Mucoid and NonmucoidBurkholderia cepaciaComplex Bacteria in Cystic Fibrosis Infections

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

In common with other members of the Burkholderia cepacia complex (BCC), Burkholderia multivorans is capable of producing exopolysaccharide (EPS) when grown on certain mannitol-rich media. The significance of the resulting mucoid phenotype and the genome-wide response to mannitol has never been characterized despite its clinical relevance following the approval of a dried-powder preparation of mannitol as an inhaled osmolyte therapy for cystic fibrosis (CF) patients. In the present study we defined the transcriptional response of B. multivorans ATCC 17616, a model genome-sequenced strain of environmental origin, to growth on mannitol-rich yeast extract media (MYEM). EPS-dependent and -independent impact of MYEM on virulence-associated traits was assessed in both strain ATCC 17616 and the CF isolate B. multivorans C1576. Our studies revealed a significant transcriptional response to MYEM encompassing approximately 23 % of predicted genes within the genome. Strikingly, this transcriptional response identified that EPS induction occurs in ATCC 17616 without the upregulation of the bce-I and bce-II EPS gene clusters, despite their pivotal role in EPS biosynthesis. Of approximately 20 differentially expressed putative virulence factors, 16 exhibited upregulation including flagella, ornibactin, oxidative stress proteins and phospholipases. MYEM-grown B. multivorans also exhibited enhanced motility, biofilm formation and epithelial cell invasion. In contrast to these potential virulence enhancements, MYEM-grown B. multivorans C1576 showed attenuated virulence in the Galleria mellonella infection model. All of the observed phenotypic responses occurred independently of EPS production, highlighting the profound impact that mannitol-based growth has on the physiology and virulence of B. multivorans.

Mannitol promotes adherence of an outbreak strain of Burkholderia multivorans via an exopolysaccharide-independent mechanism that is associated with upregulation of newly identified fimbrial and afimbrial adhesins

Burkholderia multivorans, a member of the Burkholderia cepacia complex (Bcc), is an important pathogen of the cystic fibrosis (CF) lung. Mannitol, approved as an inhaled osmolyte therapy for use in CF patients, promotes exopolysaccharide (EPS) production by the Bcc. In the present study, we investigated the role of mannitol-induced EPS in the adherence of B. multivorans. We report that mannitol promoted adherence of two representative B. multivorans strains. However, whilst this enhanced adherence was largely EPS-dependent in an environmental isolate, it was EPS-independent within a CF outbreak strain, suggesting strain-to-strain variation in adhesins. Genome sequencing of the outbreak strain enabled the identification of two distinct loci encoding putative fimbrial and afimbrial adhesins. The putative fimbriae-encoding locus was found to be widely distributed amongst clinical and environmental B. multivorans. In contrast, the locus encoding the putative afimbrial adhesin (of the filamentous haemagglutinin family, FHA) was restricted to clinical isolates. Both loci contributed to biofilm formation and mucin adherence. Furthermore, we report that mannitol promoted expression of both loci, and that the locus encoding the putative FHA-family adhesin is a key determinant of the enhanced adherence observed following growth in mannitol. Our studies provide the first characterization, to our knowledge, of B. multivorans adhesins, and in so doing highlight the strain-dependent role of EPS in the Bcc and the difficulties in assigning phenotypic traits to Bcc EPS due to the wider response to mannitol. Our observations also highlight the need to monitor the microbiological effects of inhaled mannitol therapy in Bcc-infected CF patients.

Cystic fibrosis: what to expect now in the early adult years

As a testimony to advances in patient care, more individuals with cystic fibrosis are surviving into their adult years than ever before. The clinical epidemiology of this complex multi-organ disease is evolving and has changed dramatically over the past two to three decades. This article discusses the emergence of chronic disease-related co-morbidities such as CF-related diabetes, chronic kidney disease, bone disease, arthropathy, and depression. It also provides an overview of the many challenges confronted by adult CF care providers.

Serum susceptibility in clinical isolates of Burkholderia cepacia complex bacteria: development of a growth-based assay for high throughput determination

Burkholderia cepacia complex (BCC) bacteria can cause devastating chronic infections in people with cystic fibrosis. Of particular concern is “cepacia syndrome,” a rapidly progressive and usually fatal decline in health, characterized by a necrotizing bacteremic pneumonia. An important component of defense against bloodstream infections is the bactericidal action of serum. Traditional methods to determine the capacity of bacterial isolates to resist the bactericidal effects of serum are relatively low-throughput viability assays. In this study, we developed a novel growth-based assay for serum susceptibility, which allows for high throughput analysis. We applied this assay to a range of clinical isolates of BCC as well as isolates comprising the BCC experimental strain panel. Our data demonstrate that isolates from all species of BCC examined can possess serum resistant or serum sensitive/intermediate phenotypes. Of particular clinical significance, we also found no direct link between the last saved pulmonary isolate from patients who subsequently developed “cepacia syndrome” and their capacity to resist the inhibitory effects of human serum, suggesting serum resistance cannot be used as a marker of an isolate’s capacity to escape from the lung and cause bacteremia.

Can early Burkholderia cepacia complex infection in cystic fibrosis be eradicated with antibiotic therapy?

Introduction: Organisms of the Burkholderia cepacia complex (BCC) are important pathogens in cystic fibrosis (CF). The majority of those who acquire BCC develop chronic infection but it can also result in rapid decline in a significant minority. In addition, chronic infection with Burkholderia cenocepacia in particular is regarded as a contraindication to lung transplantation in many units. Whilst aggressive antibiotic therapy is employed in CF to eradicate Pseudomonas aeruginosa before infection becomes irreversibly established, no formal assessment of such strategies has been previously reported for BCC, despite the apparent widespread adoption of this practice. Methods: UK adult CF centers were surveyed about their current approach to new BCC infection. Outcomes of eradication therapy were assessed in patients attending the Manchester Adult CF Center with new BCC isolates between 1st January 2002 and 1st May 2011. Patients with previous infection with the same strain of BCC were excluded. BCC were identified at the national reference laboratories and confirmed by species-specific PCR and RecA sequencing. Results: Routine use of therapies to attempt eradication of new BCC is commonly used in the UK (12/17 centers who responded). This typically involves a combination of intravenous and nebulised antibiotics. Of 19 eligible cases of new BCC infection, the organism has been eradicated in 7 (37%). Three of these did not receive specific eradication therapy. Of 14 patients who have received eradication therapy and completed follow up, BCC were cleared in only 4 (29%). Conclusions: Attempted eradication of new BCC is a common practice in UK adult CF centers. A minority of patients clear the infection spontaneously and the effects of eradication therapies are at best modest. Early treatment may be associated with better outcomes, though there are insufficient data to support the use of any specific treatment regimen. A prospective, systematic evaluation of treatments and outcomes is required.

Insights into the role of extracellular polysaccharides in Burkholderia adaptation to different environments

The genus Burkholderia comprises more than 60 species able to adapt to a wide range of environments such as soil and water, and also colonize and infect plants and animals. They have large genomes with multiple replicons and high gene number, allowing these bacteria to thrive in very different niches. Among the properties of bacteria from the genus Burkholderia is the ability to produce several types of exopolysaccharides (EPSs). The most common one, cepacian, is produced by the majority of the strains examined irrespective of whether or not they belong to the Burkholderia cepacia complex (Bcc). Cepacian biosynthesis proceeds by a Wzy-dependent mechanism, and some of the B. cepacia exopolysaccharide (Bce) proteins have been functionally characterized. In vitro studies showed that cepacian protects bacterial cells challenged with external stresses. Regarding virulence, bacterial cells with the ability to produce EPS are more virulent in several animal models of infection than their isogenic non-producing mutants. Although the production of EPS within the lungs of cystic fibrosis (CF) patients has not been demonstrated, the in vitro assessment of the mucoid phenotype in serial Bcc isolates from CF patients colonized for several years showed that mucoid to non-mucoid transitions are relatively frequent. This morphotype variation can be induced under laboratory conditions by exposing cells to stress such as high antibiotic concentration. Clonal isolates where mucoid to non-mucoid transition had occurred showed that during lung infection, genomic rearrangements, and mutations had taken place. Other phenotypic changes include variations in motility, chemotaxis, biofilm formation, bacterial survival rate under nutrient starvation and virulence. In this review, we summarize major findings related to EPS biosynthesis by Burkholderia and the implications in broader regulatory mechanisms important for cell adaptation to the different niches colonized by these bacteria.

Mucoid morphotype variation of Burkholderia multivorans during chronic cystic fibrosis lung infection is correlated with changes in metabolism, motility, biofilm formation and virulence

Burkholderia cepacia complex (Bcc) bacteria are opportunistic pathogens infecting hosts such as cystic fibrosis (CF) patients. Long-term Bcc infection of CF patients' airways has been associated with emergence of phenotypic variation. Here we studied two Burkholderia multivorans clonal isolates displaying different morphotypes from a chronically infected CF patient to evaluate trait development during lung infection. Expression profiling of mucoid D2095 and non-mucoid D2214 isolates revealed decreased expression of genes encoding products related to virulence-associated traits and metabolism in D2214. Furthermore, D2214 showed no exopolysaccharide production, lower motility and chemotaxis, and more biofilm formation, particularly under microaerophilic conditions, than the clonal mucoid isolate D2095. When Galleria mellonella was used as acute infection model, D2214 at a cell number of approximately 7 × 10⁶ c.f.u. caused a higher survival rate than D2095, although 6 days post-infection most of the larvae were dead. Infection with the same number of cells by mucoid D2095 caused larval death by day 4. The decreased expression of genes involved in carbon and nitrogen metabolism may reflect lower metabolic needs of D2214 caused by lack of exopolysaccharide, but also by the attenuation of pathways not required for survival. As a result, D2214 showed higher survival than D2095 in minimal medium for 28 days under aerobic conditions. Overall, adaptation during Bcc chronic lung infections gave rise to genotypic and phenotypic variation among isolates, contributing to their fitness while maintaining their capacity for survival in this opportunistic human niche.

In vitro susceptibility of Burkholderia vietnamiensis to aminoglycosides

Burkholderia cepacia complex (BCC) bacteria are opportunistic pathogens that can cause severe disease in cystic fibrosis (CF) patients and other immunocompromised individuals and are typically multidrug resistant. Here we observed that unlike other BCC species, most environmental and clinical Burkholderia vietnamiensis isolates were intrinsically susceptible to aminoglycosides but not to cationic antimicrobial peptides or polymyxin B. Furthermore, strains acquired aminoglycoside resistance during chronic CF infection, a phenomenon that could be induced under tobramycin or azithromycin pressure in vitro. In comparing susceptible and resistant B. vietnamiensis isolates, no gross differences in lipopolysaccharide structure were observed, all had lipid A-associated 4-amino-4-deoxy-L-arabinose residues, and all were resistant to the permeabilizing effects of aminoglycosides, a measure of drug entry via self-promoted uptake. However, susceptible isolates accumulated 5 to 6 times more gentamicin than a resistant isolate, and aminoglycoside susceptibility increased in the presence of an efflux pump inhibitor. B. vietnamiensis is therefore unusual among BCC bacteria in its susceptibility to aminoglycosides and capacity to acquire resistance. Aminoglycoside resistance appears to be due to decreased cellular accumulation as a result of active efflux.

Clinical significance of microbial infection and adaptation in cystic fibrosis

A select group of microorganisms inhabit the airways of individuals with cystic fibrosis. Once established within the pulmonary environment in these patients, many of these microbes adapt by altering aspects of their structure and physiology. Some of these microbes and adaptations are associated with more rapid deterioration in lung function and overall clinical status, whereas others appear to have little effect. Here we review current evidence supporting or refuting a role for the different microbes and their adaptations in contributing to poor clinical outcomes in cystic fibrosis.

Functional quorum sensing systems are maintained during chronic Burkholderia cepacia complex infections in patients with cystic fibrosis

Quorum sensing (QS) contributes to the virulence of Pseudomonas aeruginosa and Burkholderia cepacia complex lung infections. P. aeruginosa QS mutants are frequently isolated from patients with cystic fibrosis. The objective of this study was to determine whether similar adaptations occur over time in B. cepacia complex isolates. Forty-five Burkholderia multivorans and Burkholderia cenocepacia sequential isolates from patients with cystic fibrosis were analyzed for N-acyl-homoserine lactone activity. All but one isolate produced N-acyl-homoserine lactones. The B. cenocepacia N-acyl-homoserine lactone-negative isolate contained mutations in cepR and cciR. Growth competition assays were performed that compared B. cenocepacia clinical and laboratory defined wild-type and QS mutants. Survival of the laboratory wild-type and QS mutants varied, dependent on the mutation. The clinical wild-type isolate demonstrated a growth advantage over its QS mutant. These data suggest that there is a selective advantage for strains with QS systems and that QS mutations do not occur at a high frequency in B. cepacia complex isolates.