For the selective isolation of Vibrio spp., the recommended agar is which?

• A. Thiosulfate-citrate-bile salt-sucrose agar
• B. Charcoal yeast extract agar
• C. Mannitol salt agar
• D. Tinsdale agar

Answer: (A)

Selective isolation of Vibrio relies on a medium that both suppresses competing flora and differentiates Vibrio by a sugar fermentation reaction. Thiosulfate-citrate-bile salt-sucrose agar fits this need. The bile salts and high pH inhibit many Gram-positive and some non-Vibrio bacteria, giving Vibrio species a growth advantage. The sucrose in the medium, along with a pH indicator, differentiates isolates: Vibrio that ferment sucrose turn the colonies yellow due to acid production, while non–sucrose-fermenting vibrios (and many non-Vibrio organisms that might grow) yield green colonies. This combination provides both selectivity and a practical readout for presumptive Vibrio.

Other media listed aren’t ideal for Vibrio isolation. Charcoal yeast extract agar is used for fastidious organisms like Legionella and doesn’t provide the same selective pressure or differential sugar fermentation needed for Vibrio. Mannitol salt agar selects for halotolerant Staphylococcus species, not Vibrio, and Tinsdale agar is designed for Corynebacterium diphtheriae and its tellurite-based inhibition, not Vibrio.