A 62-year-old man with chronic obstructive pulmonary disease presents with purulent sputum and fever. Sputum culture grows a Gram-negative rod that is oxidase-positive and non-fermenting. On blood agar, colonies are surrounded by a greenish discoloration of the medium. Which feature most reliably distinguishes this isolate from *Burkholderia cepacia*, another common respiratory pathogen in COPD?

Explanation

## Distinguishing *Pseudomonas aeruginosa* from *Burkholderia cepacia* ### Clinical Context Both organisms are Gram-negative, oxidase-positive, non-fermenting rods that colonize the respiratory tract in COPD and cystic fibrosis patients. However, they differ in virulence, epidemiology, and antibiotic susceptibility, making rapid identification critical. ### Comparison Table | Feature | *Pseudomonas aeruginosa* | *Burkholderia cepacia* | | --- | --- | --- | | **Pigment production** | Pyocyanin (blue-green), pyoverdine (fluorescent) | Yellow, brown, or no pigment | | **Growth at 42°C** | Yes (thermophilic) | No | | **Colistin susceptibility** | Susceptible | Resistant (intrinsic) | | **Mucoid phenotype** | Common in CF patients | Less common | | **Grape-like odor** | Yes (characteristic) | No | ### Key Point: Pyocyanin Production **Key Point:** Pyocyanin is a blue-green, water-soluble phenazine pigment produced exclusively by *P. aeruginosa* under aerobic conditions. It diffuses into the agar medium, creating the characteristic greenish discoloration visible on blood agar or MacConkey plates. This is pathognomonic for *P. aeruginosa* and is absent in *B. cepacia*. ### High-Yield Distinction **High-Yield:** The greenish discoloration of the medium (not just the colony) is due to pyocyanin diffusion and is virtually diagnostic of *P. aeruginosa*. *B. cepacia* may produce yellow or brown pigments, but never the characteristic blue-green hue. ### Clinical Pearl **Clinical Pearl:** In COPD and cystic fibrosis patients, *P. aeruginosa* is more common and generally more susceptible to antipseudomonal antibiotics (e.g., fluoroquinolones, carbapenems, antipseudomonal penicillins). *B. cepacia* is intrinsically resistant to colistin and polymyxin B, making it harder to treat and epidemiologically important in CF centers (risk of cross-infection). ### Mnemonic **Mnemonic:** **PYOCYANIN** = *P*seudo*m*onas aeruginosa **Y**ields **O**xidase-positive, **C**yanin (blue-green pigment), **A**erobic, **N**on-fermenting, **I**nfects **N**ose-and-lung (respiratory tract). [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 16]