"What better microbial challenge to unite agricultural and medical microbiologists than an organism that reduces an onion to a macerated pulp, protects other crops from bacterial and fungal disease, devastates the health and social life of cystic fibrosis patients, and not only is resistant to the most famous of antibiotics, penicillin, but can use it as a nutrient!" (J. R. W. Govan, 1998)

B. cepacia is an extremely versatile organism that is truly both friend and foe to humans. It is a genuinely ubiquitous organism with its niches being soil, water, animals, plants and humans. Although originally identified as a plant pathogen, it is now recognised as a most useful organism for plant protection and plant-growth promotion. However, it has simultaneously become notorious as a naturally multi-resistant and life-threatening pathogen in immune suppressed hosts such as cystic fibrosis (CF) patients and patients with chronic granulomatous disease.

A human pathogen

Before the early 1980s, reports of human infections caused by B. cepacia were sporadic and generally restricted to hospitalised patients exposed to contaminated disinfectant and anaesthetic solutions.
A rising incidence, particularly in patients with CF, was noted in the early 1980s. Cystic fibrosis is the most common lethal inherited disease of Caucasian populations with pulmonary infections being the major cause of morbidity and mortality. Infection or colonisation by B. cepacia leads to different outcomes in different patients. However, overall, pulmonary colonisation reduces survival by 50% and about one third to a half of the patients succumb to "cepacia syndrome", a rapidly fatal necrotising pneumonia. During the 1980s and 1990s, several major outbreaks of B. cepacia infections resulted in numerous deaths in CF populations worldwide. More recently, serious outbreaks with fatalities have occurred in non-CF patients being treated in intensive care units in Europe and North America.

Relation to plants

Care and concern for the environment are leading scientists to develop biological alternatives to the present chemical strategy in the agro-industry and to reduce environmental chemical pollution. Control of plant diseases, insects and nematodes by bacteria and fungi has been proposed as an alternative or supplement to chemical pesticides. Roots and rhizospheres of various crops such as corn, maize, rice, pea, sunflower, and radish can be colonised by B. cepacia-like organisms, some of which produce a variety of antimicrobial compounds that are active against soil pathogens. Using these B. cepacia-like organisms as seed inoculants or root dips can increase crop yields significantly. Moreover, when there are no soil pathogens, a significant growth promoting effect has been reported.
The exceptional nutritional potential of some B. cepacia strains is being used in the bioremediation of hazardous waste sites and effluents. Carcinogenic or toxic products such as ethers present in gasoline, polycyclic aromatic compounds and other constituents of crude oils and coal, herbicides such as 2,4,5-trichlorophenoxyacetic acid, the principal component of Agent Orange, can be efficiently degraded by certain B. cepacia strains.

PYTHIUM-INFESTED SOIL pathogen control non-pathogen control Soil # 1 B.cepacia

Picture published with acknowledgement to Prakash K. Hebbar.

Biodiversity research in LMG

B. cepacia was first described as Pseudomonas cepacia by Walter Burkholder in 1950 as the phytopathogen responsible for the bacterial rot of onions. Taxonomic studies in the 1960s and 1970s showed that two additional pseudomonads, P. multivorans (an organism mainly found in soil and water samples) and P. kingii (an opportunistic human pathogen) represented the same species.The taxonomic heterogeneity of the genus Pseudomonas was revealed by the work of Palleroni and co-workers, which led to the gradual dissection of the genus over the following decades. In 1992, P. cepacia and several other species of rRNA group II were transferred to the new genus Burkholderia. Over the past ten years, interest in B. cepacia-like organisms led to the discovery and description of a multitude of novel species. The genus now contains 25 species (Table 1), most of which occur in soil and water.

In the early 1990s, the lack of sensitivity and specificity of various identification approaches for B. cepacia and the presence of hybrid strains with characteristics intermediate between those of typical B. cepacia and B. gladioli were reported. These data, together with the striking differences in clinical outcome, transmissibility, plant-pathogenic and biocontrol and other properties, could all have been accounted for by strain-specific characteristics but could also have pointed to an underlying taxonomic problem.
To test the latter possibility, a polyphasic taxonomic study was initiated at the Laboratorium voor Microbiology of the University of Ghent in Belgium. The initial study, published in 1997, dealt with some 80 strains and revealed that B. cepacia isolates, cultured from clinical or environmental sites, belonged to at least five distinct genomic species (genomovars), which were referred to collectively as the B. cepacia complex. Following identification of distinguishing phenotypic characteristics, the names B. multivorans and B. stabilis have been proposed for genomovars II and IV, respectively. Genomovar V was identified as B. vietnamiensis, an organism isolated from the rice rhizosphere. In the absence of differential biochemical tests to separate genomovar I (B. cepacia) from genomovar III, the latter remained unnamed.

uninoculated control R-1464 LMG 1222T LMG 2161

In that period, the CF community felt the need to coordinate their efforts in the study of this organism so the International
B. cepacia Working Group was established in 1996 as a forum for clinicians and scientists interested in advancing knowledge
of B. cepacia infection and colonisation in persons with CF through the collegial exchange of information and promotion of coordinated approaches to research (http://go.to/cepacia). The collaborative studies conducted with Prof. Dr. J. R. W. Govan (the Edinburgh Cystic Fibrosis Microbiology Laboratory and Strain Repository [United Kingdom]), Prof. Dr. David P. Speert (the Canadian Burkholderia cepacia complex Research and Referral Repository), Prof. Dr. John LiPuma (the United States Burkholderia cepacia Research Laboratory and Repository), Dr. Eshwar Mahenthiralingam (Cardiff School of Biosciences, United Kingdom), and others revealed an even more complex picture of the previously underestimated biodiversity of these bacteria.By now, up to 3000 isolates tentatively classified as B. cepacia have been examined and several new identification tools have been developed. Various novel species regularly misidentified as B. cepacia have been described, and the B. cepacia complex has been expanded to encompass at least nine genomovars. These new data confirmed that there are no phenotypic, genomic or taxonomic grounds to differentiate environmental and clinical strains of the B. cepacia complex and that the source of isolation cannot be used to assess the safety of biopesticides containing members of the B. cepacia complex. However, the first reports on the human and plant pathogenic roles and the biotechnological potential of these different genomovars suggested marked differences. It also became evident that it was necessary to establish the precise species status of B. cepacia-like organisms with biotechnological interest relative to B. cepacia-like organisms with life-threatening properties in order to provide regulatory bodies with usable criteria when they are asked to authorise the biotechnological application of strains. At present, if the principle of precaution prevails, strains with useful properties may wrongly be excluded from industrial or agricultural applications.

Experimental strain panel

The need to have a defined set of well-characterised strains representative of each genomovar led to the establishment of an experimental strain panel that was deposited in the BCCM/LMG Bacteria Collection (http://www.belspo.be/bccm/). This B. cepacia complex strain panel represents the five first genomovars and will be of assistance in the accurate identification, epidemiological analysis and systematic studies of virulence for this important group of opportunistic pathogens. The more recent discovery of novel genomovars led investigators to revise this experimental strain panel, and plans are being considered to extend the first core group of isolates to include representatives of additional special clones and clones representing the novel genomovars.

Peter A. R. Vandamme

Universiteit Gent
Laboratorium voor Farmaceutische Microbiologie
Tel.: +32 (0)9 264 80 93
Fax: +32 (0)9 264 81 95
E-mail: peter.vandamme@rug.ac.be

Further reading

• Govan J. R. W. and V. Deretic. 1996. Microbial pathogenesis in cystic fibrosis: mucoid Pseudomonas aeruginosa and Burkholderia cepacia. Microbiol. Rev. 60:539-574.
• Govan, J. R. W., J. Balandreau, and P. Vandamme. 2000. Burkholderia cepacia – friend and foe. ASM News 66:124-125.
• Govan, J. R. W. and P. Vandamme. 1998. Agricultural and medical microbiology: a time for bridging gaps. Microbiol. 144:2373-2375.
• Kersters, K., W. Ludwig, M. Vancanneyt, P. De Vos, M. Gillis, and K.-H. Schleifer. 1996. Recent changes in the classification of the pseudomonads: an overview. Syst. Appl. Microbiol. 19:465-477.
• LiPuma, J. J. 1998. Burkholderia cepacia: management issues and new insights. Clin Chest Med 19:473-486.
• LiPuma, J.J., and E. Mahenthiralingam. 1999. Commercial use of Burkholderia cepacia. Em. Inf. Dis. 5:305-306.