Pseudomonas: Introduction, Pathogenecity, Lab Diagnosis and Treatment

Mucoid Pseudomonas

Pseudomonas aeruginosa

There are 6 different types of Pseudomonas aeruginosa colonies that may be observed-

Type 1. Large and leafy

2.  smooth, circular, and coliform type Type

3. Rough Type

4.  rogose Type

5. Mucoid due to exopolysaccharide as shown above image  and

Type 6.  Dwarf and smallest

Details about Pseudomonas aeruginosa

General features

Pseudomonas aeruginosa is widely distributed in soil and water, Gram-negative rods-,aerobic, motile, produce water-soluble pigments and opportunistic pathogens.

Morphology of Pseudomonas aeruginosa

They are slender gram-negative bacillus, 1.5 – 3µm x 0.5 µm, and active motile due to having a polar flagellum. Non-sporing, non- capsulated but many strains have mucoid slime layer. Isolates from Cystic fibrosis patients have an abundance of extracellular polysaccharides composed of alginate polymers. They escape the defense mechanisms by loose capsules in which microcolonies of the bacillus are enmeshed and protected from host defense. They form round colonies with fluorescent greenish color, sweet odor, and hemolysis. Pyocyanin- nonfluorescent bluish pigment; pyoverdin- fluorescent greenish pigment; pyorubin (reddish-brown pigment), and pyomelanin (brown to black pigment). Some strains have a prominent capsule (alginate).

Culture Characteristics of Pseudomonas aeruginosa

They are obligate aerobe but grow anaerobically if nitrate is available. Growth occurs at a wide range of temperatures 6 to 42°C the optimum being 37 °C. Growth on ordinary media producing large opaque irregular colonies with distinctive musty mawkish or earthy smell. Iridescent patches with metallic sheen are seen in cultures on nutrient agar. In broth forms dense turbidity with a surface pellicle. Many organic compounds are used as carbon and nitrogen sources, but only a few carbohydrates by oxidative metabolism. Glucose was used oxidatively and lactose negative on MacConkey’s agar.

Pigment Production of Pseudomonas

Some strains produce diffusible pigments like Pyocyanin (blue), pyoverdin or fluorescein ( greenish-yellow), pyorubin (reddish-brown), and pyomelanin ( brown to black).

 Biochemical reactions of Pseudomonas

They are oxidative and non- fermentative. Glucose is utilized oxidatively. Indole, Methyl Red (MR)  and  Voges-Proskauer (VP)  and  hydrogen sulphide (H2S) tests are negative. Catalase, oxidase, and arginine tests are positive.

Toxins and enzymes

Toxic extracellular products in culture filtrates, exotoxin A and S. Exotoxin A acts as NADase resembling Diphtheria toxin. Proteases, elastase hemolysins, and enterotoxin,  Slime layer and Biofilms

 Typing and its importance

An important cause of Hospital Infections and important for epidemiological purposes. Serotyping, bacteriocins typing, pyocyanin, restriction endonuclease typing with pulse-field gel electrophoresis


They are Killed at 55°C in 1 hour.  High resistance to chemical agents. Resistance to quaternary ammonium compounds like chloroxylenol, resistant to hexachlorophene. They also grow in antiseptic bottles like Dettol as cetrimide as a selective medium. and sensitive to acids silver salts, beta glutaraldehyde.

Pathogenicity of Pseudomonas

They cause blue pus,  causing nosocomial infection, suppurative otitis, localized and generalized infections, Urinary tract infection (UTI) after catheterization, iatrogenic meningitis, post tracheostomy pulmonary infections, etc.


This organism is widely distributed in nature and is commonly present in most hospitals. It is pathogenic only when introduced into areas devoid of normal defenses, e.g. disruption of mucous membrane and skin, usage of intravenous or urinary catheters, neutropenia (as in cancer therapy). Antigenic structure, protease enzymes, and toxins Serine protease, Pili, and non-plus adhesins. metalloprotease and alkaline Capsule (alginate, glycocalyx): protease causes tissue seen in cultures from patients damage and helps bacteria with cystic fibrosis. spread lips- endotoxin, multiple phospholipase C: hemolysin immunotypes. Exotoxin A: causes tissue Pyocyanin: catalyzes necrosis and is lethal for animals production of toxic forms of (disrupts protein synthesis); oxygen that cause tissue immunosuppressive damage. It also induces IL-8production. Pyoverdin: an Exoenzyme S and T: cytotoxic to siderophore host cells.

 Susceptibility to Infections 

This bacterium is of particular concern to individuals with cystic fibrosis who are highly susceptible to Pseudomonas lung infections. It is also of grave concern to cancer and burns patients as well as those people who are immuno-compromised. The case fatality rate for individuals infected with this approaches 50%.

 Laboratory  Diagnosis of Pseudomonas aeruginosa

Diagnosis of P. aeroginosa infection depends upon isolation and laboratory identification of the bacterium. It grows well on most laboratory media and commonly is isolated on blood agar or eosin-methyl thionine blue agar. It is identified on the basis of its Gram morphology, inability to ferment lactose, a positive oxidase reaction, its fruity odor, and its ability to grow at 42°C. Fluorescence under ultraviolet light is helpful in the early identification of P. aeruginosa colonies. Fluorescence is also used to suggest the presence of P. aeruginosa in wounds.


Following antimicrobial agents are useful to get rid of this microorganism and they are gentamycin, amikacin, Cephalosporin,  cefotaxime, Ceftazidime,  Ofloxacin, Piperacillin, ticarcillin, Piperacillin tazobactam, meropenem, imipenem, tobramycin,  colistin B, and polymyxin. For local application, colistin and polymyxin are also recommended.

Further Readings

  1. Cowan & Steel’s Manual for identification of Medical Bacteria. Editors: G.I. Barron & R.K. Felthani, 3rd ed 1993, Publisher Cambridge University Press.
  2. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  3. Clinical Microbiology Procedure Handbook Vol. I & II, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  4. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr, and Sommers H.M.
  5. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  6. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  7.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
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