Production of a yellow pigment on nutrient or sheep blood agar is characteristic of which member of the Enterobacteriaceae?

• A. Enterobacter aerogenes
• B. Citrobacter freundii
• C. Cronobacter sakazakii
• D. Klebsiella oxytoca

Answer: (C)

Producing a yellow pigment on nutrient or sheep blood agar reflects a visible pigment that some Enterobacteriaceae synthesize. Among the listed organisms, Cronobacter sakazakii is the one known for forming yellow to golden pigmented colonies on these common media due to carotenoid pigment production. The others—Enterobacter aerogenes and Klebsiella oxytoca—typically appear nonpigmented or only faintly pigmented under standard conditions, so they don’t show this distinctive yellow color. This pigment clue helps differentiate Cronobacter sakazakii from the other options on routine plates. Note that Cronobacter sakazakii is an important neonatal pathogen linked to contaminated powders, but pigment appearance alone isn’t diagnostic; it’s a helpful identifying cue alongside other tests.