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27: MR-VP Tests

  • Page ID
    90573
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    Learning Objectives
    • Objective 1
    • Objective 2

    Methyl Red–Voges Proskauer (MR-VP) Test

    The MR-VP test is two separate tests that uses one broth of glucose to determine the end product of glucose fermentation. Some bacteria ferment glucose using mixed acid fermentation pathway (detected in the MR test) while other bacteria ferment glucose via the butanediol fermentation pathway (detected in the VP test).

    MRVP Broth (Glucose) –>Pyruvic acid–>Mixed acid fermentation pathway (MR+)

    OR

    MRVP Broth (Glucose) –>Pyruvic acid–>Butanediol fermentation pathway (VP+)

    methyl red test results

    Figure 1: Results of the methyl red test (MR). (Left) The medium does not turn red in the negative methyl red test indicating that the bacterial species tested does not ferment glucose using the mixed acid fermentation pathway. (Right) The medium turns red in the positive methyl red test indicating that the bacterial species does ferment glucose using the mixed acid fermentation pathway.

    Voges-Proskauer test results

    Figure 2: Results of the Voges-Proskauer test. (Left) When the color at the top of the tube is yellow, this is a negative Voges-Proskauer test indicating that the bacterial species tested does not ferment glucose using the butanediol fermentation pathway. (Right) When the color at the top of the tube is a red-brown color, this is a positive Voges-Proskauer test indicating that the bacterial species tested ferments glucose using the butanediol fermentation pathway.

    Lab Instructions

    1. Obtain a MR-VP broth tube. Use tape to label the tube with your name, your assigned organism and the name of the test media.
    2. Using a sterile inoculating loop, obtain a small amount of bacteria and inoculate the MR-VP broth.
    3. Incubate the inoculated tube in the class test tube rack until next lab session.
    4. After incubation, using a transfer pipette, transfer half of the inoculated MR-VP broth to a clean test tube. Label this tube “VP.” Leave the other half of the inoculated broth in the original MR-VP broth test tube. Label this tube “MR.”
    5. Methyl red test: Add 10 drops of Methyl Red reagent to “MR” tube. Methyl red is a pH indicator that will detect mixed acid production. Record the reactions and results on the worksheet.
    6. Voges-Proskauer test: First add 15 drops of Barritt’s A reagent (alpha-napththol) and then add 5 drops of Barritt’s B (40% KOH) to the “VP” tube. NOTE: A reversal in the order of the reagents may result in a weak-positive or false-negative reaction. Mix the tube well by flicking it with your fingers and then let it sit undisturbed in a test tube rack for 20 minutes. A positive reaction indicates the bacteria ferment glucose via the butanediol fermentation pathway.

    Attributions


    This page titled 27: MR-VP Tests is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.

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