A 62-year-old male with chronic obstructive pulmonary disease presents with productive cough, fever, and dyspnea for 5 days. Sputum is greenish and foul-smelling. Gram stain shows gram-negative rods. Culture on blood agar shows mucoid, iridescent colonies with a characteristic fruity odor. Which investigation is most appropriate to confirm the identity of the organism and guide antibiotic therapy?

Explanation

## Identification of Pseudomonas aeruginosa **Key Point:** Pseudomonas aeruginosa is a gram-negative, aerobic, motile rod that produces characteristic pigments and metabolic products. The oxidase test is the gold standard screening test, followed by biochemical identification. ### Oxidase Test — The Definitive Screening Tool The oxidase test is **positive** for P. aeruginosa (and most Pseudomonas species), making it the first-line confirmatory test. The organism oxidizes tetramethyl-p-phenylenediamine to a purple-black product within 10 seconds. **High-Yield:** Oxidase-positive gram-negative rods with mucoid colonies and fruity odor = P. aeruginosa until proven otherwise. ### API 20NE or Similar Biochemical Panel After oxidase positivity, API 20NE (Analytical Profile Index) or similar automated systems provide: - Definitive species identification - Antibiotic susceptibility patterns (especially important for P. aeruginosa, which is intrinsically resistant to many β-lactams and often multidrug-resistant) - Guidance for empiric therapy selection ### Clinical Pearls **Clinical Pearl:** P. aeruginosa in respiratory secretions often indicates: - Nosocomial infection in hospitalized patients - Chronic lung disease (CF, COPD, bronchiectasis) - Ventilator-associated pneumonia (VAP) **Clinical Pearl:** The fruity/grape-like odor is due to 2-aminoacetophenone and pyoverdine production — a helpful bedside clue. ### Why Oxidase + API? 1. **Oxidase** = rapid, specific, cost-effective screening (10 seconds) 2. **API 20NE** = comprehensive biochemical profile, species confirmation, and antibiotic susceptibility data This combination is the standard workflow in clinical microbiology laboratories and is recommended by [cite:Koneman's Textbook of Diagnostic Microbiology Ch 22]. **Mnemonic:** **OXIDASE-POSITIVE GRAM-NEGATIVE ROD** = **OPGNRod** → Think *Pseudomonas, Vibrio, Neisseria, Aeromonas*. Among these, mucoid colonies + fruity odor = P. aeruginosa.