Would you have expected Staphylococci from human skin to grow on a MSA plate? Why?
The correct answer and explanation is:
Yes, Staphylococci from human skin are expected to grow on a Mannitol Salt Agar (MSA) plate.
Explanation:
MSA is a selective and differential medium designed specifically to isolate and differentiate staphylococci. The high concentration of sodium chloride (7.5% NaCl) in the medium makes it selective, as it inhibits the growth of most bacteria that cannot tolerate this high salt concentration. Staphylococci, however, are salt-tolerant and can grow on this medium, making MSA an effective tool for isolating them.
The medium is also differential due to the presence of mannitol, a sugar alcohol that some bacteria can ferment. Staphylococci that are capable of fermenting mannitol produce acid as a byproduct, which causes the pH indicator (phenol red) in the agar to turn yellow. This color change indicates the presence of mannitol-fermenting staphylococci, such as Staphylococcus aureus. In contrast, non-mannitol fermenters, such as Staphylococcus epidermidis, will not produce acid and the agar around the colonies will remain red.
Since Staphylococci are a normal part of the human skin microbiota, they are commonly found on human skin. When a sample from human skin is inoculated onto an MSA plate, the salt-tolerant staphylococci, particularly S. aureus and S. epidermidis, are expected to grow. S. aureus will produce yellow colonies due to its ability to ferment mannitol, while S. epidermidis will produce red colonies because it does not ferment mannitol.
In conclusion, Staphylococci from human skin will grow on MSA because of their salt tolerance and ability to either ferment or not ferment mannitol, which helps in differentiating species based on their metabolic activity.