Final answer:
Mannitol Salt Agar is both selective, due to its high salt concentration inhibiting non-halotolerant bacteria, and differential, as it contains mannitol and a pH indicator to detect fermentation by color change.
Step-by-step explanation:
Mannitol Salt Agar (MSA) can be classified as both selective and differential because of its unique composition that favors the growth of certain bacteria while also distinguishing between them based on their metabolic properties. The high salt concentration (7.5% NaCl) in MSA makes it selective by inhibiting the growth of non-halotolerant bacteria. Only organisms tolerant to the high salt content, like staphylococci, can thrive in this environment.
Additionally, MSA contains mannitol as a fermentable sugar and phenol red as a pH indicator, adding a differential aspect to the medium. When mannitol is fermented by an organism, such as Staphylococcus aureus, acid by-products are produced, causing the phenol red in the agar to change color from red to yellow. This color change indicates that the organism can ferment mannitol. Bacteria that do not ferment mannitol, like some coagulase-negative staphylococci (CoNS), will not cause a color change, leaving the agar its original color.