Staphylococcus aureus, when present, could most likely be recovered from a stool sample if the primary plating medium included which selective agent?

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

Answer: (B)

The main idea is that recovering Staphylococcus aureus from a stool sample requires a medium that suppresses the abundant Gram-negative intestinal flora. Phenylethyl alcohol acts as a selective agent that inhibits Gram-negative bacteria while allowing Gram-positive organisms, like Staphylococcus aureus, to grow. This makes it the best choice for enabling recovery of Staph from mixed stool flora.

The other media listed are designed to favor or isolate Gram-negative enteric pathogens (bismuth sulfite for Salmonella Typhi, TCBS for Vibrio species, and XLD for Salmonella/Shigella). They would suppress or not support growth of Staphylococcus aureus, making them less suitable for recovering this organism from stool.