Summary Sheet Pseudomonas Aeruginosa

A small child in hospital with a burn on the forearm developed a purulent discharge at the site of the burn. Pus was aspirated and sent to the laboratory and cultured in broth. Since this infection was associated with a burn and also with hospital, therefore the first causative organism suspected would be Pseudomonas aeruginosa.

P. aeruginosa is a motile, Gram negative rods, facultative anaerobes bacteria that are non-spore forming and non-fermentative. It is a highly versatile bacterium that can utilise many different energy sources (2); hence it can grow readily on routine media including bile-containing selective media (3). Pseudomonas is distinctive in its ability to produce pigment, P. aeruginosa, in particular can be distinguished from other Pseudomonas by production of the blue-green pigment; pyocyanin.

Test HBA/O2 HBA/CO2 MAC/O2 Gram Oxidase Catalase King's A King's B

Observations

Growth Growth Lac -ve

Gram

-ve rods +ve +ve Blue pigment Yellow Pigment

Table 1: Results for 1st stage test and growth on HBA/O2, HBA/CO2, and MAC/O2

P. aeruginosa was identified on the basis of its Gram morphology (Gram -ve rod), inability to ferment lactose (lac-ve on MAC), a positive oxidase reaction and its distinctive fruity odour. To confirm P. aeruginosa, pigment production were enhance on special media (King's A and King's B). King's A media enhances pyocyanin production while King's B media enhances the production of the fluorescent pigment called pyoverdin. Fluorescence under ultraviolet light is helpful in early identification of P. aeruginosa colonies and may help identify its presence in wounds.

P. aeruginosa is a free-living bacterium that is commonly inhabit in soil, water, vegetation, and any other places that contain moisture (3); hence it is often associated with wet areas in the environments such as sinks and shower (Pitten et al., 2001). P. aeruginosa can be isolated from the skin, throat, and stool of healthy individuals. It is carried as a normal gut flora in a small percentage of healthy individuals; however, in hospital inpatients the proportion is higher (6). As an opportunistic pathogen, P. aeruginosa infects individuals that are in some way have compromised host defence; such as cystic fibrosis patients, burn victims and cancer patients that are undergoing chemotherapy. Hence it is primarily a nosocomial pathogen.

Exotoxin A is one of the most important virulence factors produced by pathogenic strains of P. aeruginosa. This toxin disrupts protein synthesis by blocking peptides elongation like diphtheria toxin (1). Exotoxin A most likely contributes to the dermatonecrosis that occurs in burn wounds, corneal damage in ocular infection and tissue damage in chronic pulmonary infections. This toxin is also immune suppressive (4). Two enzymes, LasA (serine protease) and LasB (zinc metalloprotease), act to degrade elastin, resulting damage to elastin-containing tissues and producing the lung parenchymal