Which organism is citrate-positive on citrate utilization testing?

• A. Klebsiella pneumoniae
• B. Escherichia coli
• C. Salmonella typhimurium
• D. Shigella sonnei

Answer: (A)

Citrate utilization testing checks whether an organism can use citrate as its sole carbon source. On Simmons citrate agar, those that have citrate permease can transport citrate into the cell and metabolize it, producing alkaline byproducts that raise the medium’s pH. This pH shift turns the color indicator blue and supports growth.

Klebsiella pneumoniae is citrate-positive, so it grows on this medium and the surrounding color turns blue, reflecting its ability to use citrate. The other organisms listed—Escherichia coli, Salmonella typhimurium, and Shigella sonnei—are typically citrate-negative, meaning they do not grow or do not alkalinize the medium to produce the blue color under standard test conditions.