What color indicates a positive citrate utilization test on Simmons citrate agar?

What color indicates a positive citrate utilization test on Simmons citrate agar?

• A. Blue
• B. Green
• C. Yellow
• D. Colorless

Answer: (C)

Citrate utilization on Simmons citrate agar is detected by a pH-driven color change. When an organism can use citrate, it produces alkaline byproducts that raise the medium’s pH. The indicator bromothymol blue shifts from green at neutral pH to blue in alkaline conditions, so a positive citrate utilization result appears blue. If citrate isn’t utilized, the pH stays around neutral and the green color remains. Yellow would indicate acidic conditions, which isn’t consistent with citrate metabolism, and colorless would suggest no color development or growth that doesn’t alter the pH.

Citrate utilization on Simmons citrate agar is detected by a pH-driven color change. When an organism can use citrate, it produces alkaline byproducts that raise the medium’s pH. The indicator bromothymol blue shifts from green at neutral pH to blue in alkaline conditions, so a positive citrate utilization result appears blue. If citrate isn’t utilized, the pH stays around neutral and the green color remains. Yellow would indicate acidic conditions, which isn’t consistent with citrate metabolism, and colorless would suggest no color development or growth that doesn’t alter the pH.