Burkholderia mallei Scientific classification Kingdom: Bacteria Phylum: Proteobacteria Class: Betaproteobacteria Order: Burkholderiales Family: Burkholderiaceae Genus: Burkholderia Species: B. mallei Binomial name Burkholderia mallei
Yabuuchi et al. 1993
Glanders bacillus Loeffler 1882
Bacillus mallei Zopf 1885
Actinobacillus mallei (Zopf 1885) Brumpt 1910
Pfeifferella mallei (Zopf 1885) Buchanan 1918
Malleomyces mallei (Zopf 1885) Pribram 1933
Loefferella mallei (Zopf 1885) Holden 1935
Acinetobacter mallei (Zopf 1885) Steel and Cowan 1964
Pseudomonas mallei (Zopf 1885) Redfearn et al. 1966
Burkholderia mallei is a gram-negative bipolar aerobic bacterium, a Burkholderia-genus human and animal pathogen causing Glanders; the Latin name of this disease (malleus) gave name to the causative agent species. It is closely related to B. pseudomallei, and by multilocus sequence typing, it is a subspecies of B. pseudomallei. B. mallei evolved from B. pseudomallei by selective reduction and deletions from the B. pseudomallei genome. Unlike closely related Burkholderia pseudomallei and other genus members, the bacterium is nonmotile; its shape is something in between a rod and a coccus measuring some 1.5–3 μm in length and 0.5–1μm in diameter with rounded ends.
Discovery and Early History
Wilhelm Schütz and Friedrich Löffler first isolated Burkholderia mallei in 1882.  It was isolated from an infected liver and spleen of a horse. This bacterium is also one of the first to be identified containing a type VI secretion system which is important for its pathogenicity.
Most organisms within the Burkholderiaceae Family live in soil, however, B. mallei does not. Because B. mallei is an obligate mammalian pathogen, it must cause disease in a host mammal in order to live and to be transmitted from one host to another.
B.mallei is very closely related to B. pseudomallei, being 99% identical in conserved genes when compared to B. pseudomallei. B. malllei has about 1.4 Mb less DNA than B. pseudomallei.There is speculation that B. mallei actually evolved from a strain of B. pseudomallei after the latter had infected an animal. The bacterium would have lost the genes that were not necessary for living in an animal host. This suggestion has found support from studies that compare strains of B. mallei to B. pseudomallei and indicate that their two respective genomes are very similar. The genes that allowed the bacterium to survive in a soil environment, like genes that gave B. mallei the capacity to protect against bacteriacidals antibiotic and antifungals, were likely deleted, thus the reason that B. mallei is not found outside of a host is because it lacks the genes that are necessary for survival in the soil. Genome comparisons also seem to indicate that the B. mallei is still evolving and adapting to an intracellular lifestyle.
The genome of B. mallei was sequenced in the United States by The Institute of Genomic Research (TIGR). The size of the genome is smaller than that of the closely related B. pseudomallei. The B. mallei sequence revealed a chromosome of 3.5 mega base pairs (Mb) and a 2.3 MB "megaplasmid.” Many insertion sequences and phase-variable genes were also found.The genome for B. mallei is made up of two circular chromosomes. Chromosome 1 is where genes relating to metabolism, capsule formation, and lipopolysaccharide biosynthesis are located. B. mallei has a polysaccharide capsule which indicates its potential as a pathogen. Chromosome 2 is where most of the information regarding secretion systems and virulence-associated genes are located. Multilocus sequence typing has revealed that B. mallei most likely evolved from a B. pseudomallei clone reduction. There are approximately 1000 B. pseudomellei genes absent or varying in the B. mallei genome. B. mallei’s genome also has a large amount of insertion sequences.
There is no standardized system for differentiating between B. mallei and B. pseudomallei. The methods that have been used to differentiate and identify one strain from the other include ribotyping, pulsed-field gel electrophoresis, multilocus enzyme electrophoresis, random amplified polymorphic DNA analysis, and multilocus sequence typing. Comparing the DNA of B. mallei and B. pseudomallei must be done at the 23S rDNA level, however, since there is no identifiable difference between the two species at the 16S rDNA level.
Growth in Culture
Both B. mallei and B. pseudomallei can be cultured in a lab; nutrient agar can be used to grow the bacteria. When grown in culture, B. mallei grows in smooth, grey translucent colonies. In a period of 18 hours at 37°C, a B. mallei colony can grow to approximately 0.5 – 1 mm in diameter. A B. mallei culture will not grow on MacConkey agar. Many microbiologists are unfamiliar with B. mallei and as a result it has frequently been misidentified as a Pseudomonas species or as a contaminant in a culture.
Antibiotic Resistance and Susceptibility
The bacterium is susceptible to numerous disinfectants including benzalkonium chloride, iodine, mercuric chloride, potassium permanganate, 1% sodium hypochlorite, and ethanol. The microorganism can also be destroyed by heating or UV. Antibiotics such as streptomycin, amikacin, tetracycline, doxycycline, carbapenems, ceftazidime, amoxicillin/clavulanic acid, piperacillin, chloramphenicol and sulfathiazole have been reported to be effective against the bacteria in vitro. B. mallei, like B. pseudomallei, is also resistant to a number of antibiotics including aminoglycosides, polymyxins, and beta-lactams. There is currently no vaccine available for humans or animals to protect against B. mallei infection. An animal model that will predict immune responses necessary to create immunity to the bacterium is needed before a vaccine can be developed. Mice are fairly close to humans in their susceptibility to B. mallei and would be the ideal choice of animal for creating a model for the vaccine.
B. mallei is responsible for causing Glanders disease, which historically affected animals, such as horses, mules, and donkeys, the most and rarely affected humans. Horses are considered the natural host for B. mallei infection and are highly susceptible to it. B. mallei infects and gains access to the cell of its host through lysis of the entry vacuole. B. mallei has bacterial protein dependent actin-based motility once inside the cell. It is also able to initiate host cell fusion that results in multi-nucleated giant cells (MNGCs). The consequence of MNGCs has yet to be determined, but it may allow the bacteria to spread to different cells, evade responses by the infected host’s immune system, or allow the bacteria to remain in the host longer. B. mallei is able to survive inside host cells through its capabilities in disrupting the bacteria killing functions of the cell. It leaves the vacuoles early, which allows for efficient replication of the bacteria inside the cell. Leaving the cell early also keeps the bacteria from being destroyed by lysosomal defensins and other pathogen killing agents. MNGCs may help protect the bacteria from immune responses. B. mallei’s ability to live within the host cell makes developing a vaccine against it difficult and complex. The vaccine would need to create a cell-mediated immune response as well as a humoral response to the bacteria in order to be effective in protecting against B. mallei. In regard to a vaccine against B. mallei, the closeness of B. mallei to B. pseudomallei may make it possible that a vaccine developed for either type of bacteria would be effective against the other.
Symptoms of B. mallei Infection
Horses who are chronically infected with B. mallei and have Glanders disease as a result, typically experience mucus containing nasal discharge, lung lesions, and nodules around the liver or spleen. Acute infection in horses results in a high fever, loss of fat or muscle, erosion of the surface of the nasal septum, hemorrhaging or mucus discharge. The bacterium mostly affects the lungs and airways. Human infection with B. mallei is rare, although it occasionally occurs among lab workers dealing with the bacteria or those who are frequently near infected animals. The bacteria usually infect a person through their eyes, nose, mouth, or cuts in the skin. Once a person is infected with the bacteria, they develop a fever and rigors. Eventually they will get pneumonia, pustules, and abscesses, which will prove fatal within a week to ten days if left untreated by antibiotics. The way someone is infected by the bacteria also affects the type of symptoms that will result. If the bacteria enters through the skin, a local skin infection can result, while inhaling B. mallei can cause septicemic or pulmonary infections of muscles, the liver, or spleen. B. mallei infection has a fatality rate of 95% if left untreated, and a 50% fatality rate in individuals treated with antibiotics.
Cellular Response to Infection
In the first days of B. mallei infection neutrophils, macrophages, and T cells go to the spleen in great quantities. The early cellular response to B. mallei infection involves Gr-1+ (antigen) cells, and implies their importance to immunity against this bacterial infection. T cells (nitric oxide) are actually more involved in combating B. mallei in the later stages of its infection of a host.
B. mallei has been eradicated in the United States and most Western countries, but still affects animals in Africa, Asia, the Middle East, Central America, and South America. Many Western countries were able to eliminate the disease through Glanders control programs and laws requiring notification of cases of infection to health departments and the destruction of any animal affected with B. mallei.
Potential as a Biological Weapon
B. mallei as well as B. pseudomallei have a history of being on a list of potential biological warfare agents. The Centers for Disease Control and Prevention (CDC) classifies B. mallei as a Category B critical biological agent. As a result research regarding B. mallei may only be done in biosafety level 3 facilities in the US and internationally. Even though it is so highly infective and a potential biological weapon, little research has been conducted on this bacterium.
Incidence in the United States
In March 2000, one of the first cases since the 1940s of Glanders in the United States occurred in a young microbiologist working for the U.S. Army Medical Research Institute for Infectious Diseases. The researcher had type 1 diabetes and had been working with B. mallei for about two years, however, he did not always wear gloves while conducting his research. The researcher experienced enlargement of the lymph nodes and a fever which lasted for 10 days even with antibiotic treatment. In the following weeks the researcher experienced fatigue, rigors, night sweats, and loss of weight. The next month, his symptoms seemed to disappear after treatment with clarithromycin, but after the medication was stopped the symptoms reappeared. After conducting multiple tests on cultures from the researcher’s blood and a biopsied portion of a liver abscess the bacteria was identified as B. mallei. Once it was established what the researcher was infected by, another course of antibiotics was given (imipenem and doxycycline) with 6 months of treatment. After a year the researcher made a full recovery.
This incident was the first case of a B. mallei infection in the United States since 1949. It also showed how a cut or skin abrasion is not absolutely necessary to contract the disease, as the researcher had no recollection of any cut or accident while working in the laboratory. The case was significant as it showed the difficulty that microbiology laboratories have in identifying bioweapon agents and the potential consequences if measures are not taken to prepare for an actual biological attack.
History as a Weapon of Biological Warfare
B. mallei was intentionally used to infect animals and humans during World War I. The Germans used B. mallei to infect animals that were being sent from neutral countries to the Allies with Glanders. The Germans’ plans for biological warfare started in 1915 on the east coast of the United States; they intended to infect and kill the livestock that was being sent to the Allies and facilitate the transfer of the disease to humans. The east coast was where many animals were being assembled for shipment to the Allies fighting in Europe. The Germans also targeted Romania, Norway and Spain’s animal supplies with cultures of Glanders. The German biological sabotage eventually spread to Argentina, where agents would rely on bacterial cultures from Spain to infect the cattle, horses, and mules that Argentina was supplying to the Allies. The Germans’ use of microbes as weapons is one of the only documented attacks of intentionally using biological weapons against neutral countries.
The Japanese used B. mallei in their biological warfare research units. The most notable and notorious unit, Unit 731, used the bacterium to conduct experiments on live human subjects. However, the Japanese did not end up creating a biological weapon out of B. mallei. The Japanese did actually use B. mallei to test its effectiveness in contaminating water supplies, and the results of these tests were successful.
The Russians’ biological weapons program also took an interest in B. mallei and conducted field tests with it. Some of the researchers from the program were actually infected and killed by it during the course of their research. It has been suggested that the Russians eventually used B. mallei during their war in Afghanistan against the mujahideen.
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- Burkholderia mallei genomes and related information at PATRIC, a Bioinformatics Resource Center funded by NIAID
- Pathema-Burkholderia Resource
- Centers for Disease Control and Prevention: Glanders (Burkholderia mallei) General Information
- Centers for Disease Control and Prevention: Bioterrorism Agents/Diseases
- Center for Biosecurity
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