Mannitol Salt Agar (MSA) is a specialized laboratory medium used for growing and differentiating microorganisms, primarily those from the genus Staphylococcus. MSA functions as both a selective and a differential agent. It encourages the growth of a target group of bacteria while simultaneously helping to distinguish species within that group. This makes MSA an efficient tool for the initial screening and presumptive identification of potentially pathogenic staphylococci.
MSA as a Selective Medium
The selective power of Mannitol Salt Agar lies in its high concentration of sodium chloride. MSA typically contains 7.5% salt, nearly ten times the amount found in standard bacterial growth media. This high salinity creates a hypertonic environment too harsh for most common bacteria to survive and grow.
The sodium chloride concentration inhibits the growth of non-salt-tolerant organisms, including most Gram-negative bacteria. Only halotolerant bacteria, such as those adapted to high-salt environments like human skin, are able to flourish on the agar. The high salt content effectively selects for the growth of Staphylococcus species, which tolerate these conditions.
MSA as a Differential Medium
MSA is classified as a differential medium because it allows microbiologists to visually distinguish between different types of staphylococci that grow on it. Differentiation is achieved through the inclusion of two specific ingredients: the carbohydrate mannitol and the pH indicator phenol red. The medium is initially red-orange in color due to phenol red at a neutral pH.
If a bacterium ferments the mannitol, which is the sole carbohydrate source, this metabolic process produces organic acids as byproducts. The release of these acids significantly lowers the pH of the surrounding agar. Phenol red is sensitive to this change, shifting its color from red to bright yellow when the pH drops below 6.8.
Interpreting Results for Bacterial Identification
The selective and differential functions of Mannitol Salt Agar combine to provide a powerful screening mechanism for bacterial identification. When interpreting the results, professionals look for three possible outcomes, which relate to the organism’s ability to tolerate salt and ferment mannitol. The absence of growth on the plate signifies that the organism is not salt-tolerant, meaning it is unlikely to be a Staphylococcus species.
The first positive result is characterized by bacterial growth and a color change in the medium from red to yellow. This yellow coloration indicates that the organism is salt-tolerant and has successfully fermented the mannitol, producing acid. This result strongly suggests the presence of pathogenic species, most notably Staphylococcus aureus.
The second positive result is growth on the agar with the medium remaining a pink or red color. This observation means the organism is salt-tolerant but lacks the ability to ferment mannitol, and therefore no acid was produced. This outcome typically points to non-mannitol-fermenting organisms, such as the common skin inhabitant Staphylococcus epidermidis, which is generally non-pathogenic. A presumptive identification of Staphylococcus aureus must be confirmed by additional biochemical tests.