24       Pseudomonas aeruginosa

 

Pseudomonas species are environmental organisms which occur worldwide in water and soil, and on plants. Pseudomonas aeruginosa is a Gram-negative, obligate aerobe; it is motile and can produce up to four diffusible pigments. Pyocyanin (blue-green), unique to this organism, is produced by most strains and specifically identifies P. aeruginosa. Although P. aeruginosa is an environmental organism, it is also infrequently found on the skin, on mucous membranes and in faeces of some healthy animals.

Pseudomonas aeruginosa is an opportunistic pathogen and infection is preceded by a breach in host defences such as breaks in the skin, prolonged wetting or the presence of urinary or intravenous catheters. The organism produces a variety of toxins and enzymes which promote tissue invasion and damage. Attachment to host cells is mediated by fimbriae. Colonization and replication are aided by exoenzymes, extracellular slime and outer-membrane lipopolysaccharides. Tissue damage is caused by toxins such as exotoxin A, phospholipase C, proteases and cytotoxins delivered into host cells via a type III secretion system. The cytoplasmic membranes of neutrophils are damaged by a leukocidin. Host defence mechanisms against P. aeruginosa include opsonizing antibodies and phagocytosis by macrophages. Biofilm formation is an important virulence attribute which shields the organism from phagocytosis and from the action of antimicrobial agents.

Pseudomonas aeruginosa causes a wide range of opportunistic infections (Table 24.1). Although predisposing factors are associated with the occurrence of many of these infections, some species such as farmed mink appear to be particularly susceptible to the organism. Haemorrhagic pneumonia and septicaemia, caused by P. aeruginosa, occur sporadically in ranched mink with mortality rates up to 50% in some outbreaks. Bovine mastitis associated with this organism is often linked to udder washing with contaminated water, the insertion of contaminated intramammary antibiotic tubes or contaminated wipes. Fleece-rot of sheep, a condition associated with heavy or prolonged rainfall, has been reported from the UK and Australia. Maceration of the skin following water penetration of the fleece allows colonization by P. aeruginosa, resulting in suppurative dermatitis. Specimens suitable for laboratory examination include pus, respiratory aspirates, mid-stream urine, mastitic milk and ear swabs. Blood agar and MacConkey agar plates, inoculated with suspect material, are incubated aerobically at 37°C for 24–48 hours. On blood agar, the large flat colonies with a characteristic grape-like odour resemble those of some Bacillus species. Pyocyanin production is evident on both media. On MacConkey agar, lactose is not fermented.

Table 24.1 Clinical conditions in which Pseudomonas aeruginosa may be aetiologically involved.

Host	Disease condition
Cattle	Mastitis, metritis, pneumonia, dermatitis, enteritis (calves)
Sheep	Mastitis, fleece-rot, pneumonia, otitis media
Pigs	Respiratory infections, otitis
Horses	Genital tract infections, pneumonia, ulcerative keratitis
Dogs, cats	Otitis externa, cystitis, pneumonia, ulcerative keratitis
Mink	Haemorrhagic pneumonia, septicaemia
Chinchillas	Pneumonia, septicaemia
Reptiles (captive)	Necrotic stomatitis

Due to a number of diferent mechanisms, including low permeability of its outer membrane, efflux pumps and chromosomally encoded β-lactamases, P. aeruginosa is inherently resistant to many antimicrobial agents. Thus, susceptibility testing should always be carried out on clinical isolates.