Bacteria dichotomous key

Bacteria dichotomous key DEFAULT

Education on the Halfshell: Understanding a Dichotomous Key

Lesson Goal

The students will explore the benefits of creating and using dichotomous keys as a means of identifying an organism or object.

Lesson Objective

The student will use a dichotomous key to identify seashells.

Background Information

A dichotomous key is a tool that allows the user to determine the identity of items and organisms in the natural world. It is the most widely used form of classification in the biological sciences because it offers the user a quick and easy way of identifying unknown organisms. Keys consist of a series of choices that lead the user to the correct name of a given item. “Dichotomous” means “divided into two parts.” That is why dichotomous keys always give two choices in each step. In each step, the user is presented with two statements based on characteristics of the organism. If the user makes the correct choice every time, the name of the organism will be revealed at the end.

There are two kinds of descriptions that might be presented to the user of a dichotomous key: qualitative and quantitative descriptions. Qualitative descriptions concern the physical attributes, or qualities, of the item being classified. Examples of qualitative descriptions are such phrases as “contains green striations on top surface” or “feels slick on bottom surface.” Quantitative descriptions concern values that correspond with the item being classified. Examples of quantitative descriptions are such phrases as “has 10 striations on top surface,” “has 8 legs,” or “weighs 5 grams”. Knowing the difference between these two types of descriptions can be immensely beneficial for creators and users of dichotomous keys.

There are two ways to set up a dichotomous key. One way is to present the two choices together, and the other way is to group by relationships. When the dichotomous key is set up by presenting the two choices together, it is easy to distinguish between them. However, relationships between various characteristics are not emphasized. When the dichotomous key is grouped by relationships, the choices are separated, yet it is easy to see the relationships between them. While this method may prove to be more difficult to construct, many users prefer it because it gives them more information.

Teacher Preparation

Blackline Master 1 gives students instructions on how to use a dichotomous key in general.  In this activity you  will find two styles of dichotomous keys (download Blackline Masters 2 and 3) and seashell pictures used in the keys (Blackline Master 4, pages 1-3). Pick one of the dichotomous key worksheets; both reflect the same dichotomous process and use the same seashell pictures.  Links to downloading the Blackline Masters can be found below.

Duplicate as many keys and sets of seashell pictures as necessary for each student or student group. It is suggested that if you use this activity many times, you should laminate the pictures of the shells and then cut them apart as sets for future use.

NOTE: When making copies of the seashell pictures from your blackline master, lighten the density of the copies, and if possible, use the photo/text option. If you do not lighten the density, the images will be difficult to see.

HINT: f the student is a verbal learner use the key on Blackline Master 2. If student is a visual learner use Blackline Master 3.

Blackline Masters

Alternative Assessments

  • Create a concept map from the key.
  • Design a short dichotomous key using a particular item (examples: coke tabs, empty ketchup bottles, paper clips, etc.) and have the students work through the key.
  • Have students work through an online dichotomous key.

Answer Key(s)Keys to Student Activities, versions 1 and 2 (PDF)

Extension Ideas

  • Research ways dichotomous keys are used in other areas of school, society, culture, etc.
  • In geometry class, develop a dichotomous key of a formal proof.
  • Create a collage from the different levels of the key.

Resources & Web Links

Timme, Stephen, 1991, Association for Biology Laboratory Education website, How to Construct and Use a Dichotomous Key, accessed 02/16/01,

Description: An excellent web-based activity on the construction and use of a dichotomous key that also describes the use of a dichotomous key in the field and provides a key for prairie plants.

Frontier High School, Red Rock, OK, The Dichotomous Key, accessed 02/16/01,

Description: Provides instructions on the two methods of constructing a dichotomous key as well as several online dichotomous keys. Grade level: High School.

Detka, Jon , California State University at Monteray Bay, Designing and Using a Dichotomous Key, accessed 02/16/01,

Description: Students first construct a simple dichotomous key and then use a basic key to identify some of the native plants and the most unwanted invasive weeds of California. Grade level: 3-5.

Santa Cruz Productions, Wastewater Filamentous Bacteria Dichotomous Key, accessed 02/16/01,

Description: A completely web-based dichotomous key designed to assist students in identifying wastewater bacteria.

Correlations to the Louisiana & the National Science Education Standards

Louisiana Science Frameworks
Middle School BenchmarksHigh School Benchmarks
  • SI-M-A1,A3,A4,A7
  • SI-M-B4,B6
  • PS-M-A1,A3
  • LS-M-A2,A3
  • S-M-C1
  • SI-H-A1,A3,A4,A5
  • SI-H-B2,B4
  • PS-H-A1
  • LS-H-C4,C5
National Science Education Standards
Middle School StandardsHigh School Standards
  • UC&P- Systems, order & organization
  •  LS – Diversity & adaptations of organisms


  • UC&P – Systems, order, & organization
  • PS – Structure & properties of matter

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  1. 1. Characteristics of Microbes
  2. 2. • Prokaryote• microscopic, unicellular organisms• lack nuclei and membrane-bound organelles
  3. 3. Bacterial Structures• Flagella• Pili• Capsule• Plasma Membrane• Cytoplasm• Cell Wall• Lipopolysaccharides• Teichoic Acids• Inclusions• Spores Chapter 4
  4. 4. How to Make a DichotomousKey From a Table of Bacterial Characteristics (for use in running the minimum number of tests in identifying an unknown bacterium)
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  6. 6. Dichotomous Key to Identifying Eight Bacteria Alcaligenes faecalis Mycobacterium smegmatis Citrobacter freundii Mycobacterium leprae Pseudomonas fluorescens Corynebacterium xerosis Streptococcus pneumoniae Aeromonas hydrophila
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  9. 9. Dichotomous Key to Identifying Eight Bacteria Alcaligenes faecalis Mycobacterium smegmatis Citrobacter freundii Mycobacterium leprae Pseudomonas fluorescens Corynebacterium xerosis Streptococcus pneumoniae Aeromonas hydrophila Gram stain negative positiveAlcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisAeromonas hydrophila Streptococcus pneumoniae
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  11. 11. Dichotomous Key to Identifying Eight Bacteria Alcaligenes faecalis Mycobacterium smegmatis Citrobacter freundii Mycobacterium leprae Pseudomonas fluorescens Corynebacterium xerosis Streptococcus pneumoniae Aeromonas hydrophila Gram stain negative positive Alcaligenes faecalis Mycobacterium smegmatis Citrobacter freundii Mycobacterium leprae Pseudomonas fluorescens Corynebacterium xerosis Aeromonas hydrophila Streptococcus pneumoniae Galactose fermentation positive negativeAlcaligenes faecalis Citrobacter freundiiAeromonas hydrophila Pseudomonas fluorescens
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6.1: Introduction to Dichotomous Keys

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  • Understand the uses of dichotomous keys.
  • Using the metabolic test results obtained in Lab 5, construct a dichotomous key for the identification of bacterial unknowns

Keys that are based upon successive choices between two alternatives are known as dichotomous keys (dichotomous means to “fork into two equal parts”). One use of a dichotomous key is to organize large amounts of information so identifications can be made more easily. This method is used to characterize and identify living organisms, including bacteria.

The first step in the identification of an unknown bacterium is often to determine its morphology and Gram stain reaction. This allows you to eliminate many possibilities (for example, a Gram negative bacillus is not Staphylococcus aureus or Bacillus subtilis). After determining the Gram reaction and morphology of an unknown organism, you can eliminate many possibilities, but will likely still need additional tests to determine which species you have (remember that there are lots of bacteria that look the same under the microscope). Sometimes, additional staining (like acid-fast or endospore staining, for example) can be used to further characterize bacteria. In this exercise, you will be using the metabolic tests you have learned about in Lab 5 to design a dichotomous key that will allow you determine which tests can be used to distinguish the 10 bacterial species we are using in the lab from each other. You will then be using those tests to identify your two unknowns for your midterm practical.


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Dichotomous key bacteria

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