## 1 Introduction  
The _Burkholderia_ genus are gram negative bacteria that are ubiquitous in the environment and may cause a number of diseases in plants, animals and humans. Opportunistic human infection can be particularly serious for patients with cystic fibrosis while other members are causative agents in various diseases. Conversely, _Burkholderia_ are involved in nitrogen fixation in the rhizosphere and various species have the ability to metabolize pesticides and polychlorinated biphenyls (PCBs), making them attractive from an agricultural perspective (although in this latter case concerns have been raised because of the pathogenic potential of some species). One of the most important aspects of understanding this genus as a pathogen or as beneficial microorganisms is that quorum sensing (QS) systems operating in _Burkholderia_ are important regulators of virulence and other phenotypic traits. Furthermore, QS systems appear to be of crucial importance in governing overall colonization and niche invasion. This suggests that modulation of these regulatory systems might be an attractive route for controlling virulence and hence the development of interventions designed to treat infection or (in the case of agriculture) induce beneficial activities. It is therefore crucial that we have a more complete understanding of QS systems in _Burkholderia_ and in particular the genetic basis upon which it operates. One of the challenges faced by researchers is that _Burkholderia_ are very diverse from taxonomic and genetic perspectives. For many years, _Burkholderia_ were recognized as members of the non-fluorescent pseudomonads. However, pseudomonads were found to be taxonomically heterogeneous and five species homology groups were devised on the basis of rRNA-DNA hybridization experiments [1] and the genus _Pseudomonas_ was divided into five well-defined rRNA homology groups. Subsequently, polyphasic taxonomy analyses, including 16S rRNA sequence analysis, DNA-DNA hybridization and fatty acid analysis, led to the establishment of the _Burkholderia_ genus to accommodate seven species of the _Pseudomonas_ rRNA group II (_P. cepacia_, _P. caryophylli_, _P. gladioli_, _P. mallei_, _P. pseudomallei_, _P. solanacearum_ and _P. picketti_) [2,3]. _P. solanacearum_ and _P. picketti_ were later renamed as _Ralstonia_. In the early 1980s, strains of _B. cepacia_ were increasingly being recovered from cultures of respiratory tract specimens from cystic fibrosis patients [4]. Continuous taxonomic studies allowed the designation of binomial species names of clinically-isolated _Burkholderia_ and are now referred to as the species of the _Burkholderia cepacia_ complex (BCC) [5, 6, 7]. Given their clinical importance, the BCC members as well as other pathogenic _Burkholderia_ such as _B. mallei_ and _B. pseudomallei_ have been the subject of intense analysis in terms of both their taxonomy and pathogenicity [8, 9, 10, 11, 12]. _B. pseudomallei_ is for example the causative agent of melioidosis, an infectious disease endemic in southeast Asia and northern Australia (and may occur in other tropical and subtropical regions--in 1969, several cases of melioidosis were reported in non-human primates in the USA [13]). _B. mallei_, on the other hand is the causative agent of glanders in horses and is generally transmitted to humansthrough infected animals. _B. mallei_ was also used as a bio-weapon in civil wars of America and Afghanistan [14,15]. Recently, other _Burkholderia_ species have been characterized as potentially beneficial in terms of their role in plant-growth-promotion and nitrogen fixation in the rhizosphere and/or endosphere--many of them are diazotrophs and a few are capable of nodulation in mimosa and legumes [16, 17, 18]. Although their taxonomy is continually under review, the _Burkholderia_ genus currently comprises over 60 validly described species.