Burkholderia cenocepacia is a Gram-negative, rod-shaped bacterium that is commonly found in soil and water environments and may also be associated with plants and animals, particularly as a human pathogen. It is one of over 20 species in the Burkholderia cepacia complex (Bcc) and is notable due to its virulence factors and inherent antibiotic resistance that render it a prominent opportunistic pathogen responsible for life-threatening, nosocomial infections in immunocompromised patients, such as those with cystic fibrosis or chronic granulomatous disease. The quorum sensing systems CepIR and CciIR regulate the formation of biofilms and the expression of virulence factors such as siderophores and proteases. Burkholderia cenocepacia may also cause disease in plants, such as in onions and bananas. Additionally, some strains serve as plant growth-promoting rhizobacteria.
Taxonomy
Within the Burkholderia genus, the Burkholderia cepacia complex contains over 20 related species that cause opportunistic infections and possess antibiotic resistance. Burkholderia cepacia was originally defined as a single species, but it is now one of several species in the Bcc. Although closely related, the species within the Bcc have differing severity of pathogenicity, and B. cenocepacia is one of the most intensively studied due to its higher pathogenicity and antibiotic resistance compared to other species in the complex. Exchange of genetic material between species of the Bcc has resulted in a reticulated phylogeny that presents an obstacle to diagnostic classification at the species-level. Because of this phenotypic overlap between species, previous nomenclature of Bcc species involved genomovar terms, with Burkholderia cenocepacia categorized as genomovar III of the Bcc. Within the categorization as genomovar III, there are 4 phylogenetic lineage groups: IIIA, IIIB, IIIC, and IIID. No IIIC isolates have been found in studies on the natural environment, whereas all IIID isolates studied have been in clinical isolates of B. cenocepacia.
Microbiology
The strong environmental protection response of B. cenocepacia is attributed to the biofilm formed by groups of the organism. This biofilm contains exopolysaccharides that strengthen the bacterium's resistance to antibiotics and contribute to the bacteria's virulence. It is made up of a highly branched polysaccharide unit with one glucose, one glucuronic acid, one mannose, one rhamnose, and three galactose molecules. This species in the Burkholderia cepacia complex has also created another polysaccharide with one 3-deoxy-d-manno-2-octulosonic acid and three galactose molecules. The biofilm exopolysaccharides act as a barrier to neutrophils from human immune resistance systems, undermining the neutrophil defense action by inhibiting neutrophil chemotaxis and scavenging reactive oxygen species, which are bactericidal products produced by neutrophils to destroy bacteria.
Genome
B. cenocepacia'''s genome consists of three circular chromosomes and one plasmid. Chromosome 1 contains 3.87 Mb, chromosome 2 contains 3.22 Mb, and chromosome 3 contains 0.88 Mb. The plasmid is approximately 0.09 Mb. Chromosome 3 has also been characterized as a large plasmid, or megaplasmid (pC3); unlike chromosomes 2 and 3, it does not contain essential housekeeping genes, instead coding for accessory functions such as virulence and resistance to stress. In addition to the multireplicon structure, the genome contains several insertion sequences and can rapidly mutate during infections, which contribute to B. cenocepacia's unique adaptability and ability to acquire diverse catabolic functions.