To maximize recovery of Staphylococcus aureus from a stool sample, the primary plating medium should include which selective agent?

• A. Bismuth sulfite
• B. Phenylethyl alcohol
• C. Thiosulfate-citrate-bile salts-sucrose
• D. Xylose-lysine-deoxycholate

Answer: (B)

To maximize recovery of Staphylococcus aureus from stool, you want a medium that suppresses competing bacteria while allowing Gram-positive cocci to grow. Phenylethyl alcohol serves this purpose because it inhibits most Gram-negative bacteria, while Gram-positive organisms like Staphylococcus aureus are less affected. This selective pressure reduces overgrowth by enteric Gram-negatives, making it easier to detect and recover S. aureus on the primary plate.

Other media listed are geared toward enteric pathogens such as Salmonella or Vibrio and are formulated with agents that target or inhibit different groups of organisms. Bismuth sulfite, thiosulfate-citrate-bile-salts-sucrose, and xylose-lysine-deoxycholate are designed to suppress Gram-positives and/or favor specific enteric pathogens, which would hinder recovery of Staphylococcus aureus on the primary plating.