Mannitol Salt Agar (MSA) is a specialized microbiological growth medium designed to aid in the identification of specific types of bacteria. This medium is particularly useful in clinical and research environments where the precise identification of bacterial species is important.
Key Components and Dual Function
MSA is a complex medium containing several components that give it its distinct properties. It includes nutrients and agar to support bacterial growth and provide a solid surface. The two most significant components are a high concentration of sodium chloride (salt) and the sugar alcohol mannitol, along with a pH indicator called phenol red.
The presence of these specific ingredients allows MSA to perform a dual function: it is both a selective and a differential medium. As a selective medium, it permits the growth of certain bacteria while inhibiting others. As a differential medium, it enables microbiologists to distinguish between different types of bacteria that do grow on it, often through observable changes in the medium itself.
How It Selects for Specific Bacteria
The selective nature of Mannitol Salt Agar is primarily due to its high salt concentration, typically 7.5% to 10% sodium chloride. This elevated salt level creates an osmotic pressure that is too harsh for most bacteria to tolerate, effectively inhibiting their growth.
However, certain groups of bacteria, particularly species belonging to the genus Staphylococcus, are halotolerant, meaning they can thrive in high-salt environments. These bacteria have adapted mechanisms to survive and grow in conditions that would dehydrate or damage other microorganisms. Consequently, MSA selectively encourages the growth of staphylococci while suppressing the growth of many other Gram-positive and most Gram-negative bacteria.
How It Differentiates Between Bacteria
Mannitol Salt Agar also functions as a differential medium, allowing for distinctions among the salt-tolerant bacteria that grow on it. This differentiation relies on the ability of certain bacteria to ferment mannitol, which is the sole carbohydrate source in the medium. When a bacterium ferments mannitol, it produces acidic byproducts.
The medium contains phenol red, a pH indicator that changes color depending on the acidity of its environment. Phenol red appears red at a neutral pH. If mannitol is fermented, the resulting acid lowers the pH of the medium, causing the phenol red to turn yellow. Conversely, if a bacterium grows but does not ferment mannitol, no significant acid is produced, and the medium around the colonies remains red.
Reading Results and Practical Applications
Interpreting results on a Mannitol Salt Agar plate involves observing both bacterial growth and any associated color changes in the medium. If colonies appear yellow and the surrounding medium has also turned yellow, it signifies that the bacteria have fermented mannitol, producing acid, as seen with Staphylococcus aureus.
In contrast, if salt-tolerant bacteria grow but the colonies remain red or pink with no medium color change, it indicates that these bacteria do not ferment mannitol. Staphylococcus epidermidis typically exhibits this result. The absence of growth entirely suggests that the organism cannot tolerate the high salt concentration in the medium. MSA is widely used in clinical microbiology for the presumptive identification of pathogenic Staphylococcus species, particularly S. aureus, from various samples like skin infections or food products, and in testing for staphylococci in food, dairy, environmental, and cosmetic products.