Is the statement true or false? A yellow color on mannitol salt agar indicates mannitol has been hydrolyzed by the enzyme beta-galactosidase.

The statement is false. Mannitol salt agar (MSA) is a selective differential medium used primarily to isolate and identify staphylococci, particularly Staphylococcus aureus. It contains mannitol as a fermentable carbohydrate and a high concentration of sodium chloride, which inhibits most bacteria other than staphylococci.

When a bacterium that can ferment mannitol grows on MSA, it produces acids as a byproduct of fermentation. This acid production lowers the pH of the medium, leading to a color change in the pH indicator (phenol red) present in the agar, which turns yellow in response to the acidic environment. This color change is not due to the action of beta-galactosidase, which is an enzyme involved in the metabolism of lactose in certain bacteria, but rather a direct consequence of mannitol fermentation.

Thus, the correct understanding is that yellow coloration on MSA indicates mannitol fermentation and acid production, not the activity of beta-galactosidase.