Which pair of organisms is suggested as quality control organisms for Simmons citrate agar slants?

• A. Shigella sonnei; Escherichia coli
• B. Klebsiella pneumoniae; Escherichia coli
• C. Escherichia coli; Edwardsiella tarda
• D. Morganella morganii; Edwardsiella tarda

Answer: (B)

Testing citrate utilization on Simmons citrate agar slants relies on using known reference organisms that provide both a positive and a negative result, so you can verify that the medium, incubation conditions, and color change are functioning properly. A citrate-positive organism has the citrate permease system and can use citrate as the sole carbon source, which alkalinizes the medium and changes the indicator to blue. A citrate-negative organism cannot utilize citrate and typically shows little to no growth or remains green. Klebsiella pneumoniae is a classic citrate-positive organism, producing growth with a blue color change on Simmons citrate agar. Escherichia coli is citrate-negative under standard conditions, so it will not produce the blue color change. Using this pair as quality controls ensures you can confirm both positive and negative reactions are working correctly. Other pairings do not consistently provide the standard positive/negative control combination needed for reliable QC.