Mannitol Salt Agar (MSA) What makes MSA a selective media? What does MSA select for? Staphylococcus How is a positive result for the selective property determined? The growth of bacteria What makes MSA a differential media? The mannitol will turn the media into yellow What does MSA differentiate between? Staphylococcus and other bacteria in yellow media
The Correct Answer and Explanation is:
Correct Answers:
- What makes MSA a selective media? The high concentration of salt
- What does MSA select for? Staphylococcus species
- How is a positive result for the selective property determined? By observing bacterial growth
- What makes MSA a differential media? The presence of mannitol and phenol red indicator
- What does MSA differentiate between? Mannitol-fermenting and non-mannitol-fermenting Staphylococcus species
Explanation
Mannitol Salt Agar (MSA) is both a selective and differential culture medium widely used in microbiology to isolate and identify certain types of bacteria, especially from human skin and nasal samples. Its unique formulation allows it to perform two important functions.
MSA is selective because it contains a high concentration of sodium chloride, typically around 7.5 percent. This high salt concentration inhibits the growth of most bacteria, but it allows certain halotolerant organisms, particularly members of the Staphylococcus genus, to thrive. Most other bacteria are unable to survive in such a salty environment, so their growth is suppressed. If bacterial colonies grow on MSA, it indicates that the organism is salt-tolerant, which is a characteristic feature of Staphylococcus species.
The differential aspect of MSA comes from the sugar mannitol and the pH indicator phenol red. Some Staphylococcus species, such as Staphylococcus aureus, can ferment mannitol, producing acidic byproducts. These acids lower the pH of the medium, causing the phenol red to change color from red to yellow. On the other hand, non-mannitol fermenters, such as Staphylococcus epidermidis, do not produce acid, so the color of the medium remains red or pink.
This color change allows scientists to distinguish between different types of Staphylococcus bacteria. Therefore, MSA not only selects for salt-tolerant bacteria but also differentiates between those that can ferment mannitol and those that cannot. This dual function makes it an efficient and powerful diagnostic tool in both clinical and research microbiology.
