Which organism is oxidase-positive and considered a nonfermenter and is a common cause of opportunistic infections?

• A. Aeromonas hydrophila
• B. Escherichia coli
• C. Klebsiella pneumoniae
• D. Pseudomonas aeruginosa

Answer: (D)

The key idea here is recognizing organisms that are oxidase-positive and do not ferment carbohydrates, a combination that points to Pseudomonas aeruginosa. This bacterium carries the enzyme cytochrome c oxidase, so the oxidase test is positive. It does not ferment glucose and other sugars in standard fermentation tests, instead relying on oxidative metabolism, which classifies it as a nonfermenter. In the clinical lab, this trait often correlates with growth as a nonlactose-fermenting organism on MacConkey agar and a distinctive blue-green pigment production (pyocyanin) that can help identification.

Pseudomonas aeruginosa is a classic opportunistic pathogen, notorious for causing infections in patients with weakened defenses or breached barriers—burn victims, people with cystic fibrosis, those with indwelling devices, and patients in intensive care units or on ventilators.

Other organisms listed don’t fit both traits. For example, E. coli and Klebsiella pneumoniae are lactose fermenters and oxidase-negative, so they are not oxidase-positive nonfermenters. Aeromonas hydrophila can be oxidase-positive but is generally capable of fermentation, so it does not fit the nonfermenter label.