Concept explainers
To describe: The selective and differential properties of mannitol salt agar (MSA) for the identification and isolation of Staphylococci.
Introduction: Staphylococci are Gram-positive spherical shaped bacteria that occur in the form of grape-like clusters. They consist of forty different species with twenty-four subspecies. They are non-motile, non-spore–forming bacteria, and facultative anaerobic organisms.
Explanation of Solution
Mannitol salt agar (MSA) is a selective and differential growth medium and is used for the isolation and identification of Staphylococci. MSA has a high concentration of about 7.5% salt in it. This higher salt concentration inhibits the growth of other bacteria, except Staphylococci as they could readily grow in the medium containing high salt concentration. Hence, it is used as a selective medium for the growth of Staphylococci.
Mannitol sugar in this medium is used as a substrate for fermentation, and phenol red is used as the pH indicator for detecting acid production. Staphylococcus aureus ferments mannitol sugar and alters the medium color from red to yellow. Other species of Staphylococci do not ferment mannitol; hence, it is used as the differential medium to determine the particular type of species of Staphylococci.
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