Pseudomonas: Complete Guide for Medical Microbiology

🔍 Introduction

Pseudomonas is a genus of aerobic, non-fermenting Gram-negative bacilli, widely distributed in nature. The most clinically important species is:

👉 Pseudomonas aeruginosa

It is a major opportunistic pathogen, notorious for:

• Hospital-acquired infections (HAIs)

• Multidrug resistance

• High morbidity in immunocompromised patients

  
  

🧬 Classification

• Family: Pseudomonadaceae

• Important species:

  • P. aeruginosa (most important)

  • P. fluorescens

  • P. putida

📌 Exam Pearl: P. aeruginosa = only major human pathogen among pseudomonads

🔬 Morphology

• Gram-negative slender rods

• Size: 1.5–3 µm

• Motile (single polar flagella)

• Non-spore forming

• Sometimes capsulated

📌 Seen singly, in pairs

⚙️ Cultural Characteristics

🧫 Growth Media

• Aerobic, grows easily on ordinary media

• Non-lactose fermenter on MacConkey agar

• Beta-hemolytic on blood agar

• Produces metallic sheen

  
  

  

🎨 Pigments

• Pyocyanin → Blue-green (diagnostic)

• Pyoverdin → Yellow-green fluorescence

• Pyorubin → Red

• Pyomelanin → Brown

  
  

📌 Exam Pearl: Pyocyanin production = diagnostic of P. aeruginosa

👃 Odor

• Characteristic grape-like / fruity smell

🧪 Biochemical Reactions

Test	Result
Oxidase	✅ Positive
Catalase	✅ Positive
Indole	❌ Negative
Urease	❌ Negative
Glucose	Oxidative (not fermentative)

📌 Key Concept: Non-fermenter + oxidase positive = think Pseudomonas

🧫 Additional Important Biochemical Tests

Test	Result	Importance
O/F Test (Hugh–Leifson)	Oxidative (O+/F−)	Confirms non-fermenter
Nitrate Reduction	✅ Positive (to N₂ gas)	Key diagnostic feature
Citrate Utilization	✅ Positive	Uses citrate as carbon source
TSI Agar	K/K (alkaline/alkaline)	No sugar fermentation
Gelatin Liquefaction	✅ Positive	Proteolytic activity
Arginine Dihydrolase	✅ Positive	Important differentiating test
Lysine Decarboxylase	❌ Negative	Helps differentiate from Enterobacteriaceae
Ornithine Decarboxylase	❌ Negative	Usually negative
ONPG Test	❌ Negative	No lactose fermentation
H₂S Production	❌ Negative	Differentiates from Salmonella
Motility Test	✅ Positive	Polar flagella
Acetamide Utilization	✅ Positive	Highly specific for P. aeruginosa
Growth at 42°C	✅ Positive	Important identification feature

🧠 Virulence Factors

• Exotoxin A → inhibits protein synthesis (like diphtheria toxin)

• Elastase → tissue destruction

• Phospholipase C

• Pyocyanin → ROS damage

• Alginate (biofilm) → chronic infection (CF patients)

• Endotoxin (LPS)

📌 Biofilm formation = major cause of antibiotic resistance

🦠 Pathogenesis

• Opportunistic pathogen

• Infects:

  • Immunocompromised

  • Burn patients

  • ICU patients

  • Cystic fibrosis patients

    
    

🏥 Clinical Manifestations

🔴 1. Skin & Soft Tissue

• Burn wound infection (blue-green pus)

• Ecthyma gangrenosum (pathognomonic)

• Folliculitis (hot tub)

  
  

🫁 2. Respiratory Infections

• Ventilator-associated pneumonia

• Chronic infection in cystic fibrosis

  
  

🚽 3. Urinary Tract Infection

• Catheter-associated UTI

  
  

🧠 4. CNS

• Meningitis (post neurosurgery)

  
  

❤️ 5. Septicemia

• High mortality

• Seen in neutropenic patients

  
  

👂 6. Ear Infection

• Malignant otitis externa (diabetics)

  
  

👁️ 7. Eye Infection

• Keratitis (contact lens users)

  
  

🧫 Laboratory Diagnosis

1️⃣ Specimens

• Pus, sputum, urine, blood, CSF

  
  

2️⃣ Microscopy

• Gram-negative rods

  
  

3️⃣ Culture

• Non-lactose fermenting colonies

• Pigment production

  
  

4️⃣ Special Media

• Cetrimide agar (selective)

  
  

5️⃣ Identification

• Oxidase positive

• Pigment production

• Growth at 42°C (important feature)

📌 Exam Pearl: Growth at 42°C helps differentiate from other pseudomonads

💊 Antibiotic Sensitivity & Resistance

🚨 Resistance Mechanisms

• Efflux pumps

• Beta-lactamases

• Porin mutations

• Biofilm

  
  

💉 Treatment Options

Anti-pseudomonal drugs:

• Piperacillin-tazobactam

• Ceftazidime

• Cefepime

• Carbapenems (Imipenem, Meropenem)

• Aminoglycosides (Amikacin)

• Fluoroquinolones (Ciprofloxacin)

📌 Exam Pearl: Ceftazidime = classic anti-pseudomonal cephalosporin

🧬 Infections in Special Conditions

🧫 Cystic Fibrosis

• Mucoid strains

• Biofilm → chronic infection

  
  

🔥 Burns

• Leading cause of mortality

  
  

🧠 Clinical Case Correlation

👉 Case 1:Burn patient with greenish pus + fruity odor→ Likely Pseudomonas aeruginosa

👉 Case 2:Diabetic with severe ear pain → skull base involvement→ Malignant otitis externa

👉 Case 3:Neutropenic patient with black necrotic skin lesions→ Ecthyma gangrenosum

⚠️ Infection Control

• Strict hospital hygiene

• Sterilization of equipment

• Avoid contaminated water sources

  
  

🎯 Exam Pearls (High-Yield)

• Blue-green pigment = Pyocyanin

• Oxidase positive + non-fermenter

• Growth at 42°C

• Grape-like odor

• Ecthyma gangrenosum = hallmark

• Cetrimide agar = selective medium

• Major cause of nosocomial infections

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📚 References (Standard Textbooks)

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 & Textbook of Diagnostic Microbiology

5. Mackie & McCartney Practical Medical Microbiology