Summary Members of the genus Burkholderia are versatile organisms that occupy a surprisingly wide range of ecological niches. These bacteria are exploited for biocontrol, bioremediation and plant growth promotion purposes, but safety issues regarding human infections, especially in cystic fibrosis patients, have not been solved. This minireview gives an overview of the taxonomic and ecological diversity of the genus with particular emphasis on strains belonging to the Burkholderia cepacia complex and addresses the important question whether ‘good’ and ‘bad’ strains are actually the same. The genus Burkholderia contains over 30 species, which occupy remarkably diverse ecological niches, ranging from contaminated soils to the respiratory tract of humans. The Burkholderia cepacia complex is ubiquitous in nature and can be found in soil, water (including sea water), the rhizosphere of plants, in humans and various animal species and in the hospital environment. Burkholderia cepacia complex isolates have been exploited for various purposes, including biological control of plant pathogens, bioremediation of recalcitrant xenobiotics and plant growth promotion. Unfortunately, some Burkholderia species have been involved in human infections, and safety issues regarding these human infections are hampering the wide-spread biotechnological applications. The purpose of this minireview is to give an overview of the remarkable diversity of the genus Burkholderia and to shed some light on the important question whether or not clinical and environmental B. cepacia complex isolates are the same. A phylogenetic tree based on 16S rRNA gene sequences, showing the positions of all of the Burkholderia species and representatives of related genera, is shown in Fig. 1.

Diversity and significance of Burkholderia species occupying diverse ecological niches

The taxonomic position of nitrogen-fixing strains that were isolated from rhizosphere macerates of rice cultivated in the Binh Thanh region of Vietnam was determined by using polyphasic taxonomy. We determined the phylogenetic relationships of these organisms by performing DNA-rRNA hybridization experiments with a labeled rRNA probe from the type strain of Burkholderia cepacia, and we found that they belong to a single rRNA complex. Other members of this rRNA complex were also studied, and the N2-fixing strains were found to be closely related to B. cepacia. In addition, all members of the rRNA complex containing B. cepacia were studied by performing auxanographic and DNA-DNA hybridization experiments. Phenotypically and genotypically, the N2-fixing isolates constitute a single cluster together with two strains of clinical origin. These organisms constitute a new Burkholderia species, for which the name Burkholderia vietnamiensis is proposed; the type strain of this species is TVV75 (= LMG 10929). All members of this species can fix nitrogen. On the basis of our polyphasic taxonomy results and previously published data we concluded that the genus Burkholderia should be restricted to the following species: B. cepacia (the type species), Burkholderia mallei, Burkholderia pseudomallei, B. vietnamiensis, Burkholderia gladioli, Burkholderia caryophylli, Burkholderia plantarii, Burkholderia glumae, Burkholderia vandii, Burkholderia cocovenenans comb. nov., and Burkholderia andropogonis comb. nov. On the basis of genotypic and phenotypic results [Alcaligenes] eutrophus, [Burkholderia] solanacearum, and [Burkholderia] pickettii belong to two other clusters whose internal structures must be studied further.

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Polyphasic Taxonomy in the Genus Burkholderia Leading to an Emended Description of the Genus and Proposition of Burkholderia vietnamiensis sp. nov. for N2-Fixing Isolates from Rice in Vietnam

Both in natural and in managed ecosystems, bacteria are common inhabitants of the phytosphere and the internal tissues of plants. Probably the most diverse and environmentally adaptable plant-associated bacteria belong to the genus Burkholderia. This genus is well-known for its human, animal and plant pathogenic members, including the Burkholderia cepacia complex. However, it also contains species and strains that are beneficial to plants and can be potentially exploited in biotechnological processes. Here we present an overview of plant-associated Burkholderia spp. with special emphasis on beneficial plant-Burkholderia interactions. A discussion of the potential for utilization of stable plant-Burkholderia spp. associations in the development of low-input cropping systems is also provided.

Diversity and occurrence of Burkholderia spp. in the natural environment.

Recent Changes in the Classification of the Pseudomonads: an Overview

Burkholderia is an incredibly diverse and versatile Gram-negative genus, within which over 80 species have been formally named and multiple other genotypic groups likely represent new species. Phylogenetic analysis based on the 16S rRNA gene sequence and core genome ribosomal multilocus sequence typing analysis indicates the presence of at least three major clades within the genus. Biotechnologically, Burkholderia are well-known for their bioremediation and biopesticidal properties. Within this review, we explore the ability of Burkholderia to synthesise a wide range of antimicrobial compounds ranging from historically characterised antifungals to recently described antibacterial antibiotics with activity against multiresistant clinical pathogens. The production of multiple Burkholderia antibiotics is controlled by quorum sensing and examples of quorum sensing pathways found across the genus are discussed. The capacity for antibiotic biosynthesis and secondary metabolism encoded within Burkholderia genomes is also evaluated. Overall, Burkholderia demonstrate significant biotechnological potential as a source of novel antibiotics and bioactive secondary metabolites.

Burkholderia: an update on taxonomy and biotechnological potential as antibiotic producers.

An aerobic, gram-negative bacterium that causes seedling blight of rice was isolated from diseased rice seedlings and bed soil in nursery boxes in Chiba and Niigata Prefectures, Japan. The cells are nonsporeforming and nonencapsulated straight rods (0.7 to 1.0 by 1.4 to 1.9 μm) that occur singly, in pairs, or in short chains and are motile with one to three polar flagella. The bacterium displays the characteristics of the genus Pseudomonas and can be clearly differentiated from Pseudomonas glumae and Pseudomonas avenae, which are known to cause seedling diseases in rice grown in nursery boxes, and from other bacteria. The deoxyribonucleic acid base composition is 64.8 mol% guanine plus cytosine. A new species, Pseudomonas plantarii sp. nov., is proposed. The type strain is NIAES 1723 (JCM 5492, AZ 8201). All strains of the new species produce the disease-causing substance tropolone (a nonbenzenoid aromatic compound with a seven-membered ring) and a reddish brown pigment which was considered to be a derivative of the substance.

Chikafusa Fukazawa

Papers

Pseudomonas plantarii sp. nov., the Causal Agent of Rice Seedling Blight