Pseudomonas aeruginosa

Gram stain of pseudomonas cells
Gram stain of pseudomonas cells

Kingdom: Bacteria
phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: Pseudomonadales
Family: Pseudomonadadaceae
Genus: Pseudomonas
Species: aeruginosa

Pseudomonas aeruginosa is member of the Gamma Proteobacteria class of Bacteria. It is a Gram-negative, aerobic rod belonging to the bacterial family Pseudomonadaceae. Members of the genus Pseudomonas which are cleaved into eight groups, Pseudomonas aeruginosa is a species of its group which contains 12 other members.

Like other members of the genus, Pseudomonas aeruginosa is a free-living bacterium, commonly found in soil and water. However, it occurs regularly on the surfaces of plants and occasionally on the surfaces of animals. Members of the genus are well known to plant microbiologists because they are one of the few groups of bacteria that are true pathogens of plants. However, Pseudomonas aeruginosa has become increasingly recognized as an emerging opportunistic pathogen of clinical relevance.

Pseudomonas aeruginosa is an opportunistic pathogen, meaning that it exploits a weakness in the host’s defences to initiate an infection. The bacterium almost never infects uncompromised tissues, yet there is hardly any tissue that it cannot infect if the tissue defences are compromised in some manner. It causes urinary tract infections, respiratory system infections (especially in those of cystic fibrosis), dermatitis, soft tissue infections, bacteremia, bone and joint infections, gastrointestinal infections and a range of systemic infections, particularly in patients with severe burns and in cancer and AIDS patients who are immunosuppressed or immunocomprimised. Pseudomonas aeruginosa infection is a serious and potentially fatal problem in patients with cancer, cystic fibrosis, and burns.


  • Pseudomonas aeruginosa is a Gram-negative rod measuring 0.5 to 0.8 µm by 1.5 to 3.0 µm. Almost all strains are motile by means of a single polar flagellum.

  • The bacterium is ubiquitous in soil and water, and on surfaces in contact with soil or water. Its metabolism is respiratory and never fermentative, but it will grow in the absence of O2 if NO3 is available as a respiratory electron acceptor.

  • The typical Pseudomonas bacterium in nature might be found in a biofilm, attached to some surface (example medical device such as catheter) or substrate, or in a planktonic form, as a unicellular organism, actively swimming by means of its flagellum (see video to right). Pseudomonas is one of the most vigorous, fast-swimming bacteria seen in pond water samples.

  • Pseudomonas aeruginosa has very simple nutritional requirements. It is often observed "growing in distilled water", which is evidence of its minimal nutritional needs.

  • Its optimum temperature for growth is 37 degrees, and it is able to grow at temperatures as high as 42 degrees.

  • It is tolerant to a wide variety of physical conditions, including temperature. It is resistant to high concentrations of salts and dyes, weak antiseptics, and many commonly used antibiotics- which it continues to become more resistant to.

  • Pseudomonas aeruginosa has a preference for growth in moist environments, which is probably a reflection of its natural existence in soil and water.

Resistance to Antibiotics

Pseudomonas aeruginosa is notorious for its resistance to antibiotics and is, therefore, a particularly dangerous and dreaded pathogen. The bacterium is naturally resistant to many antibiotics due to the permeability barrier afforded by its Gram-negative outer membrane. Also, its tendency to colonize surfaces in a biofilm form makes the cells impermeable to antibiotics.
Since its natural habitat is the soil, living in association with the bacilli, actinomycetes and molds, it has developed resistance to a variety of their naturally-occuring antibiotics.
Only a few antibiotics are effective against Pseudomonas aeruginosa, including fluoroquinolones, gentamicin and imipenem, and even these antibiotics are not effective against all strains. The futility of treating Pseudomonas infections with antibiotics is most dramatically illustrated in cystic fibrosis patients, virtually all of whom eventually become infected with a strain that is so resistant that it cannot be treated.

An article on the growth of Pseudomonas aeruginosa biofilms in cystic fibrosis patients.

BBC article from 2002, reporting on the cystic fibrosis "superbug"