A sample from bluish purulent head lesions yields a Gram-negative, motile, non-spore-forming oxidase-positive rod. Which organism is most likely?

A sample from bluish purulent head lesions yields a Gram-negative, motile, non-spore-forming oxidase-positive rod. Which organism is most likely?

• A. Pseudomonas aeruginosa
• B. Acinetobacter baumannii
• C. Neisseria meningitidis
• D. Bacteroides fragilis

Answer: (A)

The key idea here is matching a Gram-negative rod that is motile, non-spore-forming, and oxidase-positive, with a clinical clue of bluish-purulent lesions. Pseudomonas aeruginosa fits all of these features. It is a Gram-negative rod that moves with a single polar flagellum (motile) and does not form spores. It is oxidase-positive, which helps distinguish it from many other Gram-negative rods. The bluish color of the purulent material is a classic clue because Pseudomonas produces pigmented compounds, especially pyocyanin, giving a blue-green hue to infections. In contrast, Acinetobacter baumannii is nonmotile and typically oxidase-negative; Neisseria meningitidis is a Gram-negative diplococcus, not a rod, and isn’t the right match; Bacteroides fragilis is an anaerobic rod that is oxidase-negative. So the combination of a motile, oxidase-positive Gram-negative rod with bluish-purulent lesions most strongly points to Pseudomonas aeruginosa.

The key idea here is matching a Gram-negative rod that is motile, non-spore-forming, and oxidase-positive, with a clinical clue of bluish-purulent lesions. Pseudomonas aeruginosa fits all of these features. It is a Gram-negative rod that moves with a single polar flagellum (motile) and does not form spores. It is oxidase-positive, which helps distinguish it from many other Gram-negative rods. The bluish color of the purulent material is a classic clue because Pseudomonas produces pigmented compounds, especially pyocyanin, giving a blue-green hue to infections.

In contrast, Acinetobacter baumannii is nonmotile and typically oxidase-negative; Neisseria meningitidis is a Gram-negative diplococcus, not a rod, and isn’t the right match; Bacteroides fragilis is an anaerobic rod that is oxidase-negative. So the combination of a motile, oxidase-positive Gram-negative rod with bluish-purulent lesions most strongly points to Pseudomonas aeruginosa.