Pseudomonas & Burkholderia
1-introduction
Pseudomonads are Gram-negative, aerobic, rod-shaped bacteria with widespread occurrence in nature, especially in damp biotopes.
The relevant virulence factors are: exotoxin A, exoenzyme S, cytotoxin, various metal proteases, and two types of phospholipase C. the lipopolysaccharide of the outer membrane also plays an important role in the pathogenesis. Pseudomonas infections occur only in patients with weakened immune defense systems, The most important species in this group from a medical point of view is P. aeruginosa.
P. aeruginosa frequently contributes to nosocomial infections, notably pneumonias in cystic fibrosis, colonization of burn wounds, endocarditis in drug addicts, postoperative wound infection, urinary tract infection,
sepsis.
Numerous other Pseudomonas species and the species of the genera Burkholderia are occasionally found in pathogenic roles in immunosuppressed patients. B. mallei causes malleus (glanders) and B. pseudomallei causes melioidois ,a disease of wild rodents caused by the bacterium. It can be transmitted to humans, possibly by rat fleas, causing pneumonia, multiple abscesses, and septicemia, It is often fatal.
2-Classification of Some of the Medically Important Pseudomonads
Order: Pseudomonadales
Family: Pseudomonadaceae
Genus : Pseudomonas
Genus and species
I Fluorescent group Pseudomonas aeruginosa
Pseudomonas fluorescens
Pseudomonas putida
Nonfluorescent group Pseudomonas stutzeri
Pseudomonas mendocina
II Burkholderia pseudomallei
Burkholderia mallei
Burkholderia cepacia
3-Morphology and identification
3-1-Typical Organisms
P aeruginosa is motile and rod-shaped with one to several polar flagella. , measuring about 0.6 x 2?m. It is gram-negative and occurs as single bacteria, in pairs, and occasionally in short chains.
3-2-culture Characteristics
P aeruginosa is an obligate aerobe that grows readily on many types of culture media. Some strains hemolyze blood. P aeruginosa forms smooth round colonies with a fluorescent greenish color due to the fluorescent pigment pyoverdin . It often produces the nonfluorescent bluish pigment pyocyanin, which diffuses into the agar. Other Pseudomonas species do not produce pyocyanin.Some strains produce the dark red pigment pyorubin or the black pigment pyomelanin. Some strains can produce a viscous extracellular slime layer.
Gram stain of Pseudomonas aeruginosa, which are about 0.6 x 2?. Original magnification x 1000.
Pseudomonas aeruginosa on a 10 cm Mueller-Hinton agar plate. Individual colonies are 3–4 mm in diameter. The organism produces pyocyanin, which is blue, and pyoverdin, which is green. Together these pigments produce the blue green color that is seen in the agar around the pseudomonas growth
Variation in colony morphology of Pseudomonas aeruginosa. A: Green-gray colonies 6–8 mm in diameter on a 10 cm blood agar plate; the blood in the agar around the colonies shows hemolysis. B: Silver toned dry colonies on a similar blood agar plate; no hemolysis is present (the dark shadow in the lower part of the picture is from a label on the back of the petri dish).
3-3-Growth Characteristics
P aeruginosa grows well at 37–42°C; its growth at 42°C helps differentiate it from other Pseudomonas species in the fluorescent group. It is oxidase-positive. It does not ferment carbohydrates, but many strains oxidize glucose. Identification is usually based on colonial morphology, oxidase positivity, the presence of characteristic pigments, and growth at 42°C.
3-4-Diagnostic Laboratory Tests
3-4-1-Specimens
Specimens from skin lesions, pus, urine, blood, spinal fluid, sputum, and other material should be obtained as indicated by the type of infection.
3-4-2-Smears
Gram-negative rods are often seen in smears. There are no specific morphologic characteristics that differentiate pseudomonads in specimens from enteric or other gram-negative rods.
3-4-3-Culture
Specimens are plated on blood agar and the differential media commonly used to grow the enteric gram-negative rods. Pseudomonads grow readily on most of these media, but they may grow more slowly than the enterics. P aeruginosa does not ferment lactose and is easily differentiated from the lactose-fermenting bacteria. Culture is the specific test for diagnosis of P aeruginosa infection.
The combination of characteristic oxidase positive colonies, pyocyanin production and the ability to grow at 42°C is sufficient to distinguish P. aeruginosa from other Pseudomonas species.