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

Cloaking antibodies are prevalent in Burkholderia cepacia complex infection and their removal restores serum killing

Introduction

The Burkholderia cepacia complex encompasses a group of gram-negative opportunistic pathogens that cause chronic lung infections in people with cystic fibrosis. Distinct from other respiratory pathogens, Burkholderia causes a unique clinical disease in a subset of patients known as 'cepacia syndrome', fulminant pneumonia accompanied by bacteraemia and sepsis with a mortality rate of up to 75%. Due to the bacteraemia associated with this disease, the mechanisms that allow Burkholderia to resist the bactericidal effects of serum complement-depending killing are vital. Antibodies usually promote serum killing; however, we have described 'cloaking antibodies', specific for lipopolysaccharides that paradoxically protect serum-sensitive bacteria from complement-mediated lysis. Cloaking antibodies that protect Pseudomonas aeruginosa have been found in 24%-41% of patients with chronic lung diseases. The presence of these antibodies is also associated with worse clinical outcomes. Here, we sought to determine the relevance of cloaking antibodies in patients with Burkholderia infection.

Methods

Twelve Burkholderia spp. were isolated from nine pwCF and characterised for susceptibility to healthy control serum. Patient serum was analysed for the titre of the cloaking antibody. The ability of the patient serum to prevent healthy control serum (HCS) killing of its cognate isolates was determined.

Results

We found that several of the Burkholderia strains were shared between patients. Ten of the 12 isolates were highly susceptible to HCS killing. Four of nine (44%) patients had cloaking antibodies that protected their cognate strain from serum killing. Depleting cloaking antibodies from patient serum restored HCS killing of Burkholderia isolates.

Discussion

Cloaking antibodies are prevalent in patients with Burkholderia pulmonary infection and protect these strains from serum killing. Removal of cloaking antibodies via plasmapheresis, as previously described for individuals with life-threatening Pseudomonas infection, may be a useful new strategy for those with serious and life-threatening Burkholderia infection.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

Insertion sequences drive the emergence of a highly adapted human pathogen

Pseudomonas aeruginosa is a highly adaptive opportunistic pathogen that can have serious health consequences in patients with lung disorders. Taxonomic outliers of P. aeruginosa of environmental origin have recently emerged as infectious for humans. Here, we present the first genome-wide analysis of an isolate that caused fatal haemorrhagic pneumonia. In two clones, CLJ1 and CLJ3, sequentially recovered from a patient with chronic pulmonary disease, insertion of a mobile genetic element into the P. aeruginosa chromosome affected major virulence-associated phenotypes and led to increased resistance to the antibiotics used to combat the infection. Comparative genome, proteome and transcriptome analyses revealed that this ISL3-family insertion sequence disrupted the genes for flagellar components, type IV pili, O-specific antigens, translesion polymerase and enzymes producing hydrogen cyanide. Seven-fold more insertions were detected in the later isolate, CLJ3, than in CLJ1, some of which modified strain susceptibility to antibiotics by disrupting the genes for the outer-membrane porin OprD and the regulator of β-lactamase expression AmpD. In the Galleria mellonella larvae model, the two strains displayed different levels of virulence, with CLJ1 being highly pathogenic. This study revealed insertion sequences to be major players in enhancing the pathogenic potential of a P. aeruginosa taxonomic outlier by modulating both its virulence and its resistance to antimicrobials, and explains how this bacterium adapts from the environment to a human host.

The Mla Pathway Plays an Essential Role in the Intrinsic Resistance of Burkholderia cepacia Complex Species to Antimicrobials and Host Innate Components

Antibiotic resistance is a threat to our modern society, and new strategies leading to the identification of new molecules or targets to combat multidrug-resistant pathogens are needed. Species of the genus Burkholderia, including the Burkholderia cepacia complex (Bcc), Burkholderia pseudomallei, and Burkholderia mallei, can be highly pathogenic and are intrinsically resistant to multiple classes of antibiotics. Bcc species are nonetheless sensitive to extracellular products released by Pseudomonas aeruginosa in interspecies competition. We screened for Burkholderia transposon mutants with increased sensitivity to P. aeruginosa spent medium and identified multiple mutants in genes sharing homology with the Mla pathway. Insertional mutants in representative genes of the Bcc Mla pathway had a compromised cell membrane and were more sensitive to various extracellular stresses, including antibiotics and human serum. More precisely, mla mutants in the Bcc species Burkholderia cenocepacia and Burkholderia dolosa were more susceptible to Gram-positive antibiotics (i.e., macrolides and rifampin), fluoroquinolones, tetracyclines, and chloramphenicol. Genetic complementation of mlaC insertional mutants restored cell permeability and resistance to Gram-positive antibiotics. Importantly, Bcc mla mutants were not universally weaker strains since their susceptibilities to other classes of antibiotics were unaffected. Although cell permeability of homologous mla mutants in Escherichia coli or P. aeruginosa was also impaired, they were not more sensitive to Gram-positive antibiotics or other antimicrobials as was observed in Bcc mla mutants. Together, the data suggest that the Mla pathway in Burkholderia may play a different biological role, which could potentially represent a Burkholderia-specific drug target in combination therapy with antibiotic adjuvants.IMPORTANCE The outer membrane of Gram-negative bacteria acts as an effective barrier against toxic compounds, and therefore compromising this structure could increase sensitivity to currently available antibiotics. In this study, we show that the Mla pathway, a system involved in maintaining the integrity of the outer membrane, is genetically and functionally different in Burkholderia cepacia complex species compared to that in other proteobacteria. Mutants in mla genes of Burkholderia cenocepacia or Burkholderia dolosa were sensitive to Gram-positive antibiotics, while this effect was not observed in Escherichia coli or Pseudomonas aeruginosa The Mla pathway in Burkholderia species may represent an ideal genus-specific target to address their intrinsic antimicrobial resistances.