Your instructor will show you an example of a simple dichotomous key using geometric shapes (see below). The key is based on looking for features that distinguish one shape from another. Similarly, unknown bacteria can be identified by looking for tests that distinguish one type of bacteria from another based on their different properties. Dichotomous keys for bacterial unknowns are based on Gram reactions, morphologies and metabolic assays like the ones you have learned in lab. Once you understand the concept of developing a dichotomous key, you will design your own keys that will allow you to identify your particular unknowns based on these characteristics. There are many ways to make a useful key, so your key may not resemble those made by other students.
Although additional staining can sometimes be used as part of a dichotomous key, you will be only using the metabolic tests for this exercise.
Once you have completed your dichotomous key, review the gram reaction and morphologies of your two unknowns. Based on these results, decide which media should be used to identify your unknowns, and write them down in the space provided on the back of your dichotomous key. When you are finished, turn it in to your instructor. You can do this during the lab period (please make sure it is legible), or you can take it home to work on it, provided you return it to your instructor at least 3 days before your next scheduled lab. Your instructor will check it to make sure it is correct and that you have identified the correct media to use to identify your unknowns. Your key will be graded and returned to you by the next lab period when you will inoculate your chosen media with your unknowns. Points will be deducted for late dichotomous keys.
Figure 6.2.1: Sample dichotomous key