What medium is specifically used to isolate Staphylococcus aureus?

What medium is specifically used to isolate Staphylococcus aureus?

• A. Nutrient agar
• B. Mannitol salt agar
• C. MacConkey agar
• D. XLD agar

Answer: (B)

Mannitol salt agar is specifically designed to isolate Staphylococcus aureus due to its unique composition. This medium contains a high concentration of salt (7.5% sodium chloride), which inhibits the growth of most other bacteria, allowing Staphylococcus species, particularly Staphylococcus aureus, to thrive. Furthermore, mannitol salt agar contains mannitol, a sugar alcohol that Staphylococcus aureus can ferment. When S. aureus ferments mannitol, it produces acid, which lowers the pH of the medium and causes a color change in the phenol red indicator present in the agar. This reaction results in a yellow halo around the colonies, making S. aureus easily identifiable. In contrast, nutrient agar provides a general growth medium for a wide variety of bacteria but lacks the specific elements needed to isolate S. aureus. MacConkey agar is selective for gram-negative bacteria, primarily aiding in the isolation of enteric (intestinal) bacteria and does not support the growth of Staphylococcus species. XLD agar is used to isolate enteric gram-negative bacteria as well, specifically for those that can ferment xylose, and it is not suitable for the isolation of Staphylococcus aureus. Thus,

Mannitol salt agar is specifically designed to isolate Staphylococcus aureus due to its unique composition. This medium contains a high concentration of salt (7.5% sodium chloride), which inhibits the growth of most other bacteria, allowing Staphylococcus species, particularly Staphylococcus aureus, to thrive.

Furthermore, mannitol salt agar contains mannitol, a sugar alcohol that Staphylococcus aureus can ferment. When S. aureus ferments mannitol, it produces acid, which lowers the pH of the medium and causes a color change in the phenol red indicator present in the agar. This reaction results in a yellow halo around the colonies, making S. aureus easily identifiable.

In contrast, nutrient agar provides a general growth medium for a wide variety of bacteria but lacks the specific elements needed to isolate S. aureus. MacConkey agar is selective for gram-negative bacteria, primarily aiding in the isolation of enteric (intestinal) bacteria and does not support the growth of Staphylococcus species. XLD agar is used to isolate enteric gram-negative bacteria as well, specifically for those that can ferment xylose, and it is not suitable for the isolation of Staphylococcus aureus.

Thus,