Lab 6 - Prokaryotic Identification and Flow Charts

Following this exercise the student should be able to:

1.      Explain key characteristics in differentiating prokaryotes

2.      Interpret flow charts for identification


3.  Create flow charts for identification



On the chart below briefly describe what or how these identification characteristics are used to make conclusions. (See the first box for a sample answer)


                                      Criteria for Classification of Prokaryotes



Microscopic Morphology



Growth Characteristics

Metabolic Pathways



Location in Broth

Displays aerobic optimums

Cell Shape

Cell Wall

pH tolerance

Carbon requirements

DNA base


Colony Appearance




Gram Stain

Temperature requirements

End Products Fermentation/ Respiration







Internal Structures






Sensitivity to chemicals

Sugar needs










 Antibiotic sensitivity





















Many different characteristics are used to identify bacteria. Identification charts/tables are difficult to follow. As a result microbiologists often depend upon flow charts to arrive at a preliminary identification for an organism. Flow charts are created from known results concerning bacteria. When unknown bacteria are subjected to a battery of tests the flow chart can help direct and minimize the investment of time, energy and media. These flow charts are called dichotomous keys.

Each step in the flow chart asks a question indicated by the information in the square. It is helpful to ask a yes no question and to always place the yes or positive answer on the left and the no or negative answer on the right.


Do you recognize the questions as you flow down the chart? What a question that could be asked at the Eukaryotic box to get answers that bifurcate (divide into two) organisms that are identified as algae versus those that are identified as protozoans?

Remember these are presumptive flow charts – there are often answers that do not fit nicely into the flow, which would then require more information. Can you think of an organism that would not be classified cleanly by this flow chart from the kingdom Protista?

Now create a simple dichotomous key or flow chart for the Multicelled organism identification pathway. What are the three kingdoms that will represent all multicelled organisms?




What question(s) would help divide them into those kingdoms? You may be surprised to find that there are different ways that this flow chart can be constructed.







When we apply this identification or decision making principle using flow charts, we are able to identify bacteria and decide what kinds of biochemical testing to perform. Using just the tests we have currently worked with (Gram Stain, Differential and Selective Media) we can construct a simple flow chart.







Here is a simple flow chart insert the appropriate species in the final Gram positive boxes. An unknown bacteria is Gram stained and identified as a Gram positive coccus, in clusters - what is the identity of the organism?







Go back to lab 5. Using the data on the table with the results, create a flow chart on the next page that identifies bacteria using the selective and differential nature of MSA, EMB and/or MacConkey agar.  




Using the key that you created determine the identity of a bacteria isolated from the urine of an ICU patient that grows on EMB and MacConkey Agar and produces a color change indicating lactose fermentation. 



Review  - Look at the flow chart (dichotomous key) in the text. Answer the critical thinking question and explain your answer. The instructor will hand out additional flow charts which have more detail and will be used throughout the rest of the semester.

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Date last updated 07/23/2011
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