Pseudomonas aeruginosa was first obtained in pure culture by Gessard in 1882 from cutaneous wounds which had a blue green discolouration1 and is the major human pathogen from a large genus of strictly aerobic Gram-negative rods which are widely distributed in nature.2 The majority of P aeruginosa strains produce at least two pigments, a fluorescent yellow pigment and a blue pigment called pyocyanin, which together give the characteristic colour noted above when the bacterium is grown on agar.3 P aeruginosa is motile by means of a single flagellum and thrives in moist environments; it is extremely versatile biochemically and can grow in many habitats including soil, surface waters, plants and various foods such as vegetables eaten by man.2 4 In hospitals P aeruginosa can be found in sinks, respirators, humidifiers, etc, and is occasionally found on the hands of medical personnel.2

P aeruginosa is an opportunistic pathogen which only causes disease in patients with impaired host defences. The patient’s defences may be generally weakened by debility or cancer, or there may be specific humeral or cellular defects. Neutropenic patients are especially susceptible to pseudomonas infection and to subsequent septicaemia. Alternatively, the body’s defences may be specifically breached as in corneal ulceration or skin burns, or artificially overcome as with assisted ventilation or by an indwelling urinary catheter.4 Patients with bronchiectasis are particularly prone to chronic infection, and delayed mucociliary clearance may be responsible.5 6 The use of broad spectrum antibiotics may kill commensal flora or more antibiotic-sensitive pathogenic species causing infection, and promote colonisation by the intrinsically resistant pseudomonas.7 P aeruginosa is particularly associated with progressive and ultimately fatal chronic respiratory infection in cystic fibrosis. Clues about the biological basis of this host-bacterial interaction which occurs almost inevitably are just being discovered. …