Bacteriology 102: Dichotomous Keys

The lab manual referred to herein (with its page numbers and experiments) is referenced here.

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exposed to such keys written out in paragraph form such as in the following example, adapted from the very old and outdated text Freshwater Algae of the United States by Gilbert M. Smith (McGraw-Hill, 1950):

Differentiation of Genera in the Family Astasiaceae of the Euglenophytes:
1.  With one flagellum	2
1.  With two flagella	Distigma
2.  Cells strongly plastic	Astasia
2.  Cells rigid or slightly plastic	3
3.  Cells radially symmetrical	Rhabdomonas
3.  Cells not radially symmetrical	Menoidium

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 can make use of a dichotomous key in Experiment 7 to differentiate our twelve known species such that relevant testing and ultimate identification of our Exp. 7.2 unknowns are facilitated. The following shows how such a key can be made; it is left incomplete with only "EF" and "PF" (Enterococcus faecalis and Pseudomonas fluorescens) fully differentiated from the rest. 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, morphology, catalase reaction, glucose fermentation) before other tests are utilized (lactose fermentation, amylase, etc.).

We also find a dichotomous key useful to differentiate the genera in Experiments 12, 13 and 14. This key is found on page 150 of the lab manual and is reproduced here. As noted on the key, it is incomplete in that all of the different genera, especially the "enterics," are not differentiated fully; we may complete the key on the web at a later time.

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, you will be constructing your own key for the 12 genera shown on page 106 to help you speedily identify your three unknowns.

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Page last modified on 6/21/01 at noon.

John Lindquist, Department of Bacteriology

University of Wisconsin – Madison