Student Sheet 18 – Koch’s Postulates

Robert Koch was a bacteriologist who lived from 1843 to 1910. In his work on diseases, he was trying to establish whether a microbe, obtained from a diseased patient, was in fact the cause of the disease. This led him to formulate ‘Koch’s Postulates’ – a set of conditions which need to be fulfilled to establish which organism is causing a particular disease.

Many plant pathogens are safe to use in a school or college laboratory (but simple safety precautions must be observed – see safety notice below). If you attempt to isolate a pathogen from infected fruit, vegetable, or other plant tissue, you are likely to obtain a plate with several organisms growing on it. This is because, once a plant becomes weakened by disease, secondary saprophytic organisms often invade. So, when identifying which is the disease-causing organism, the first step is to demonstrate Koch’s Postulates.

To demonstrate Koch’s Postulates, you must do the following:

(i) Describe and record the symptoms shown.

(ii) Isolate the suspected pathogen from the infected plant material and establish a pure culture.

(iii) Use the pure culture to infect new plant material.

(iv) Describe and record the symptoms shown by the new plant. Check that these are the same as your original observations.

(v) Re-isolate the organism. Check that this is the same as that isolated previously.

This protocol allows students to demonstrate and test Koch’s Postulates, using apples infected with the fungus Penicillium expansum. The required experimental work extends over 3 to 4 weeks, but the essential steps can be demonstrated in a single practical session if students are provided with cultures and infected fruit which represent different stages in the sequence of steps.