Plant host and sugar alcohol induced exopolysaccharide biosynthesis in the Burkholderia cepacia complex

Abstract

The species that presently constitute the Burkholderia cepacia complex (Bcc) have multiple roles; they include soil and water saprophytes, bioremediators, and plant, animal and human pathogens. Since the first description of pathogenicity in the Bcc was based on sour skin rot of onion bulbs, this study returned to this plant host to investigate the onion-associated phenotype of the Bcc. Many Bcc isolates, which were previously considered to be non-mucoid, produced copious amounts of exopolysaccharide (EPS) when onion tissue was provided as the sole nutrient. EPS production was not species-specific, was observed in isolates from both clinical and environmental sources, and did not correlate with the ability to cause maceration of onion tissue. Chemical analysis suggested that the onion components responsible for EPS induction were primarily the carbohydrates sucrose, fructose and fructans. Additional sugars were investigated, and all alcohol sugars tested were able to induce EPS production, in particular mannitol and glucitol. To investigate the molecular basis for EPS biosynthesis, we focused on the highly conserved bce gene cluster thought to be involved in cepacian biosynthesis. We demonstrated induction of the bce gene cluster by mannitol, and found a clear correlation between the inability of representatives of the Burkholderia cenocepacia ET12 lineage to produce EPS and the presence of an 11 bp deletion within the bceB gene, which encodes a glycosyltransferase. Insertional inactivation of bceB in Burkholderia ambifaria AMMD results in loss of EPS production on sugar alcohol media. These novel and surprising insights into EPS biosynthesis highlight the metabolic potential of the Bcc and show that a potential virulence factor may not be detected by routine laboratory culture. Our results also highlight a potential hazard in the use of inhaled mannitol as an osmolyte to improve mucociliary clearance in individuals with cystic fibrosis.

Growth on mannitol-rich media elicits a genome-wide transcriptional response in Burkholderia multivorans that impacts on multiple virulence traits in an exopolysaccharide-independent manner

“…The fact that mannitol induces EPS in B. multivorans whilst mannose does not is striking given that D-mannose and Dmannitol are each a single reaction away from D-fructose, which itself can promote EPS production (Bartholdson et al, 2008) through the action of D-mannose isomerase and Dmannitol 2-dehydrogenase respectively. Both enzymic activities have previously been reported in Burkholderia (Pseudomonas) cepacia (Allenza et al, 1982;Allenza et al, 1990).…”

“…Microarray analysis was performed using custom-made 4644 K microarrays based on the genome of B. multivorans ATCC 17616, using genomic DNA as a common reference. Two test conditions were evaluated: (i) growth on MYEM as the specific mannitol-containing medium that induces EPS production in B. multivorans (Bartholdson et al, 2008) and (ii) growth on NM-YEM as the equivalent mannose-containing agar which does not induce EPS production in B. multivorans (Bartholdson et al, 2008). In both cases, the RiboPure-Bacteria Kit (Ambion) was used to extract total RNA from B. multivorans ATCC 17616 following 16 h growth.…”

“…Mannitol is one of several sugars and sugar alcohols that can induce the overproduction of exopolysaccharide (EPS) by members of the BCC, resulting in a profoundly mucoid phenotype (Bartholdson et al, 2008), and MYEM has been widely used in studies defining the biological and structural properties of this EPS. The most common EPS produced by members of the BCC is cepacian (Herasimenka et al, 2007), the biosynthetic enzymes for which are encoded by the bce-I and bce-II gene clusters (Moreira et al, 2003;Ferreira et al, 2010).…”

Phase variation has a role in Burkholderia ambifaria niche adaptation

“…The role of EPSs in plant-Burkholderia interaction is not well documented. However, onion tissues induce EPS production from Burkholderia, probably through the carbohydrates produced by the onion (Bartholdson et al, 2008). In some plant-bacterial associations (for example, Rhizobium), EPSs are involved in the attachment of bacteria to the roots.…”

Mechanisms Controlling the Expression of the Exopolysaccharide of Burkholderia and Role in Niche Adaptation

“…If EPS produced by Bcc can both neutralize oxidative species and interact with non-oxidative antimicrobial peptides, then it might be an important virulence factor, since it would leave neutrophils without means to clear the offending bacteria. Against the importance of EPS in virulence is the study of Bartholdson and colleagues where Bcc clinical and environmental strains were assessed for EPS production and onion tissue maceration ability (Bartholdson et al, 2008). According to their results no correlation could be established between EPS production and onion tissue maceration.…”