Overview
Pseudomonas is a mostly saprophytic genus found widely in soil, water and other moist environments. It has emerged as one of the most important causes of health-care associated and opportunistic infections worldwide, with most clinical isolates showing resistance to many commonly used antibiotics.
Pseudomonas aeruginosa, the type species, is a strict aerobe, motile Gram-negative rod belonging to the family Pseudomonadaceae. Because it thrives in compromised hosts — burns patients, ICU patients, cystic fibrosis patients — recognising its distinctive pigments and biochemical profile is a critical laboratory skill.
Learning Objectives
After this lesson you will be able to…- Classify Pseudomonas and describe its morphological characteristics.
- Enumerate the key biochemical characteristics used for identification.
- Explain the mechanisms of virulence and pathogenicity, including exotoxin A and biofilm formation.
- Enumerate the range of infections caused by Pseudomonas in different organ systems.
- Describe how to culture and identify Pseudomonas from clinical specimens.
- Discuss antibiotic susceptibility patterns and mechanisms of drug resistance.
Clinical Story
Why This MattersA patient in the burns ward develops a wound with a distinct sweet, fruity odour and a blue-green discolouration of the dressing. The nursing staff sends a wound swab to microbiology. On Gram stain, thin Gram-negative rods are seen; culture on nutrient agar produces flat, spreading colonies with a bluish-green pigment diffusing into the medium. The oxidase test is strongly positive. The technologist recognises this classic picture instantly as Pseudomonas aeruginosa — a leading cause of burn wound infection worldwide.
Core Concepts
Pseudomonas belongs to Class Gamma Proteobacteria, Order Pseudomonadales, Family Pseudomonadaceae. There are eight recognised groups (P. aeruginosa, P. chlororaphis, P. fluorescens, P. pertucinogena, P. putida, P. stutzeri, P. syringae, P. incertae sedis) among 191 valid species; P. aeruginosa is the type species.
Morphologically, Pseudomonas is a slender, rod-shaped (0.5–0.8 µm × 1.5–3.0 µm) Gram-negative organism, motile by polar flagella (sometimes more than two). Mucoid strains, particularly from cystic fibrosis patients, produce a pseudocapsule (glycocalyx) of polysaccharide that protects against host defenses.
Pseudomonas is a strict (obligate) aerobe but can grow anaerobically if nitrate is available as an electron acceptor. Optimum temperature is 37°C; it grows on ordinary media including nutrient agar and even in distilled water. Colonies are large, opaque, flat with irregular margins and a distinctive fruity odour.
Diagnostic pigments:
- Pyoverdin — fluorescent, greenish-yellow.
- Pyocyanin — blue, bluish-green.
- Pyorubin — red.
- Pyomelanin — brown.
Pseudomonas has an oxidative metabolism — being non-fermentative, it does not produce acid from peptone water sugars via fermentation. Key reactions: oxidase positive, catalase positive, nitrates reduced to nitrites, arginine dihydrolase positive, glucose utilized oxidatively (OF test). Indole, methyl red (MR), Voges-Proskauer (VP) and H₂S production are all negative. The oxidase test is the commonest screening biochemical test used in the lab.
Pseudomonas usually cannot infect a normal host but readily infects any tissue or organ system in an immunocompromised host. Infection proceeds through three steps: bacterial attachment/colonization, local invasion, and disseminated systemic disease.
Key virulence factors include exotoxin A (inactivates elongation factor 2, halting protein synthesis and killing the cell), exoenzyme Exo U (damages cell membranes), elastase, alkaline protease, cytotoxin (leukocidin), hemolysins (phospholipase and lecithinase), and the pigment pyocyanin which impairs nasal ciliary function.
Pseudomonas forms biofilm through quorum sensing — when a critical bacterial density is reached, cells communicate via signalling molecules, triggering gene expression that produces slime, glycocalyx, exotoxins and enzymes. Bacteria embedded in biofilm are protected from host defenses and antibiotics, and biofilms on medical devices are a major cause of chronic opportunistic and nosocomial infections.
Pseudomonas causes respiratory infections (pneumonia, especially in cystic fibrosis and neutropenic patients), bacteremia/septicemia (accounts for 25% of hospital-acquired Gram-negative BSIs), otitis externa ('swimmer's ear'), CNS infections, hospital-acquired UTI (third leading cause, 12%), endocarditis, bone/joint infections, and skin/soft tissue infections including burns.
Pseudomonas is inherently resistant to many antibiotics via multidrug efflux pumps, chromosomal resistance genes (mexAB, mexXY), and low membrane permeability in biofilms. It remains sensitive to aminoglycosides, ceftazidime/cefotaxime, fluoroquinolones, piperacillin/ticarcillin and colistin.
Key Biochemical Panel
Identification Data⚠️ Values summarised from standard microbiology references; always confirm with your laboratory's SOPs.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Bluish-green pigmented colonies with fruity odour on MacConkey/nutrient agar | Presumptive P. aeruginosa (pyocyanin production) | Confirm with oxidase test, OF test, arginine dihydrolase; report to clinician especially in burns/ICU patients |
| Pale, non-lactose-fermenting colonies on MacConkey agar, oxidase positive | Consistent with Pseudomonas species | Perform full biochemical panel and susceptibility testing |
| Mucoid colonies from cystic fibrosis sputum | Suggests chronic biofilm-forming, more virulent strain | Correlate with clinical status; consider anti-pseudomonal combination therapy |
| Multidrug resistant isolate from ICU patient | Possible efflux pump-mediated or biofilm-associated resistance | Perform full antibiogram (Kirby-Bauer); escalate infection control measures |
Common Errors & How to Avoid Them
Cause: Several other Gram-negative organisms are also oxidase positive.
Prevention: Confirm with a full panel: pigment production, fruity odour, OF test, arginine dihydrolase and growth characteristics before finalising identification.
Cause: Pigments like pyocyanin and pyoverdin may be under-produced on some media or with inadequate incubation.
Prevention: Use nutrient agar and adequate incubation time; examine plates under normal and, where available, UV light for fluorescent pyoverdin.
Cause: Standard susceptibility testing on planktonic organisms may not reflect true resistance of biofilm-embedded bacteria on catheters/prosthetics.
Prevention: Correlate lab susceptibility results with clinical response; consider device removal in refractory device-associated infections.
Laboratory Tips from the Bench
The fruity, grape-like odour of Pseudomonas cultures is a strong bedside/bench clue — train your nose alongside your eyes when working up suspicious Gram-negative rod isolates.
Always test oxidase directly from a MacConkey agar colony when Pseudomonas is suspected, as it is the fastest and most reliable initial screening step in a busy lab.
Remember Pseudomonas pigments with 'VBRM': pyoVerdin (green-yellow, fluorescent), pyocyanin (Blue), pyoRubin (red), pyoMelanin (brown).
Important Notes
Burns patients, neutropenic cancer patients, cystic fibrosis patients and ICU/ventilated patients are at particularly high risk for serious, sometimes fatal, Pseudomonas infections.
Because Pseudomonas is inherently resistant to many agents, always perform full susceptibility testing rather than relying on empirical regimens for serious infections.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeDevelops a foul-smelling, bluish-green discolouration of the burn wound dressing with increasing wound exudate and low-grade fever. No prior antibiotic exposure this admission.
The characteristic blue-green pigment (pyocyanin), fruity odour, and oxidase-positive Gram-negative rod growth from a burn wound are classic for Pseudomonas aeruginosa infection — one of the most common and serious pathogens in burns patients.
- →Burn patients are highly susceptible to Pseudomonas due to loss of the skin barrier.
- →Pigment production and fruity odour are strong presumptive identification clues.
- →Full antibiotic susceptibility testing is essential given the organism's intrinsic multidrug resistance.
Frequently Asked Questions
It combines multiple mechanisms: multidrug efflux pumps, low outer-membrane permeability, chromosomally-encoded resistance genes (e.g. mexAB, mexXY), and protective biofilm formation, especially on medical devices.
Yes — Pseudomonas can be a harmless commensal in healthy individuals; it typically only causes disease when host defenses are compromised (burns, neutropenia, cystic fibrosis, catheterization, mechanical ventilation).
The OF (oxidation-fermentation) test showing an oxidative reaction and a positive arginine dihydrolase test are used as confirmatory steps alongside colony morphology and pigment.
Quick Revision
10-Minute ReviewKey Takeaways
- Pseudomonas aeruginosa is a Gram-negative, oxidase-positive, strictly aerobic rod that is a leading opportunistic and nosocomial pathogen.
- Distinctive pigments (pyocyanin, pyoverdin) and a fruity odour aid rapid presumptive identification.
- Key virulence factors include exotoxin A, elastase, alkaline protease and biofilm formation via quorum sensing.
- Pseudomonas primarily infects immunocompromised hosts — burns, cystic fibrosis, neutropenia and device-associated infections are classic settings.
- The organism is intrinsically resistant to many antibiotics, making susceptibility testing essential for effective treatment.
- Laboratory diagnosis relies on culture on blood/MacConkey agar, colony/pigment inspection, oxidase and OF testing.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology Module — Lesson 28: Pseudomonas.
- Collee JG, et al. Mackie & McCartney Practical Medical Microbiology, 14th ed.
- Murray PR, et al. Manual of Clinical Microbiology.