Burkholderia

Burkholderia is a genus of Proteobacteria whose pathogenic members include the "Burkholderia cepacia" complex, which attacks humans and "Burkholderia mallei", responsible for glanders, a disease that occurs mostly in horses and related animals; "Burkholderia pseudomallei", causative agent of melioidosis; and "Burkholderia cepacia", an important pathogen of pulmonary infections in people with cystic fibrosis (CF).
The "Burkholderia" (previously part of "Pseudomonas") genus name refers to a group of virtually ubiquitous Gram-negative, obligately aerobic, rod-shaped bacteria that are motile by means of single or multiple polar flagella, with the exception of "Burkholderia mallei", which is nonmotile. Members belonging to the genus do not produce sheaths or prosthecae and are able to use poly-beta-hydroxybutyrate (PHB) for growth. The genus includes both animal and plant pathogens, as well as some environmentally important species. In particular, "B. xenovorans" (previously named "Pseudomonas cepacia" then "B. cepacia" and "B. fungorum") is renowned for being catalase positive (affecting patients with chronic granulomatous disease) and its ability to degrade chlororganic pesticides and polychlorinated biphenyls. The conserved RNA structure anti-hemB RNA motif is found in all known bacteria in this genus.
Due to their antibiotic resistance and the high mortality rate from their associated diseases, "B. mallei" and "B. pseudomallei" are considered to be potential biological warfare agents, targeting livestock and humans.
History.
The genus was named after Walter H. Burkholder, plant pathologist at Cornell University. The first species placed in the genus were transfers from "Pseudomonas", on the basis of various biochemical tests.
Until recently, the genus "Burkholderia" was inclusive of all "Paraburkholderia" species. However, the genus "Paraburkholderia" is phylogenetically distinct, and can be distinguished from all "Burkholderia" species on the basis of molecular signatures that are uniquely found for each genus.
Taxonomy.
"Burkholderia" species form a monophyletic group within the Burkholderiales order of the Betaproteobacteria. Currently, the 48 validly named species can be distinguished from related genera (i.e. "Paraburkholderia") and all other bacteria by conserved signature indels in a variety of proteins. These indels represent exclusive common ancestry shared among all "Burkholderia" species.
The genus has three distinct monophyletic clusters. One group consists of all species belonging to the "Burkholderia cepacia" complex, another clade comprises "B. pseudomallei" and closely related species, and the last clade encompasses of most of the phytogenic species within the genus, including "B. glumae" and "B. gladioli ". Conserved signature indels are specific for each of these subgroups within the genus that aid in demarcating members of this extremely large and diverse genus.
Research.
Recently, research in "Burkholderia" species has investigated a range of topics and characteristics including metabolomic response to antibiotics, contact-dependent interactions between bacterial communities, and genomic potential to yield beneficial products.
In "Burkholderia" species, certain antibiotics such as trimethoprim has been shown to induce and upregulate a large amount of the metabolome, inducing over 100 silent secondary metabolite gene clusters in "Burkholderia thailandensis." These global activators can be used as a source of investigation into how the metabolomes of pathogenic bacterial species respond to antibiotic stress and how bacterial species can vary in response to them. It has been shown that closely related cystic fibrosis-associated "Burkholderia" species respond to trimethoprim with differing levels of expression of various secondary metabolites, highlighting the personalized nature of metabolomics in related bacterial strains.
Research focused on interbacterial signaling using "Burkholderia" has shown that contact-dependent growth inhibition plays a significant role in mediating cell to cell communication specifically in "B. thailandensis". In this interaction, cells release protein toxins to the surrounding environment, and only those with a corresponding protective protein (usually bacteria of the same strain) will not have its growth inhibited or die. Furthermore, recipient cells that have the corresponding protein then undergo changes to gene expression and phenotype that promotes community formation in the form of biofilms. This occurs even if the recipient cell was not of the same bacterial strain which highlights the importance of this system. The genes that encode the protein toxins and the rest of the contact-dependent inhibition system can become mobile in the form of a transposon that can transfer between cells and is critical to communal aspect of the system. Thus, contact-dependent signaling plays a significant role in bacterial self recognition and community formation.
"Burkholderia" species have been shown to be a potential source of beneficial products such as antimicrobials and biosurfactants. Along with the related genus "Pseudomonas", "Burkholderia" can synthesize a particular class of biosurfactant called rhamnolipids. Rhamnolipids synthesized by "Burkholderia" have differing chemical characteristics (compared to those synthesized by "Pseudomonas") and thus have the potential for novel applications.
Species.
List of species: