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14.1: Lab Practicals

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    During the course of the semester, there will be two lab practical exams: the Midterm Practical and the Final Practical. The midterm practical will occur in several steps beginning in week 5 and extending through week 10. The final practical will occur on the last day of class.

    The practical exams are each worth 20% of your lab grade. Each practical exam has a different format.

    The Midterm Practical is an open book exam in which you will use Gram staining and metabolic tests to determine the identity of two bacterial unknowns, and write a lab report based on your results. A more complete description of what is required for the midterm and lab report can be found below.

    The Final Practical is a cumulative exam that is not open book. Twenty five stations will be set up around the lab, and you will be expected to answer questions about the material at these stations. You may be asked to identify microorganisms, analyze the results of a metabolic test or interpret the results of an experiment. Since the exam is cumulative, it can cover material from the first lab through the last lab of the class. The Microbiology Review site on Blackboard contains material that will help you review the material that you have covered throughout the semester.

    Midterm Practical

    This first lab practical involves the identification of two unknown bacteria using both Gram staining and metabolic assays. There are 3 parts to this lab practical—Part I: Gram staining (Lab 5); Part II: Preparation of dichotomous key (Lab 6) and metabolic testing (Lab 7); Part III: a written lab report (due on Lab 10). Each part of the lab practical is graded separately (see below). This first lab practical is an open book exam. You are being tested on your ability to correctly perform procedures and interpret results. Although you can use your books and your notes, you cannot seek the advice or assistance of other students or the instructor!

    Schedule for Midterm Practical

    Lab 5: You will be given two unknown bacteria and asked to Gram stain them following the procedures you have learned in previous labs. After Gram staining, you will observe the cells under the microscope, and determine their morphology and Gram reaction. At the end of the lab period you will submit your results (see sample sheet below) to your instructor for grading. Your slide will be saved and may also be photographed for use in your lab report.


    You will be required to show the results of your Gram staining procedure to your instructor. Make sure you don’t wait until the last minute to call the instructor over to view your slides!

    Lab 6: You will learn how to construct a dichotomous key for bacterial identification. Using what you know about the Gram reactions of your unknown bacteria as well as information about the metabolic reactions of bacterial species obtained in Lab 5, you will construct a dichotomous key that can be used to identify your 2 unknown bacteria. You will also choose the media that you will need to use for the inoculations you will do in the next lab. You will turn in the dichotomous key for grading and the instructor will return it to you on Lab 7 (corrected if necessary).

    Lab 7: You will inoculate your unknown bacteria in the media that you chose in Lab 6.

    Lab 8: You will analyze the results of your metabolic tests. Using these results and your dichotomous key, you will identify your two unknown bacteria.

    Lab 10: You will turn in the Lab Report for the Midterm Practical.

    Midterm Practical Grading

    Point Distribution—out of 100%:

    Gram reactions: 25%

    Metabolic assays: 25%

    Written lab report: 50% (see Lab Report Rubric)

    Grading Breakdown:

    Part I: Gram Reactions (25/100)

    Morphology for unknown A correct – 5 points

    Morphology for unknown B correct- 5 points

    Gram reaction for unknown B: color and interpretation is correct -5 points

    Gram reaction for unknown B: color and interpretation is correct- 5 points

    Unknown A slide shown to professor -2.5 points

    Unknown B slide shown to professor - 2.5 points

    Part II: Metabolic Assays (25/100)

    Dichotomous key- complete and correct - 5 points

    Media required based on student’s key complete and correct - 5 points

    Metabolic test results recorded and interpreted correctly - 15 points

    Part III: Lab Report (50/100)

    14.1: Lab Practicals is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Joan Petersen & Susan McLaughlin.

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