A negative citrate reaction is characteristic of which organism?

A negative citrate reaction is characteristic of which organism?

• A. Citrobacter freundii
• B. Enterobacter aerogenes
• C. Serratia marcescens
• D. Shigella boydii

Answer: (D)

Citrate utilization testing shows whether an organism can use citrate as its sole carbon source. The medium uses bromothymol blue as a pH indicator: if citrate is metabolized, alkaline end products form and the color shifts from green to blue; if citrate isn’t utilized, the medium stays green. Shigella boydii is typically citrate-negative, meaning it cannot use citrate under the test conditions, so the medium remains green. The other organisms listed are generally citrate-positive and would turn the medium blue, reflecting citrate metabolism.

Citrate utilization testing shows whether an organism can use citrate as its sole carbon source. The medium uses bromothymol blue as a pH indicator: if citrate is metabolized, alkaline end products form and the color shifts from green to blue; if citrate isn’t utilized, the medium stays green.

Shigella boydii is typically citrate-negative, meaning it cannot use citrate under the test conditions, so the medium remains green. The other organisms listed are generally citrate-positive and would turn the medium blue, reflecting citrate metabolism.