How to create a dichotomous key for bacteria

Dichotomous Key

As you have learned, biochemical test results can be used to identify unknown species of microorganisms. You have analyzed results from several biochemical tests now, and so you should be aware that biochemical tests are performed to determine which enzymes/metabolic pathways are present and active in a particular species. When this is done using an unidentified (unknown) organism, you can compare test results to many known species to determine the identity of the unknown species. You will be doing this in the Bacterial Unknowns assignment soon.

First, to prepare for the process of identifying an unknown species, you must construct a dichotomous key that you will later use.

STEP 1: Read This Introduction to Dichotomous Keys
Dichotomous means “dividing into two parts.” As discussed in lecture, a dichotomous key is a flow chart that is constructed to organize information that is useful to distinguish between different species. It consists of a series of characteristics of which there are two varieties, so that different species can be divided based on differences in these characteristics.

When constructing a dichotomous key that may be used to identify bacteria, we start by dividing a group of bacteria into two groups based on a certain characteristic, such as the Gram-stain reaction. Then, each of the two subgroups is subdivided into two groups based on a second characteristic, and so on. For example, a dichotomous key used to identify unknowns from a group of four Gram-negative cocci bacteria might look like the following:

example dichotomous key.jpg

As you can see, in order to construct a dichotomous key, you need several test results from known species.

STEP 2: Examine the Data
Carefully review the “Summary Biochemical Test Results” data for the twelve species in the Excel file below. In this file are biochemical test results from twelve different bacterial species. These are the data that you will use to build your dichotomous keys.

Unknown Summary List.xlsxPreview the document

List of the twelve species included:

Escherichia coli
Enterobacter aerogenes
Klebsiella pneumoniae
Proteus vulgaris
Serratia marcescens
Staphylococcus aureus
Staphylococcus epidermidis
Enterococcus faecalis
Kocuria rhizophila
Bacillus subtilis
Corynebacterium xerosis
Bacillus cereus
STEP 3: Construct Your Dichotomous Keys
In order to identify unknown organisms from the list of twelve bacteria, you will need to develop three dichotomous keys (flowcharts), one for each of the following groups of bacteria:

Gram-Negative Bacilli
Gram-Positive Cocci
Gram-Positive Bacilli
Keep the following in mind as you build your dichotomous keys:

The tests that you include in your keys should include tests that are represented in the Summary Biochemical Test Results file.
Hint: select which tests to include by determining which ones will be most helpful to subdivide members of each group.
There’s more than one way to build a dichotomous key from the given data. That means that your dichotomous keys might look different from someone else’s, but that doesn’t mean that either of your keys is “wrong.” There are multiple ways to organize the given information. You might choose to include different tests in your dichotomous keys than someone else chose.

Summary of test results landsca
Anaerobic Growth Nitrate H2S Indole Urea DNase Catalase PR Glucose PR Lactose PR Mannitol PR Sucrose Citrate MR VP
Gram Negative Rods:
E. coli + + - + - - + AG AG AG AG - + -
E. aerogenes + + - - - - + A A A A + - +
K. pneumoniae + + - - + - + AG AG AG AG + - +
P. vulgaris + + + + + + + AG - - A - + -
S. marcescens + + - - - + + A - A A + - +
Gram Positive Cocci:
S. aureus + + - - (+) + + A A A A - + +
S. epidermidis + + - - (+) - + A A - A - + +
E. faecalis + - - - - - - A A A A - + +
K. rhizophila - - - - + - + - (amb) - - - - - +
Gram Positive Rods:
B. subtilis - + - - - - + A - + A - - +
C. xerosis + + - - - - + - - - - - - -
B. cereus + + - - - - + A - - A - + +
amb = ambiguious ()=hard to tell