially as scientific technology improves. The case against animal testing, from the Physicians Committee for Responsible Medicine: http://www.pcrm.org/sites/default/files/pdfs/research/research/Problems-Associated-with-Animal-Experimentation.pdf
After students have designed their experiments, reveal that they have rediscovered the experiments of Robert Koch, who proved that B. anthracis was the cause of anthrax outbreaks in cattle in 19th-century Germany. Koch’s postulates are as follows:
1.) Microbe must be found in sick individuals but NOT in healthy ones.
2.) Microbe is cultured from sick individuals.
3.) Microbe should cause disease when introduced to healthy individuals.
4.) Exact same microbe is cultured from newly sick individuals.
The first two steps are to establish correlation. The third step establishes causation. The fourth double-checks correlation.
Allow for students to solidify their knowledge by practicing with the steps using tuberculosis as an example. The last question on the handout asks: What is a potential problem with using Koch’s postulates?
Students should be able to identify at least one problem, including: the ethics problem of infecting humans and animals, not all diseases are caused by bacteria, bacteria can mutate and change.