The enzyme involved in the Mannitol-salt agar (MSA) test is mannitol dehydrogenase. This enzyme mediates the oxidation of mannitol to fructose 6-phosphate.

Here's how it works:

1. Mannitol is added to the agar as a carbon source.

2. Staphylococcus aureus, a common pathogen, possesses mannitol dehydrogenase.

3. If the bacteria can utilize mannitol, the enzyme oxidizes it to fructose 6-phosphate.

4. This reaction releases acid, which lowers the pH of the agar.

5. The indicator phenol red in the agar turns yellow in acidic conditions.

Therefore, a yellow halo around the bacterial growth on MSA indicates that the bacteria can ferment mannitol and produce acid. This is a key characteristic of *Staphylococcus aureus*, which is used to differentiate it from other staphylococcal species.