Bacteriology 102: Dichotomous Keys
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A dichotomous key shows how various biological entities can be differentiated from each other by indicating their opposing reactions in carefully-chosen tests and observations. In a previous biology course, you were probably exposed to such keys written out in paragraph form such as in the following example, adapted from the very old and outdated text Freshwater Algae of the United States by Gilbert M. Smith (McGraw-Hill, 1950):
With the first pair of observations (numbered "1"), one genus (Distigma) is immediately differentiated from the others by being the only one with two flagella per cell. Those genera with one flagellum need further differentiation which is accomplished by succeeding pairs of observations (numbered "2" and "3"); sooner or later, all of the genera are differentiated from each other.
Dichotomous keys can also apply to non-biological entities as shown in the following example where six objects are fully differentiated based on some obvious physical characteristics. In this key, the inverted tree form is followed, and each individual object is at the very end of its own "branch."
In bacteriology, such keys are occasionally seen, but it cannot be emphasized enough that differentiation of genera – such as in the multi-genera Family Enterobacteriaceae – is not always so clear-cut, and a table showing the reactions (sometimes "variable") of the many tests necessary to differentiate the organisms is often preferred over a dichotomous key. Go to such texts as Bergey's Manual or the Manual of Clinical Microbiology to see many examples of such tables.
We illustrate the usefulness of a dichotomous key in our Experiment 7 to differentiate our set of known species such that relevant testing and ultimate identification of the unknown organisms (derived from the same list) are facilitated. The following shows what we may come up with. (Click on the image to see the associated pdf handout in full/expandable view in separate window.)
There is no rule that says any particular test or observation has to be found at the same level across the width of the key, but the most "primary" tests should be used first (such as gram reaction, cellular morphology, catalase reaction, glucose fermentation) before other tests are utilized (lactose fermentation, amylase, etc.).
Dichotomous keys must not be confused with flow charts. One can run any number of tests at any given time and obtain results for more than one level of the key. For Experiment 17 – as indicated in the "pdf handout" above – you will be constructing your own key for the 12 genera listed in the experiment to help you speedily identify your three unknowns.