Burkholderia mallei

Burkholderia mallei is a Gram-negative, bipolar, aerobic bacterium, a human and animal pathogen of genus Burkholderia causing glanders; the Latin name of this disease (malleus) gave its name to the species causing it. 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 B. pseudomallei and other genus members, B. mallei is nonmotile; its shape is coccobacillary measuring some 1.5–3.0 μm in length and 0.5–1.0 μm in diameter with rounded ends. Wilhelm Schütz and Friedrich Löffler first isolated B. 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. In 1885, the German Botanist and Bacteriologist, Wilhelm Zopf (1846–1909) gave the pathogen its binomial name, after analyzing samples of the bacterium. He further refined his observations with the pathogen in 1886. Most organisms within the Burkholderiaceae live in soil; however, B. mallei does not. Because B. mallei is an obligate mammalian pathogen, it must infect a host mammal 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. B. mallei may have 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 bactericidals, antibiotics, and antifungals, were likely deleted.