Burkholderia pseudomallei[a] (also known asPseudomonas pseudomallei) is aGram-negative, bipolar,aerobic,motile rod-shapedbacterium.[2] It is a soil-dwelling bacterium endemic intropical andsubtropical regions worldwide, particularly in Thailand and northern Australia.[3] It was reported in 2008 that there had been an expansion of the affected regions due to significant natural disasters, and it could be found in Southern China, Hong Kong, and countries in the Americas.[4]B. pseudomallei, amongst other pathogens, has been found in monkeys imported into the United States from Asia forlaboratory use, posing a risk that the pathogen could be introduced into the country.[5]
Although it is mainly a soil-dwelling bacterium, one study showed thatBurkholderia pseudomallei survived in distilled water for 16 years, demonstrating that it is capable of living in water if a specific environment is provided.[6] It is resistant to a variety of harsh conditions including nutrient deficiency, extreme temperature or pH.[7] It infects humans, causing the diseasemelioidosis;[8] mortality is 20–50% even with treatment. TheCDC classifies it as a "Tier 1 select agent" with potential as abioterrorism agent.[5] It infects other animals, most commonly livestock such as goats, pigs, and sheep, less frequently.[9] It is also capable of infecting plants in a laboratory setting.[10]
Burkholderia pseudomallei measures 2–5 μm in length and 0.4–0.8 μm in diameter and is capable of self-propulsion usingflagella. The bacteria can grow in a number of artificial nutrient environments, especiallybetaine- andarginine-containing ones.
In vitro, optimal proliferation temperature is reported around 40 °C in neutral or slightly acidic environments (pH 6.8–7.0). The majority of strains are capable of oxidation, not fermentation, ofsugars without gas formation (most importantly,glucose andgalactose; older cultures are reported to also metabolizemaltose andstarch). Bacteria produce bothexo- andendotoxins. The role of the toxins identified in the process of melioidosis symptom development has not been fully elucidated.[11]
Burkholderia pseudomallei is notfastidious and grows on a large variety of culture media (blood agar,MacConkey agar,EMB, etc.).Ashdown's medium (orBurkholderia cepacia medium) may be used for selective isolation.[12]Cultures typically become positive in 24 to 48 hours (this rapid growth rate differentiates the organism fromB. mallei, which typically takes a minimum of 72 hours to grow). Colonies are wrinkled, have a metallic appearance, and possess an earthy odor. OnGram staining, the organism is aGram-negative rod with a characteristic "safety pin" appearance (bipolar staining). On sensitivity testing, the organism appears highly resistant (it is innately resistant to many antibiotics includingcolistin andgentamicin) and that again differentiates it fromB. mallei, which is in contrast, exquisitely sensitive to many antibiotics. For environmental specimens only, differentiation from the nonpathogenicB. thailandensis using anarabinose test is necessary (B. thailandensis is never isolated from clinical specimens).[13] The laboratory identification ofB. pseudomallei has been described in the literature.[14]
The classic textbook description ofB. pseudomallei in clinical samples is of an intracellular, bipolar-staining, Gram-negative rod, but this is of little value in identifying the organism from clinical samples.[14] Some[15] suggest theWayson stain is useful for this purpose, but this has been shown not to be the case.[16]
Laboratory identification ofB. pseudomallei can be difficult, especially in Western countries where it is rarely seen. The large, wrinkled colonies look like environmental contaminants, so are often discarded as being of no clinical significance.Colony morphology is very variable and a single strain may display multiple colony types,[17][18] so inexperienced laboratory staff may mistakenly believe the growth is not pure. The organism grows more slowly than other bacteria that may be present in clinical specimens, and in specimens from nonsterile sites, is easily overgrown. Nonsterile specimens should, therefore, be cultured in selective media (e.g., Ashdown's[19][20] orB. cepacia medium).[12] For heavily contaminated samples, such as feces, a modified version of Ashdown's that includesnorfloxacin,amoxicillin, andpolymyxin B has been proposed.[21] In blood culture, the BacT/ALERT MB system (normally used for culturingmycobacteria) by bioMérieux has been shown to have superior yields compared to conventional blood culture media.[22]
Even when the isolate is recognized to be significant, commonly used identification systems may misidentify the organism asChromobacterium violaceum or other nonfermenting, Gram-negative bacilli such asBurkholderia cepacia orPseudomonas aeruginosa.[23][24] Again, because the disease is rarely seen in Western countries, identification ofB. pseudomallei in cultures may not actually trigger alarms in physicians unfamiliar with the disease.[25] Routine biochemical methods for identification of bacteria vary widely in their identification of this organism: theAPI 20NE system accurately identifiesB. pseudomallei in 99% of cases,[26] as does the automated Vitek 1 system, but the automated Vitek 2 system only identifies 19% of isolates.[24]
The pattern of resistance to antimicrobials is distinctive, and helps to differentiate the organism fromP. aeruginosa. The majority ofB. pseudomallei isolates are intrinsically resistant to all aminoglycosides (via an efflux pump mechanism),[27] but sensitive to co-amoxiclav:[28] this pattern of resistance almost never occurs inP. aeruginosa and is helpful in identification.[29] Unfortunately, the majority of strains in Sarawak, Borneo, are susceptible to aminoglycosides and macrolides, which means the conventional recommendations for isolation and identification do not apply there.[30]
Molecular methods (PCR) of diagnosis are possible, but not routinely available for clinical diagnosis.[31][32] Fluorescencein situ hybridisation has also been described, but has not been clinically validated, and it is not commercially available.[33] In Thailand, alatex agglutination assay is widely used,[26] while a rapid immunofluorescence technique is also available in a small number of centres.[34]
Burkholderia pseudomallei infection in humans is calledmelioidosis or Whitmore's disease. It is spread though direct contact with water or soil that holds the bacteria. There have been few cases of transmission of the bacteria perinatally.[40] Its mortality is 20 to 50% even with treatment.[28]
The antibiotic of choice isceftazidime.[28] While variousantibiotics are activein vitro (e.g.,chloramphenicol,doxycycline,co-trimoxazole), they have been proven to be inferiorin vivo for the treatment of acute melioidosis.[41] Disc diffusion tests are unreliable when looking for co-trimoxazole resistance inB. pseudomallei (they greatly overestimate resistance) andEtests oragar dilution tests should be used in preference.[42][43] The actions of co-trimoxazole and doxycycline are antagonistic, which suggests these two drugs ought not to be used together.[44]
The organism is intrinsically resistant togentamicin[45] andcolistin, and this fact is helpful in the identification of the organism.[46]Kanamycin is used to killB. pseudomallei in the laboratory, but the concentrations used are much higher than those achievable in humans.[47]
Burkholderia pseudomallei is an opportunistic pathogen and, since its an environmental organism, has no requirement to pass through an animal host to replicate. From the point of view of the bacterium, human infection is a developmental "dead end".[48]
Strains which cause disease in humans differ from those causing disease in other animals, by possessing certaingenomic islands.[49] It may have the ability to cause disease in humans via DNA acquired from other microorganisms.[49] Its mutation rate is also high, and the organism continues to evolve even after infecting a host.[50]
Burkholderia pseudomallei is able to invade cells - being an intracellular pathogen.[51] It is able to polymeriseactin, and to spread from cell to cell, causing cell fusion and the formation of multinucleated giant cells.[52] It possesses a uniquely fusogenictype VI secretion system that is required for cell-cell spread and virulence in mammalian hosts.[53] The bacterium also expresses a toxin called lethal factor 1.[54]B. pseudomallei is one of the first Proteobacteria to be identified as containing an active type VI secretion system. It is also the only organism identified that contains up to six different type VI secretion systems.[55]
Burkholderia pseudomoallei can go through transformation. The bacteria is able to uptake a free plasmid using electroporation and the plasmid material will integrate into the host DNA when they are electrocompetent.[61]
^abWalsh AL, Wuthiekanun V (1996). "The laboratory diagnosis of melioidosis".Br J Biomed Sci.53 (4):249–53.PMID9069100.
^Brundage WG, Thuss CJ, Walden DC (March 1968). "Four fatal cases of melioidosis in U. S. soldiers in Vietnam. Bacteriologic and pathologic characteristics".The American Journal of Tropical Medicine and Hygiene.17 (2):183–91.doi:10.4269/ajtmh.1968.17.183.PMID4869109.
^Ashdown LR (1979). "An improved screening technique for isolation ofPseudomonas pseudomallei from clinical specimens".Pathology.11 (2):293–7.doi:10.3109/00313027909061954.PMID460953.
^Roesnita B; Tay ST; Puthucheary SD; Sam IC. (2012). "Diagnostic use ofBurkholderia pseudomallei selective media in a low prevalence setting".Trans R Soc Trop Med Hyg.106 (2):131–3.doi:10.1016/j.trstmh.2011.10.007.PMID22112687.
^Ruppitsch W, Stöger A, Indra A, et al. (March 2007). "Suitability of partial 16S ribosomal RNA gene sequence analysis for the identification of dangerous bacterial pathogens".Journal of Applied Microbiology.102 (3):852–9.doi:10.1111/j.1365-2672.2006.03107.x.PMID17309636.S2CID24843231.
^White NJ, Dance DA, Chaowagul W, Wattanagoon Y, Wuthiekanun V, Pitakwatchara N (September 1989). "Halving of mortality of severe melioidosis by ceftazidime".Lancet.2 (8665):697–701.doi:10.1016/S0140-6736(89)90768-X.PMID2570956.S2CID28919574.
^Lumbiganon P, Tattawasatra U, Chetchotisakd P, et al. (2000). "Comparison between the antimicrobial susceptibility ofBurkholderia pseudomallei to trimethoprim-sulfamethoxazole by standard disk diffusion method and by minimal inhibitory concentration determination".J Med Assoc Thai.83 (8):856–60.PMID10998837.
^Kespichayawattana W, Intachote P, Utaisincharoen P, Stitaya Sirisinha S (2004). "VirulentBurkholderia pseudomallei is more efficient than avirulentBurkholderia thailandensis in invasion of and adherence to cultured human epithelial cells".Microbial Pathogenesis.36 (5):287–9.doi:10.1016/j.micpath.2004.01.001.PMID15043863.
