Cetrimide Agar: Principle, Uses, Composition & Pseudomonas Identification

🔍 Introduction

Cetrimide agar is a selective and differential culture medium primarily used for isolation and identification of👉 Pseudomonas aeruginosa

It is especially important in:

• Clinical microbiology labs

• Hospital infection control

• Environmental & water testing

  
  

  

  
  

⚙️ Principle

🧪 Key Concept

• Contains cetrimide (cetyltrimethylammonium bromide)

• A quaternary ammonium compound (QAC)

  
  

🧠 Mechanism of Action

• Disrupts bacterial cell membrane permeability

• Causes leakage of cellular contents → death of susceptible bacteria

• Pseudomonas aeruginosa survives due to:

  • Intrinsic resistance

  • Efflux pumps

  • Low outer membrane permeability

👉 Result: Selective growth of Pseudomonas

🧫 Composition

Component	Function
Cetrimide	Selective agent (inhibits other bacteria)
Peptone	Nutrient source
Magnesium chloride	Enhances pigment production
Potassium sulfate	Promotes pyocyanin production
Agar	Solidifying agent

📌 Important: Mg²⁺ + sulfate ions → stimulate pigment synthesis

🎨 Colony Characteristics

On cetrimide agar, Pseudomonas aeruginosa shows:

• Blue-green colonies → due to pyocyanin

• Yellow-green fluorescence → due to pyoverdin

• Smooth, flat colonies

• Metallic sheen (sometimes)

• Grape-like odor

  
  
  
  

🔬 Differential Features

Feature	Observation
Pigment production	Enhanced
Fluorescence	Seen under UV light
Selectivity	Suppresses most bacteria
Specificity	High for P. aeruginosa

🧪 Laboratory Uses

1️⃣ Clinical Microbiology

• Isolation from:

  • Burn wounds

  • Pus

  • Urine

  • Respiratory samples

    
    

2️⃣ Hospital Infection Control

• Detects contamination in:

  • ICU equipment

  • Ventilators

  • Catheters

    
    

3️⃣ Water Microbiology

• Detection of Pseudomonas in:

  • Drinking water

  • Swimming pools

  • Dialysis fluids

    
    

🧠 Procedure (Step-wise)

1. Collect clinical specimen

2. Inoculate onto cetrimide agar

3. Incubate at 37°C for 18–24 hours

4. Observe:

   • Pigment production

   • Colony morphology

   • Fluorescence under UV

     
     

⚠️ Limitations

• Not 100% specific:

  • Some non-aeruginosa pseudomonads may grow

• Pigment production may vary in some strains

• Requires confirmation with:

  • Oxidase test

  • Biochemical tests

  • Molecular methods

    
    

⚖️ Comparison with Other Media

Medium	Purpose	Pseudomonas Growth
MacConkey agar	Lactose fermentation	Non-lactose fermenter
Blood agar	General growth	β-hemolysis
Cetrimide agar	Selective for Pseudomonas	Strong growth + pigment

🧠 Clinical Case Correlation

👉 Burn patient with greenish discharge

• Culture on cetrimide agar → blue-green colonies

  
  

  ✔ Confirms Pseudomonas aeruginosa

  
  

🎯 Exam Pearls

• Cetrimide agar = selective medium for Pseudomonas aeruginosa

• Contains quaternary ammonium compound

• Enhances pyocyanin production

• Shows fluorescence under UV

• Used in water testing + hospital surveillance

  
  

📚 References

1. Ananthanarayan & Paniker’s Textbook of Microbiology

2. Apurba Sastry – Essentials of Medical Microbiology

3. Bailey & Scott’s Diagnostic Microbiology

4. Koneman’s Color Atlas of Diagnostic Microbiology

5. Mackie & McCartney Practical Medical Microbiology