Pseudomonas aeruginosa is n aerobic, oxidase-positive, gram-negative, bacillus that produces a blue-green pigment (pyocyanin) and has an odor of trimethylamine (urine-like). It is ubiquitous in nature and has been isolated environmentally from fruits, vegetables, ornamental plants, solutions, body lotions, faucets, sinks, soil, sponges, mops and water. It has been isolated from the respiratory tract, burns, wounds, blood, urogenital tract, spinal and joint fluid of humans.

P. aeruginosa is not particularly distinctive as a pseudomonad, but there are a few characteristics that are noteworthy and relate to its pathogenesis. The organism can be isolated from soil and water, particularly in enrichments for denitrifying bacteria. Although the bacterium is respiratory and never fermentative, it will grow in the absence of O₂ if NO₃ is available as a respiratory electron acceptor. P. aeruginosa possesses the metaboli versatility for which pseudomonads are so renowned. Organic growth factors are not required, and it can use more than thirty organic compounds for growth. Pseudomonas aeruginosa is often observed growing in "distilled water" which is evidence of its minimal nutritional requirements. Its optimum temperature for growth is 37 degrees, and it is able to grow at temperatures as high as 42 degrees. Its tolerance to a wide variety of physical conditions, including temperature, contributes to its ecological success as an opportunistic pathogen. Pseudomonas aeruginosa does, however, show a predilection for growth in moist environments, a reflection of its natural existence in soil and water.

Pseudomonas aeruginosa is primarily a nosocomial pathogen. According to the CDC, the overall incidence of P. aeruginosa infections in US hospitals averages about 0.4 percent (4 per 1000 discharges), and the bacterium is the fourth most commonly-isolated nosocomial pathogen accounting for 10.1 percent of all hospital-acquired infections. It is a major cause of nosocomial (hospital-acquired) infections.