Dichotomous charts---simple taxonomic flow charts used to classify organisms---are used to separate observed organisms based on a variety of factors. Simply put, the flow chart tests every organism for a specific trait and then subsequent sub-traits until species identification is possible. For instance, you could classify a microorganism based on its shape and then, following the flow chart, characterise the organism based on its reaction to certain substances or behaviour until you have reached the bottom of the flow chart and can reliably establish a species name for the organism. Dichotomous charts vary depending on the factors of interest in the experiment, but they can be very useful and are generally very easy to create.
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Things you need
- Unknown organism
Establish the first criterion for your microorganism. In the case of microorganisms, the first criterion used for classification is often Gram staining. Gram positive organisms are those that can retain the crystal violet dye used in Gram staining, while Gram negative organisms do not exhibit the dye colour. Thus, establish Gram staining as your first criteria. All Gram positive organisms will be classified to the left side of your flow chart, while all Gram negative microorganisms will follow the flow chart to the right.
Establish a second criterion in your flow chart. Returning to the example, assuming the goal is to identify certain microorganisms that are Gram negative and have the ability to ferment lactose, we would use the ability to ferment lactose as the second condition in your dichotomous chart. Once again, the group of Gram negative bacteria would be divided by this characteristic and the flow chart would continue under the group that is able to ferment lactose in order to further classify these microorganisms and establish species names.
Create a new dichotomous condition. As an example, you can use indole production (the ability to separate indole from the amino acid tryptophan) as a third condition. The organisms analysed would then be divided into two groups of Gram negative, lactose fermenting microorganisms---one with the ability to produce indole and one without it.
Create new bifurcations in you chart. Indole-producing microorganisms, for instance, could be further divided by their use of citrate as a sole carbon source. Non-indole-producing species could be divided by their methyl red reaction.
Identify the species based on the set of characteristics. Once you have completed several levels in your dichotomous chart, you will usually be able to identify the species from a list of known characteristics of known species of microorganisms. For example, a Gram negative, lactose-fermenting bacteria that produces indoles and uses citrate as a sole carbon source is very likely Citrobacter intermedius. Similarly, a Gram negative, lactose-fermenting bacterium that does not produces indoles and has a negative methyl red reaction is very likely Enterobacter aerogenes.
Repeat the process with all the microorganisms you need to classify, adding steps and criteria as needed depending on the organisms you want to identify and the factors that are of interest in your experiment.
Tips and warnings
- Before you begin to select criteria for your chart, think about the traits that interest you in your classification and the organisms you are likely to encounter. This will organise your chart and save you time when selecting steps.
- Remember that you can apply the same condition to different branches of your dichotomous chart.
- Be sure that you are selecting traits and criteria that are relevant to your study and useful in identifying species. Other traits will make your design unnecessarily long and complicated.
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