Dichotomous Keys in Microbiology

Laboratory Exercises in Microbiology

Dichotomous Keys

Objectives

• Understand the uses of dichotomous keys.

• Use metabolic test results from Lab 5 to construct a dichotomous key for identifying bacterial unknowns.

Introduction

• Definition of Dichotomous Keys:

  • Dichotomous keys are diagrams based on successive choices between two alternatives.

  • The term "dichotomous" is derived from the Greek word meaning "to fork into two equal parts."

• Purpose:

  • Organize large amounts of information to make identifications easier.

  • Used to characterize and identify living organisms, including bacteria.

• Initial Steps in Identification:

  1. Morphology and Gram Stain Reaction:

  • The first step in identifying an unknown bacterium is often to determine its morphology and Gram stain reaction.

  • This initial determination allows for the elimination of many possibilities.

    • Example provided: A Gram-negative bacillus cannot be Staphylococcus aureus or Bacillus subtilis.

  1. Need for Further Testing:

  • After determining the Gram reaction and morphology, several possibilities may still need to be eliminated.

  • Many bacteria can appear similar under the microscope.

  • Additional staining techniques (e.g., acid-fast or endospore staining) can be used to characterize bacteria further.

Practical Application

• In this exercise, the metabolic tests learned in Lab 5 will be used to:

  • Design a dichotomous key that helps distinguish between 10 bacterial species used in the laboratory.

  • Identify two unknown bacteria for the midterm practical.

Procedure

• Example Demonstration:

  • The instructor will show an example of a simple dichotomous key using geometric shapes as a demonstration.

  • Just like shapes can be identified by distinct features, unknown bacteria can be identified by tests that reveal unique properties.

• Components of Dichotomous Keys for Bacteria:

  • Based on:

  • Gram reactions

  • Morphologies

  • Metabolic assays learned in lab

• Students will develop their own keys to identify specific unknowns based on these characteristics.

Key Creation Reminder

• There are numerous ways to create a useful dichotomous key, and the resulting keys may differ between students.

• While additional staining can be utilized in some cases, only metabolic tests will be applied in this particular exercise.

Completion and Submission

• After finishing the dichotomous key, students must:

  • Review the Gram reactions and morphologies of their two known bacteria.

  • Decide on which media should be used to identify the unknowns.

  • Record the media types and specific numbers needed in the designated space on the back of the dichotomous key.

  • Submit the completed key to the instructor:

  • Can be submitted during lab periods or brought home for completion.

  • Should be returned to the instructor at least 3 days before the next scheduled lab.

• Instructor's Review:

  • The instructor will verify the accuracy of the key and the identification of the correct media for the unknowns.

• Grading Policy:

  • The keys will be graded and returned during the next lab period when students will inoculate chosen media with their unknowns.

  • Late submissions will incur point deductions.