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5.3: Results

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    Record your results for the metabolic tests in the tables on the following pages. At the end of this lab there is also a table where you can record test results for all of the organisms that will be on your first practical exam. It is important to obtain results all of the organisms in the table, as you will need this information to identify your unknown organisms for your midterm practical.

    A. Carbohydrate Fermentation

    Observe the results of your own carbohydrate fermentation test, as well as the tests done by your table partners. You can compare your inoculated tubes with the negative controls in the front of the class at the instructor’s table. Record the results in the table below.

    The following convention is used when noting the results of fermentation experiments.

    A = acid

    G = gas

    AG means that both acid and gas are present

    If neither acid nor gas is present, you can write “negative”

    Bacteria Glucose Lactose Sucrose
    Proteus vulgaris
    Escherichia coli
    Bacillus subtilis
    Streptococcus faecalis

    B. Gelatin Hydrolysis

    Observe your gelatin deeps and those of your table partners. Record your results in the table below.

    Bacteria Liquid or solid Result (+/-)
    Staphylococcus aureus
    Serratia marcescens
    Streptococcus faecalis
    Bacillus subtilis

    C. Starch Hydrolysis

    Add enough Gram’s iodine to the plate to ensure that the entire areas of growth and the agar surrounding it are covered (you should also add iodine to the negative control area). Once you have added the iodine, make sure you do not tip the plate too much! Observe the appearance of your plate. Record your results in the table below.


    Bacteria Color change? (Y/N) Starch present or absent? Presence (+) or absence (-) of amylase
    A. Proteus vulgaris
    B. Escherichia coli
    C. Bacillus subtilis
    D. Negative control

    D. Casein Hydrolysis

    Observe your casein plate. It is helpful to hold it up to the light so you can detect the clear zones. Make a drawing of your results the circle below. Indicate the location of the bacterial growth and draw any clear zones that are present. Record your results in the table below.


    Bacteria Clear zone? Result (+/-)
    A. Enterobacter aerogenes
    B. Bacillus subtilis
    C. Negative control

    E. Urea Hydrolysis

    Observe your urea slants. Record the results in the table below.

    Bacteria Color Result (+/-)
    Escherichia coli
    Proteus vulgaris

    If the urea slant is pink, what can you say about the pH of the medium?_______________ This pH change is due to what molecule? __________________

    F. Citrate Utilization

    Observe your Simmon’s citrate slants. Record the results in the table below.

    Bacteria Color Growth? Result (+/-)
    Escherichia coli
    Serratia marcescens

    G. Catalase Activity

    Add a dropper full of H2O2 to the surface of the slant. Record your results below.

    Bacteria Presence of bubbles Result (+/-)
    Staphylococcus aureus
    Streptococcus faecalis

    H. Tryptophan Hydrolysis

    Flick your inoculated culture to stir up the bacteria. Use a sterile swab to obtain some bacteria. Make sure you swirl the swab around in the broth so you pick up a large quantity of bacteria. Rub the swab on one area of the BBL Dry Slide. Record any color changes that occur between 30 – 60 seconds. Do not wait too long before you observe your result, as false positives can occur if results are not read right away.

    Record your results in the table below.

    Bacteria Color Result (+/-)
    Escherichia coli
    Enterobacter aerogenes

    This page titled 5.3: Results 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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