Pink colonies on MacConkey agar, citrate positive, LOA: -++, gram negative bacillus

Pink colonies on MacConkey agar, citrate positive, LOA: -++, gram negative bacillus

• A. Klebsiella pneumoniae
• B. Klebsiella oxytoca
• C. Enterobacter aerogenes
• D. Enterobacter cloacae

Answer: (A)

MacConkey agar shows lactose fermentation by turning the colonies pink; combined with a Gram-negative rod that is citrate positive, you’re looking at a lactose-fermenting enteric organism that can use citrate. Klebsiella pneumoniae fits this profile best: it is a gram-negative bacillus, a strong lactose fermenter that yields pink colonies on MacConkey, and it is citrate positive. It also tends to form large, mucoid, non-motile colonies due to its prominent capsule, which helps distinguish it from some Enterobacter species that are more motile and can have slightly different colony appearances. While the other organisms listed can also ferment lactose and be citrate positive, the characteristic pink MacConkey growth together with citrate positivity and the typical Klebsiella morphology makes Klebsiella pneumoniae the best match for this profile.

MacConkey agar shows lactose fermentation by turning the colonies pink; combined with a Gram-negative rod that is citrate positive, you’re looking at a lactose-fermenting enteric organism that can use citrate. Klebsiella pneumoniae fits this profile best: it is a gram-negative bacillus, a strong lactose fermenter that yields pink colonies on MacConkey, and it is citrate positive. It also tends to form large, mucoid, non-motile colonies due to its prominent capsule, which helps distinguish it from some Enterobacter species that are more motile and can have slightly different colony appearances. While the other organisms listed can also ferment lactose and be citrate positive, the characteristic pink MacConkey growth together with citrate positivity and the typical Klebsiella morphology makes Klebsiella pneumoniae the best match for this profile.