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1.29: Mannitol Salt Agar

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    Learning Objectives
    • Define selective medium and differential medium.
    • Explain how mannitol salts agar is selective.
    • Explain how mannitol salts agar is differential.
    • Successfully utilize mannitol salts agar to characterize/identify bacterial species and interpret results.


    Mannitol Salts Agar

    Mannitol salts agar is a microbiological medium that is both selective and differential. Selective means the medium will only allow certain microorganisms to grow. Differential means the medium will show some characteristic of the microorganisms growing on it and can be used to differentiate between different species.

    Mannitol salts agar is selective since it has a high salt concentration and will only allow halophilic (salt-loving species) or halotolerant (salt-tolerant) species to grow on it. This medium contains 7.5% NaCl (salt), whereas typical media contains about 0.5% NaCl.

    • species that grow on mannitol salts agar: mannitol utilization and salt resistance positive
    • species that do not grow on mannitol salts agar: mannitol utilization/salt resistance negative

    Mannitol salts agar is differential since it will detect acid production as a result of fermentation of mannitol (a sugar in the medium) in species that can ferment mannitol to produce acid. The medium contains a pH indicator, phenol red, that is red-orange when the pH is neutral (around pH 7). If a species ferments mannitol and produces acid, the pH of the medium decreases and phenol red will become yellow.

    • medium remains red-orange after growth and growth observed: mannitol fermentation negative
    • medium is yellow after growth: mannitol fermentation positive


    mannitol salts agar results showing mannitol fermentation negative result and mannitol fermentation positive result

    Figure 1: Mannitol salts agar. (Left) The uninoculated medium has a light red to red-orange color. (Middle) The medium with Staphylococcus epidermidis growing on it shows the appearance with a mannitol fermentation negative species and appears red to red-orange. (Right). The medium with Staphylococcus aureus growing on it shows the appearance with a mannitol fermentation positive species and appears yellow in the region of the medium where the bacteria is growing.


    Use of mannitol salts agar is useful for differentiation of species of Staphylococcus and Micrococcus. Mannitol fermentation by pathogenic staphylococci, such as Staphylococcus aureus, is indicated by the media changing to yellow, whereas Staphylococcus epidermidis, a non-pathogenic species of staphylococci, does not produce a yellow color.


    Laboratory Instructions

    1. Obtain a mannitol salts agar petri plate and label with your name/group name or number and "mannitol salts agar."
    2. Draw a straight line on the underside of the petri plate to separate the plate into two halves. Label one side Staphylococcus aureus and the other side Staphylococcus epidermidis.
    3. Aseptically inoculate each half of the plate with the corresponding bacterial species with a streak that is a straight line in the center of each half of the petri plate.
    4. Invert the petri plate and incubate for 24 hours.
    5. Observe, record, and interpret results.


    Results & Questions


    bacterial species growth (+/-) salt resistance and mannitol utilization (+/-) medium color surrounding growth mannitol fermentation (+/-)
    Staphylococcus aureus        
    Staphylococcus epidermidis        
    1. Fill in the table above with results and interpretations.
    2. Define selective medium.
    3. Explain how mannitol salts agar is a selective medium.
    4. Define differential medium.
    5. Explain how mannitol salts agar is a differential medium.
    6. Explain how phenol red, a component of mannitol salts agar medium, detects if a species conducts mannitol fermentation.


    This page titled 1.29: Mannitol Salt Agar 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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