Last updated on June 4th, 2021
Pseudomonas species are a group of aerobic, non-spore-forming, Gram-negative rods found in the environment and soil. Only a few species are pathogenic, of which Pseudomonas aeruginosa is most common. Pseudomonas aeruginosa is an opportunistic pathogen and causes urinary tract infections, respiratory system infections, dermatitis, soft tissue infections, bacteremia, bone and joint infections, gastrointestinal infections, and a variety of systemic infections, mostly in immunosuppressed patients.
Due to its ability to survive under a variety of environmental conditions, it easily colonizes surgical instruments, catheter tubes, respiratory ventilators and is transmitted from patient to patient in healthcare facilities leading to nosocomial infections.
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How to identify P.aeruginosa in laboratory
In the laboratory, P.aeruginosa can be isolated from any sample (urine, pus, blood, ear swabs, tissue biopsies, body fluids, etc.)
They grow well on standard broth and solid media such as blood agar, chocolate agar, and MacConkey agar, which are recommended to isolate Pseudomonas species from clinical specimens. Selective agar containing inhibitors such as cetrimide can also be used for isolation and presumptive identification.
Colony morphology:
- On MacConkey agar colonies of P.aeruginosa are flat, 2-3 mm, smooth, non-lactose fermenting colonies with irregular margin (leafy margin) and slightly pigmented (greenish pigmentation).
- 2. On Blood agar: It is documented that, P. aeruginosa isolates may produce three colony types on blood agar depending on the source of isolation.
- Natural isolates from soil or water typically produce a small, rough colony.
- Clinical samples, yield one or another of two smooth colony types. On blood agar, Pseudomonas appear as give large colonies with metallic sheen, mucoid, rough, or pigmented (pyocyanin), and often β-hemolytic
- One type has a fried-egg appearance which is large, smooth, with flat edges and an elevated appearance.
- Another type, frequently obtained from respiratory and urinary tract secretions, has a mucoid appearance, which is attributed to the production of alginate slime.
- Cetrimide Agar is used as a selective medium for the isolation of Pseudomonas aeruginosa from pus, sputum and drains, etc. Pseudomonas aeruginosa produces yellow-green to blue colonies and fluoresces under UV light.
Gram stain
Pseudomonas aeruginosa is a Gram-negative rod measuring 0.5 to 0.8 µm by 1.5 to 3.0 µm.
Biochemical characteristics
- Pseudomonas aeruginosa is often preliminarily identified by its typical odor in vitro. The smell is described as grape-like, tortilla-like, or “Philadelphus coronarius-like” (production of aminoacetophenone).
- Catalase-positive
- Rapid oxidase-positive within 10 seconds (exception P. luteola and P. oryzihabitans)
- Motile by means of one or more polar flagella.
- It is not an active fermenter of carbohydrates and produces acid, but no gas, in glucose and is lactose-negative. Alkaline slant/alkaline deep (K/K) reaction in TSI or KIA agar.
- Few strains of P. aeruginosa secret a variety of pigments, including pyocyanin (blue-green), pyoverdine (yellow-green and fluorescent), pyomelanin (brown to black), and pyorubin (red-brown).
- Strict aerobic respiratory metabolism with oxygen but in some cases, nitrate has been used as an alternative that allows anaerobic growth.
- Can grow in temperatures up to 42°C.The combination of pyocyanin production and the ability to grow at 42°C is sufficient to distinguish P.aeruginosa from other Pseudomonas spp. (e.g., P.fluorescens, P.putida, P.stutzeri, P.putrefaciens).
All strains of P.aeruginosa may not produce Pyocyanin.
Distinguishing Characteristics from Other Species
| Species | Pyocyanin production | Fluorescein Production | Growth at 42°C | Arginine dehydrolase | Lysine decarboxylase | Gelatin liquefaction | Aesculin hydrolysis | Lactose utilization (oxidative) | Maltose utilization (oxidative) | Reduction of nitrate to nitrite |
| P.aeruginosa | + | + | + | + | – | + | – | – | – | + |
| P.fluorescens | – | + | – | + | – | + | – | – | – | – |
| P.putida | – | + | – | + | – | – | – | – | – | – |
| P.maltophila | – | – | + | – | + | + | + | – | + | – |
| P.stutzeri | – | – | + | – | – | – | – | – | + | + |
| P.cepacia | – | – | + | – | + | + | + | + | + | – |
| P.pseudomallei | – | – | + | + | – | + | + | + | + | + |
| P.mallei | – | – | + | + | – | + | * | * | +/- | + |
References and further readings:
- Public Health England. (2015). Identification of Pseudomonas species and other Non-Glucose Fermenters. UK Standards for Microbiology Investigations. ID 17 Issue 3. https://www.gov.uk/uk-standards-for-microbiology-investigations-smi-quality-and-consistency-in-clinical-laboratories
- Mackie and McCartney “Practical medical Microbiology”21st edition, Churchill Livingstone
- Image source: https://www.microbiologyinpictures.com/
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