A fermentative gram-negative bacillus that is oxidase positive, motile, and grows well on MacConkey agar is most consistent with which organism?

A fermentative gram-negative bacillus that is oxidase positive, motile, and grows well on MacConkey agar is most consistent with which organism?

• A. Aeromonas hydrophila
• B. Pseudomonas aeruginosa
• C. Stenotrophomonas maltophilia
• D. Yersinia enterocolitica

Answer: (A)

This item tests recognizing a fermentative, oxidase-positive, motile Gram-negative rod that can grow on MacConkey agar. Aeromonas hydrophila fits this profile: it is a Gram-negative bacillus that uses fermentation (facultative anaerobe), is oxidase positive, and has motility due to flagella. It grows well on MacConkey, which helps distinguish it from non-fermenters. The other organisms don’t fit all three features together. Pseudomonas aeruginosa is oxidase-positive and motile but relies on oxidative metabolism (not fermentative), so the fermentative clue points away from it. Stenotrophomonas maltophilia is oxidase-negative, so it doesn’t match the oxidase-positive requirement. Yersinia enterocolitica is typically oxidase-negative and often less clearly matches the fermentative profile. Hence, the combination of fermentative metabolism with oxidase positivity and motility on MacConkey strongly points to Aeromonas hydrophila.

This item tests recognizing a fermentative, oxidase-positive, motile Gram-negative rod that can grow on MacConkey agar. Aeromonas hydrophila fits this profile: it is a Gram-negative bacillus that uses fermentation (facultative anaerobe), is oxidase positive, and has motility due to flagella. It grows well on MacConkey, which helps distinguish it from non-fermenters.

The other organisms don’t fit all three features together. Pseudomonas aeruginosa is oxidase-positive and motile but relies on oxidative metabolism (not fermentative), so the fermentative clue points away from it. Stenotrophomonas maltophilia is oxidase-negative, so it doesn’t match the oxidase-positive requirement. Yersinia enterocolitica is typically oxidase-negative and often less clearly matches the fermentative profile. Hence, the combination of fermentative metabolism with oxidase positivity and motility on MacConkey strongly points to Aeromonas hydrophila.