A 42-year-old man with a history of recurrent respiratory tract infections is admitted to a tertiary care hospital in Delhi with fever (38.5°C), productive cough with greenish sputum, and dyspnea. Chest X-ray shows bilateral lower lobe infiltrates. Sputum culture is requested. The laboratory receives the specimen and inoculates it on routine blood agar and MacConkey agar. After 24 hours of incubation at 35°C in 5% CO₂, no growth is observed on either medium. However, the clinician suspects *Pseudomonas aeruginosa* based on the clinical presentation and sputum appearance. Which selective medium should be used to enhance isolation of *P. aeruginosa* from this sputum specimen?

Explanation

## Selective Media for *Pseudomonas aeruginosa* Isolation ### Why Cetrimide Agar is the Correct Choice **Key Point:** Cetrimide agar (also called Pseudomonas isolation agar or Pseudomonas aeruginosa medium) is the selective medium of choice for primary isolation of *P. aeruginosa* from mixed bacterial populations, especially from respiratory specimens. **High-Yield:** Cetrimide agar contains: - Cetrimide (cetyl trimethyl ammonium bromide) — a quaternary ammonium compound - Selective agent that inhibits most gram-positive bacteria and many gram-negative bacteria - Allows *P. aeruginosa* to grow preferentially - Nalidixic acid (in some formulations) — further enhances selectivity ### Characteristics of Cetrimide Agar | Feature | Details | |---------|----------| | **Selectivity** | Inhibits gram-positive bacteria, most gram-negatives except *Pseudomonas* | | **Colony appearance** | Green or blue-green pigment (pyoverdine/pyocyanin) | | **Pigment production** | Characteristic greenish discoloration of medium | | **Growth conditions** | 35–37°C, aerobic | | **Incubation time** | 24–48 hours | | **Clinical significance** | Greenish sputum is a classic sign of *P. aeruginosa* infection | ### Why Routine Media Failed **Clinical Pearl:** *P. aeruginosa* grows on blood agar and MacConkey agar, but in mixed sputum cultures, it may be overgrown by faster-growing commensals (e.g., *Staphylococcus aureus*, *Streptococcus pneumoniae*, *Haemophilus influenzae*). Selective media suppress these competitors and allow *P. aeruginosa* to predominate. ### Mechanism of Selectivity ```mermaid flowchart TD A[Sputum specimen with mixed flora]:::outcome --> B[Inoculated on Cetrimide agar]:::action B --> C{Organism susceptibility to cetrimide?}:::decision C -->|Susceptible| D[Growth inhibited]:::urgent C -->|Resistant| E[Growth allowed]:::action D --> F[Gram-positive cocci suppressed]:::outcome D --> G[Most gram-negatives suppressed]:::outcome E --> H[*P. aeruginosa* grows preferentially]:::outcome H --> I[Green/blue-green colonies]:::outcome ``` ### Identification of *P. aeruginosa* on Cetrimide Agar **Mnemonic — "P. aeruginosa on Cetrimide: GREEN & MEAN":** - **G**reen pigment (pyoverdine) - **R**ound, mucoid or non-mucoid colonies - **E**nzymes: oxidase-positive (key confirmatory test) - **E**xotoxins: virulent pathogen - **N**on-lactose fermenter - **M**otile (flagellated) - **E**arthy/grape-like odor - **A**erobic - **N**eeds selective media in mixed cultures ### Clinical Significance in This Case The greenish sputum is pathognomonic for *P. aeruginosa* infection. The patient's recurrent respiratory infections and bilateral infiltrates suggest chronic *P. aeruginosa* colonization (possibly underlying bronchiectasis or cystic fibrosis). Cetrimide agar will: 1. Suppress contaminating flora 2. Allow *P. aeruginosa* to grow preferentially 3. Facilitate rapid identification via pigment and oxidase test 4. Enable antimicrobial susceptibility testing (critical for *P. aeruginosa* due to high antibiotic resistance) ### Other Selective Media for Gram-Negatives | Medium | Target Organism | Selectivity | |--------|-----------------|-------------| | **Cetrimide agar** | *Pseudomonas aeruginosa* | Cetrimide inhibits most bacteria except *P. aeruginosa* | | **MacConkey agar** | Enterobacteriaceae | Bile salts inhibit gram-positive bacteria | | **Thayer-Martin** | *Neisseria gonorrhoeae* | Antibiotics inhibit normal flora | | **Sabouraud agar** | Fungi | Low pH inhibits bacteria | [cite:Mackie McCartney Practical Medical Microbiology 14e Ch 12]