Burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative, bipolar, aerobic, motile rod-shaped bacterium. A human and animal pathogen, B. pseudomallei causes melioidosis.

B. pseudomallei measures 2–5 μm in length and 0.4–0.8μm in diameter and are capable self-propulsion using flagellae. The bacteria can grow in a number artficial nutrient environments, especially betaine- and arginine-containing.

In vitro, optimal proliferation temperature is reported around 40°C in pH-neutral or slightly acidic environments (pH 6.8–7.0). The majority of strains are capable of fermentation of sugars without gas formation (most importantly, glucose and galactose, older cultures are reported to also metabolize maltose and starch). Bacteria produce both exo- and endo-toxins. The role of the toxins identified in the process of melioidosis symptom development has not been fully understood.

Identification
B. pseudomallei grows on a large variety of culture media (blood agar, McConkey's, EMB, etc.). Ashdown's medium (or Burkholderia cepacia medium) may be used for selective isolation. Cultures typically become positive in 24 to 48 hours (this rapid growth rate differentiates the organism from B. mallei, which typically takes a minimum of 72 hours to grow), and colonies are wrinkled, have a metallic appearance, and possess an earthy odour. On Gram staining, the organism is a Gram-negative rod with a characteristic "safety pin" appearance (bipolar staining). On sensitivity testing, the organism appears highly resistant (it is innately resistant to a large number of antibiotics including gentamicin) and that again differentiates it from B. mallei, which is in contrast, exquisitely sensitive to a large number of antibiotics. For environmental specimens only, differentiation from the non-pathogenic B. thailandensis using an arabinose test is necessary (B. thailandensis is never isolated from clinical specimens).

Laboratory identification of B. pseudomallei can be difficult, especially in Western countries where B. pseudomallei is rarely seen. The large wrinkled colonies look like environmental contaminants and are therefore often discarded as being of no clinical significance. The organism grows more slowly than other bacteria that may be present in clinical specimens, and in specimens from non-sterile sites, is easily overgrown. Non-sterile specimens should therefore be cultured in selective media (e.g., Ashdown's or B. cepacia medium ). Even when the isolate is recognised to be significant, commonly used identification systems may misidentify the organism as Chromobacterium violaceum or other non-fermenting gram-negative bacilli such as Burkholderia cepacia or Pseudomonas aeruginosa. Again, because the disease is rarely seen in western countries, identification of the bacterium B. pseudomallei in cultures may not actually trigger alarm bells in physicians unfamiliar with the disease.

Molecular methods (PCR) of diagnosis are possible, but not routinely available for clinical diagnosis.

Disinfection
B. pseudomallei is susceptible to numerous disinfectants including benzalkonium chloride, iodine, mercuric chloride, potassium permanganate, 1% sodium hypochlorite, 70% ethanol, 2% glutaraldehyde and to a lesser extents, phenolic preparations. The microorganism can also be destroyed by heating to above 74°C for 10 min or by UV irradiation.

Treatment
The current first choice antibiotic is ceftazidime. While various antibiotics are active in vitro (e.g., chloramphenicol, doxycycline, co-trimoxazole), they have been proven to be inferior in vivo for the treatment of acute melioidosis.

The organism is intrinsically resistant to gentamicin and to colistin, and this fact is sometimes used to help in the identification of the organism.

Medical importance
B. pseudomallei is the cause of melioidosis. There are number of different forms of melioidosis, but sepsis due to B. pseudomallei has a mortality rate of 80 percent if untreated.

Patogenicity mechanisms
B. pseudomallei is one of the first proteobacteria to be identified as containing an active Type 6 secretion system. it is also the single organism identified that probably contains up to 6 different independent type 6 secretion system (Gil Shalom, Jonathan G. Shaw and Mark S, Thomas. In vivo expression technology identifies a type VI secretion system locus in Burkholderia pseudomallei that is induced upon invasion of macrophages. Microbiology, Vol. 153, pp2689-2699, (2007).)

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