Which test result indicates that an unknown microorganism can utilize citrate as its sole carbon source?

• A. A red ring forms at the top of the medium.
• B. The medium becomes cloudy without a color change.
• C. The medium changes from green to blue.
• D. Gas bubbles are produced in the Durham tube.

Answer: (C)

Verified step by step guidance

1. Understand the purpose of the citrate utilization test: it determines whether a microorganism can use citrate as its sole carbon source for growth.
2. Recall that the citrate test uses Simmons' citrate agar, which contains citrate as the only carbon source and a pH indicator (bromothymol blue) that is green at neutral pH and turns blue in alkaline conditions.
3. Recognize that if the microorganism can utilize citrate, it will metabolize citrate and produce alkaline byproducts, raising the pH and causing the medium to change color from green to blue.
4. Note that a red ring at the top, cloudiness without color change, or gas bubbles in a Durham tube are not indicative of citrate utilization; these results correspond to other biochemical tests.
5. Conclude that the positive citrate utilization test is indicated by the medium changing from green to blue, confirming the microorganism's ability to use citrate as its sole carbon source.