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

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

Answer: (A)

This item tests using a combination of oxidase reaction, motility, and fermentative ability to identify Gram-negative bacilli that grow on MacConkey agar. The organism described is oxidase positive, motile, and fermentative, with good growth on MacConkey, which fits Aeromonas hydrophila. Pseudomonas aeruginosa is oxidase positive and motile but nonfermentative, so the fermentative clue doesn’t fit. Stenotrophomonas maltophilia is oxidase negative and nonfermentative. Yersinia enterocolitica is oxidase negative and has motility characteristics that don’t align with the fermentative, oxidase-positive profile. Thus, Aeromonas hydrophila best matches all the given traits.