What is the selective agent in a manitol salt medium?

The selective agent in mannitol salt medium is sodium chloride (NaCl). This high concentration of salt inhibits the growth of most bacteria, allowing only salt-tolerant organisms, particularly staphylococci, to thrive. Additionally, mannitol serves as a carbohydrate source, and some staphylococci can ferment it, producing acid and changing the color of the pH indicator in the medium.