Production of exopolysaccharide by Burkholderia cenocepacia results in altered cell-surface interactions and altered bacterial clearance in mice

Functional analysis of the Burkholderia cenocepacia J2315 BceAJ protein with phosphomannose isomerase and GDP-D-mannose pyrophosphorylase activities

The bceA(J) gene from the cystic fibrosis isolate Burkholderia cenocepacia J2315 encodes a 56-kDa bifunctional protein, with phosphomannose isomerase (PMI) and guanosine diphosphate (GDP)-mannose pyrophosphorylase (GMP) activities, a new member of the poorly characterised type II PMI class of proteins. Due to the lack of homology between the type II PMIs and the human PMI, this class of proteins are being regarded as interesting potential targets to develop new antimicrobials. The BceA(J) protein conserves the four typical motifs of type II PMIs: the pyrophosphorylase signature, the GMP active site, the PMI active site and the zinc-binding motif. After overproduction of BceA(J) by Escherichia coli as a histidine tag derivative, the protein was purified to homogeneity by affinity chromatography. The GMP activity is dependent on the presence of Mg(2+) or Ca(2+) as cofactors, while the PMI activity uses a broader range of divalent ions, in the order of activation Mg(2+) > Ca(2+) > Mn(2+) > Co(2+) > Ni(2+). The kinetic parameters K(m), V(max) and K(cat)/K(m) for the PMI and GMP activities were determined. Results suggest that the enzyme favours the formation of GDP-mannose instead of mannose catabolism, thus channelling precursors to the formation of glycoconjugates.

Pseudomonas aeruginosa and Burkholderia cenocepacia infections in patients affected by cystic fibrosis: serum resistance and antibody response

Pseudomonas aeruginosa and Burkholderia cenocepacia are opportunistic pathogens causing important chronic pulmonary infections in patients affected by cystic fibrosis (CF). The interplay of bacterial and host factors involved in the establishment and evolution of these infections needs further clarification. We investigated the susceptibility of P. aeruginosa and B. cenocepacia derived from CF patients or from the environment to hyperimmune sera obtained from the same CF patients and evaluated the amount of specific antibodies present in these sera. Our data indicate that the bactericidal activity of human serum against these two bacteria is mostly complement-mediated, and that the mucous layer probably confers serum-resistance to B. cenocepacia. The mean amount of antibodies against P. aeruginosa was higher than that against B. cenocepacia. The contribution of these data to the assessment of the importance of the humoral immune response in CF pulmonary infections by Pseudomonas and Burkholderia is briefly discussed.

Differential mucoid exopolysaccharide production by members of the Burkholderia cepacia complex

We demonstrate that all nine species of the Burkholderia cepacia complex can express the mucoid phenotype. A survey of clinical isolates showed that strains of B. cenocepacia, the most virulent species of the complex, are most frequently nonmucoid. Additionally, isolates from patients with chronic infections can convert from mucoid to nonmucoid.

Exopolysaccharides produced by Inquilinus limosus, a new pathogen of cystic fibrosis patients: novel structures with usual components

The major cause of morbidity and mortality in patients with cystic fibrosis, an autosomal recessive disorder, is chronic microbial colonisation of the major airways that leads to exacerbation of pulmonary infection. Several different microbes colonise cystic fibrosis lungs, and Pseudomonas aeruginosa is one of the most threatening, since the establishment of mucoid (alginate producing) strains is ultimately associated with the patient's death. Very recently a new bacterium, named Inquilinus limosus, was repeatedly found infecting the respiratory tract of cystic fibrosis patients. Its multi-resistance characteristic to antibiotics might result in the spreading of I. limosus infection among the cystic fibrosis community, as recently happened with strains of the Burkholderia cepacia complex. Since exopolysaccharides are recognised as important virulence factors in lung infections, the primary structure of the polysaccharide produced by I. limosus strain LMG 20952(T) was investigated as the first step in understanding its role in pathogenesis. The structure was determined by means of methylation analysis, acid degradations, mass spectrometry and NMR spectroscopy. The results showed that the bacterium produced a mixture constituted of the following polymers: [3)-[4,6-O-(1-carboxyethylidene)]-beta-D-Glcp(1-->]n; [2)-[4,6-O-(1-carboxyethylidene)]-alpha-D-Manp(1-->]n. Both polymers were completely substituted with pyruvyl ketal groups, a novel structural characteristic not previously found in bacterial polysaccharides. The absolute configuration of all pyruvyl groups was S. Inspection of possible local conformations assumed by the two polysaccharide chains showed features, which might provide interesting clues for understanding structure-function relationships.

Virulence of Burkholderia cepacia complex strains in gp91phox-/- mice

In cystic fibrosis (CF), infection with Burkholderia cepacia complex (Bcc) strains may cause long-term asymptomatic airway colonization, or severe lung infection leading to rapid pulmonary decline. To assess the virulence of Bcc strains, we established a lung infection model in mice with a null allele of the gene involved in X-linked chronic granulomatous disease (CGD). CGD mice, challenged intratracheally with 10(3) cells of the epidemic Burkholderia cenocepacia strain J2315, died within 3 days from sepsis after bacteria had multiplied to 3.3 x 10(8) cells. Infected mice developed neutrophil-dominated lung abscesses. Other B. cenocepacia strains and a B. cepacia strain were less virulent and one B. multivorans and one B. vietnamensis CF isolate were both avirulent. Bcc mutants, defective in exopolysaccharide synthesis or quorum sensing revealed diminished or no abscess formation and mortality. Immunofluorescence staining of Bcc-infected murine and CF lung tissues revealed colocalization of Bcc and neutrophils, suggesting Bcc persistence within neutrophils in CGD and CF. In vitro, Bcc cells were rapidly killed during aerobic neutrophil phagocytosis; however, the pathogens survived in neutrophils with blocked nicotinamide adenine dinucleotide phosphate oxidase activity and under anaerobic conditions. We conclude that the Bcc infection model in CGD mice is well suited for the assessment of Bcc virulence.

Oxygen, cyanide and energy generation in the cystic fibrosis pathogen Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative, rod-shaped bacterium that belongs to the gamma-proteobacteria. This clinically challenging, opportunistic pathogen occupies a wide range of niches from an almost ubiquitous environmental presence to causing infections in a wide range of animals and plants. P. aeruginosa is the single most important pathogen of the cystic fibrosis (CF) lung. It causes serious chronic infections following its colonisation of the dehydrated mucus of the CF lung, leading to it being the most important cause of morbidity and mortality in CF sufferers. The recent finding that steep O2 gradients exist across the mucus of the CF-lung indicates that P. aeruginosa will have to show metabolic adaptability to modify its energy metabolism as it moves from a high O2 to low O2 and on to anaerobic environments within the CF lung. Therefore, the starting point of this review is that an understanding of the diverse modes of energy metabolism available to P. aeruginosa and their regulation is important to understanding both its fundamental physiology and the factors significant in its pathogenicity. The main aim of this review is to appraise the current state of knowledge of the energy generating pathways of P. aeruginosa. We first look at the organisation of the aerobic respiratory chains of P. aeruginosa, focusing on the multiple primary dehydrogenases and terminal oxidases that make up the highly branched pathways. Next, we will discuss the denitrification pathways used during anaerobic respiration as well as considering the ability of P. aeruginosa to carry out aerobic denitrification. Attention is then directed to the limited fermentative capacity of P. aeruginosa with discussion of the arginine deiminase pathway and the role of pyruvate fermentation. In the final part of the review, we consider other aspects of the biology of P. aeruginosa that are linked to energy metabolism or affected by oxygen availability. These include cyanide synthesis, which is oxygen-regulated and can affect the operation of aerobic respiratory pathways, and alginate production leading to a mucoid phenotype, which is regulated by oxygen and energy availability, as well as having a role in the protection of P. aeruginosa against reactive oxygen species. Finally, we consider a possible link between cyanide synthesis and the mucoid switch that operates in P. aeruginosa during chronic CF lung infection.

The Burkholderia cepacia bceA gene encodes a protein with phosphomannose isomerase and GDP-D-mannose pyrophosphorylase activities

The bceA gene is part of the Burkholderia cepacia IST408 exopolysaccharide (EPS) biosynthetic cluster. It encodes a 55.3-kDa bifunctional protein (type II PMI family) with phosphomannose isomerase (PMI) and GDP-mannose pyrophosphorylase (GMP) activities. GMP activity is strongly dependent on the presence of Ca(2+) or Mn(2+), while PMI activity can use a broader variety of divalent cations (Ca(2+)>Mn(2+)>Mg(2+)>Co(2+)>Ni(2+)). The lack of a functional bceA gene does not affect EPS production yield in a non-polar insertion bceA mutant. The in silico search for putative bceA homologues revealed the presence of 2-5 bceA orthologues in the Burkholderia genomes available. This suggests that in B. cepacia IST408 putative bceA functional homologues may compensate the bceA mutation. However, the viscosity of aqueous solutions prepared with the EPS produced by the bceA mutant was significantly reduced compared with wild-type biopolymer and the mutant forms biofilms with a size reduced by 6-fold.

Functional analysis of Burkholderia cepacia genes bceD and bceF, encoding a phosphotyrosine phosphatase and a tyrosine autokinase, respectively: role in exopolysaccharide biosynthesis and biofilm formation

The biosynthesis of the exopolysaccharide (EPS) cepacian by Burkholderia cepacia complex strains requires the 16.2-kb bce cluster of genes. Two of the clustered genes, bceD and bceF, code for two proteins homologous to phosphotyrosine phosphatases and tyrosine kinases, respectively. We show experimental evidence indicating that BceF is phosphorylated on tyrosine and that the conserved lysine residue present at position 563 in the Walker A ATP-binding motif is required for this autophosphorylation. It was also proved that BceD is capable of dephosphorylating the phosphorylated BceF. Using the artificial substrate p-nitrophenyl phosphate (PNPP), BceD exhibited a V(max) of 8.8 mumol of PNPP min(-1) mg(-1) and a K(m) of 3.7 mM PNPP at 30 degrees C. The disruption of bceF resulted in the abolishment of cepacian accumulation in the culture medium, but 75% of the parental strain's EPS production yield was still registered for the bceD mutant. The exopolysaccharide produced by the bceD mutant led to less viscous solutions and exhibited the same degree of acetylation as the wild-type cepacian, suggesting a lower molecular mass for this mutant biopolymer. The size of the biofilm produced in vitro by bceD and bceF mutant strains is smaller than the size of the biofilm formed by the parental strain, and this phenotype was confirmed by complementation assays, indicating that BceD and BceF play a role in the establishment of biofilms of maximal size.

Burkholderia cenocepacia ZmpB is a broad-specificity zinc metalloprotease involved in virulence

In previous studies we characterized the Burkholderia cenocepacia ZmpA zinc metalloprotease. In this study, we determined that B. cenocepacia has an additional metalloprotease, which we designated ZmpB. The zmpB gene is present in the same species as zmpA and was detected in B. cepacia, B. cenocepacia, B. stabilis, B. ambifaria, and B. pyrrocinia but was absent from B. multivorans, B. vietnamiensis, B. dolosa, and B. anthina. The zmpB gene was expressed, and ZmpB was purified from Escherichia coli by using the pPROEXHTa His(6) Tag expression system. ZmpB has a predicted preproenzyme structure typical of thermolysin-like proteases and is distantly related to Bacillus cereus bacillolysin. ZmpB was expressed as a 63-kDa preproenzyme precursor that was autocatalytically cleaved into mature ZmpB (35 kDa) and a 27-kDa prepropeptide. EDTA, 1,10-phenanthroline, and Zn(2+) cations inhibited ZmpB enzyme activity, indicating that it is a metalloprotease. ZmpB had proteolytic activity against alpha-1 proteinase inhibitor, alpha(2)-macrogobulin, type IV collagen, fibronectin, lactoferrin, transferrin, and immunoglobulins. B. cenocepacia zmpB and zmpA zmpB mutants had no proteolytic activity against casein and were less virulent in a rat agar bead chronic infection model, indicating that zmpB is involved in B. cenocepacia virulence. Expression of zmpB was regulated by both the CepIR and CciIR quorum-sensing systems.

Spontaneous reversion of Mycobacterium abscessus from a smooth to a rough morphotype is associated with reduced expression of glycopeptidolipid and reacquisition of an invasive phenotype

Mycobacterium abscessus is an increasingly important cause of human disease; however, virulence determinants are largely uncharacterized. Previously, it was demonstrated that a rough, wild-type human clinical isolate (390R) causes persistent, invasive infection, while a smooth isogenic mutant (390S) has lost this capability. During serial passage of 390S, a spontaneous rough revertant was obtained, which was named 390V. This revertant regained the ability to cause persistent, invasive infection in human monocytes and the lungs of mice. Glycopeptidolipid (GPL), which plays a role in environmental colonization, was present in abundance in the cell wall of 390S, and was associated with sliding motility and biofilm formation. In contrast, a marked reduction in the amount of GPL in the cell wall of 390R and 390V was correlated with cord formation, a property associated with mycobacterial virulence. These results indicate that the ability to switch between smooth and rough morphologies may allow M. abscessus to transition between a colonizing phenotype and a more virulent, invasive form.

Exopolysaccharides from Burkholderia cenocepacia inhibit neutrophil chemotaxis and scavenge reactive oxygen species

Bacteria belonging to the Burkholderia cepacia complex are important opportunistic pathogens in compromised hosts, particularly patients with cystic fibrosis or chronic granulomatous disease. Isolates of B. cepacia complex may produce large amounts of exopolysaccharides (EPS) that endow the bacteria with a mucoid phenotype and appear to facilitate bacterial persistence during infection. We showed that EPS from a clinical B. cenocepacia isolate interfered with the function of human neutrophils in vitro; it inhibited chemotaxis and production of reactive oxygen species (ROS), both essential components of innate neutrophil-mediated host defenses. These inhibitory effects were not due to cytotoxicity or interference with intracellular calcium signaling. EPS also inhibited enzymatic generation of ROS in cell-free systems, indicating that it scavenges these bactericidal products. B. cenocepacia EPS is structurally distinct from Pseudomonas aeruginosa alginate, yet they share the capacity to scavenge ROS and inhibit chemotaxis. These properties could explain why the two bacterial species resist clearance from the infected cystic fibrosis lung.

The role of nutrient presence on the adhesion kinetics of Burkholderia cepacia G4g and ENV435g

The adhesion kinetics of Burkholderia cepacia G4g and ENV435g have been investigated in a radial stagnation point flow (RSPF) system under well-controlled hydrodynamics and solution chemistry. The sensitivity of adhesion behavior to nutrient condition was also examined. Supplementary cell characterization techniques were conducted to evaluate the viability, hydrophobicity, electrophoretic mobility, size, and charge density of cells grown in both nutrient rich Luria broth (LB) and nutrient poor basal salts medium (BSM). Comparable adhesion kinetics were observed for the wild-type (G4g) and mutant (ENV435g) grown in the same medium; however, the attachment efficiency increased with the level of nutrient presence for both cell types by approximately 60%. Nutrient condition altered deposition due to its impact on the surface charge characteristics and size of the cells. Adhesion behavior was consistent with expectations based on classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for colloidal interactions, as the adhesion efficiency increased with ionic strength. However, the results also suggest the involvement of non-DLVO type interactions that influence cell adhesion. Systematic experimentation with B. cepacia in the RSPF system demonstrated that the ENV435g mutant is not "adhesion deficient"; rather, adhesion for both the G4g and ENV435g was a function of the nutrient condition and resulting cell surface chemistry.

Update on the Burkholderia cepacia complex

PURPOSE OF REVIEW

The Burkholderia cepacia complex is comprised of a group of related bacterial species that are capable of causing life-threatening respiratory tract infection in persons with cystic fibrosis. This article reviews advances in our understanding of Burkholderia cepacia complex infection in cystic fibrosis, focusing on the taxonomy, clinical microbiology, and epidemiology, as well as the natural history and clinical outcomes associated with Burkholderia cepacia complex infection.

RECENT FINDINGS

Each of the nine species of the Burkholderia cepacia complex has now received a formal species name. These names are the preferred nomenclature, replacing the former 'genomovar' designations. Studies from several countries reiterate that two species, Burkholderia cenocepacia and Burkholderia multivorans, account for most Burkholderia cepacia complex infection in cystic fibrosis. Bacterial genotyping studies indicate that specific Burkholderia cepacia complex strains infect multiple cystic fibrosis patients, implying that they may have an enhanced capacity for interpatient spread. Emerging clinical outcomes data suggest that at least some of these so-called transmissible or epidemic strains are also more virulent in the cystic fibrosis host. Ongoing research is aimed at gaining a better understanding of Burkholderia cepacia complex ecology, defining Burkholderia cepacia complex virulence factors and pathogenic mechanisms, and determining the relative virulence of distinct strains.

SUMMARY

Significant advances in our understanding of the Burkholderia cepacia complex serve as a critical foundation for further efforts that ultimately will enable better infection control and the development of novel therapeutics to treat Burkholderia cepacia complex infection in persons with cystic fibrosis.

Burkholderia cenocepaciaInduces Neutrophil Necrosis in Chronic Granulomatous Disease

Burkholderia cepacia complex is a life-threatening group of pathogens for patients with chronic granulomatous disease (CGD), whose phagocytes are unable to produce reactive oxygen species (ROS). Unlike other CGD pathogens, B. cepacia complex is particularly virulent, characteristically causing septicemia, and is the bacterial species responsible for most fatalities in these patients. We found that a nonmucoid Burkholderia cenocepacia (a predominant species in the B. cepacia complex) isolate was readily ingested by normal human neutrophils under nonopsonic conditions and promoted apoptosis in these cells. The proapoptotic effect was not due to secreted bacterial products, but was dependent on bacterial viability. Phagocytosis was associated with a robust production of ROS, and the apoptotic neutrophils could be effectively cleared by monocyte-derived macrophages. The proapoptotic effect of B. cenocepacia was independent of ROS production because neutrophils from CGD patients were rendered apoptotic to a similar degree as control cells after challenge. More importantly, neutrophils from CGD patients, but not from normal individuals, were rendered necrotic after phagocytosis of B. cenocepacia. The extreme virulence of B. cepacia complex bacteria in CGD, but not in immunocompetent hosts, could be due to its necrotic potential in the absence of ROS.

The multifarious, multireplicon Burkholderia cepacia complex

The Burkholderia cepacia complex (Bcc) is a collection of genetically distinct but phenotypically similar bacteria that are divided into at least nine species. Bcc bacteria are found throughout the environment, where they can have both beneficial and detrimental effects on plants and some members can also degrade natural and man-made pollutants. Bcc bacteria are now recognized as important opportunistic pathogens that can cause variable lung infections in cystic fibrosis patients, which result in asymptomatic carriage, chronic infection or 'cepacia syndrome', which is characterized by a rapid decline in lung function that can include invasive disease. Here we highlight the unique characteristics of the Bcc, focusing on the factors that determine virulence.