Final answer:
Mannitol Salt Agar (MSA) is a selective medium that inhibits the growth of bacteria susceptible to osmotic stress. Staphylococci spp., such as Staphylococcus aureus, can grow on MSA and ferment mannitol, while other bacteria are unable to do so.
Step-by-step explanation:
Mannitol Salt Agar (MSA) is a selective medium that inhibits the growth of bacteria susceptible to osmotic stress due to its high concentration of salt (7.5%). The medium also contains mannitol, which can be fermented by some bacteria, and phenol red as an indicator for acid. While Staphylococci spp., such as Staphylococcus aureus, can grow on MSA and ferment mannitol, other bacteria are unable to do so.
The reason lies in the high concentration of salt (7.5% NaCl) present in Mannitol Salt Agar, which creates an osmotically challenging environment that inhibits the growth of non-halotolerant bacteria. Staphylococci, however, are well-adapted to tolerate high salt concentrations and can thrive on this medium.
Furthermore, Mannitol Salt Agar contains mannitol, which certain bacteria such as S. aureus can ferment, leading to a color change in the indicator phenol red from red to yellow, signifying acid production. Conversely, bacteria such as B. cereus do not ferment mannitol and retain the deep red color of the medium.