Paul J. Brett

Molecular Insights into Burkholderia pseudomallei and Burkholderia mallei Pathogenesis

Burkholderia pseudomallei and Burkholderia mallei are closely related gram-negative bacteria that can cause serious diseases in humans and animals. This review summarizes the current and rapidly expanding knowledge on the specific virulence factors employed by these pathogens and their roles in the pathogenesis of melioidosis and glanders. In particular, the contributions of recently identified virulence factors are described in the context of the intracellular lifestyle of these pathogens. Throughout this review, unique and shared virulence features of B. pseudomallei and B. mallei are discussed.

The Cluster 1 Type VI Secretion System Is a Major Virulence Determinant in Burkholderia pseudomallei

ABSTRACT The Burkholderia pseudomallei K96243 genome encodes six type VI secretion systems (T6SSs), but little is known about the role of these systems in the biology of B. pseudomallei. In this study, we purified recombinant Hcp proteins from each T6SS and tested them as vaccine candidates in the BALB/c mouse model of melioidosis. Recombinant Hcp2 protected 80% of mice against a lethal challenge with K96243, while recombinant Hcp1, Hcp3, and Hcp6 protected 50% of mice against challenge. Hcp6 was the only Hcp constitutively produced by B. pseudomallei in vitro; however, it was not exported to the extracellular milieu. Hcp1, on the other hand, was produced and exported in vitro when the VirAG two-component regulatory system was overexpressed in trans. We also constructed six hcp deletion mutants (Δhcp1 through Δhcp6) and tested them for virulence in the Syrian hamster model of infection. The 50% lethal doses (LD50s) for the Δhcp2 through Δhcp6 mutants were indistinguishable from K96243 (<10 bacteria), but the LD50 for the Δhcp1 mutant was >103 bacteria. The hcp1 deletion mutant also exhibited a growth defect in RAW 264.7 macrophages and was unable to form multinucleated giant cells in this cell line. Unlike K96243, the Δhcp1 mutant was only weakly cytotoxic to RAW 264.7 macrophages 18 h after infection. The results suggest that the cluster 1 T6SS is essential for virulence and plays an important role in the intracellular lifestyle of B. pseudomallei.

Molecular and Physical Characterization of Burkholderia mallei O Antigens

Burkholderia mallei lipopolysaccharide (LPS) has been previously shown to cross-react with polyclonal antibodies raised against B. pseudomallei LPS; however, we observed that B. mallei LPS does not react with a monoclonal antibody (Pp-PS-W) specific for B. pseudomallei O polysaccharide (O-PS). In this study, we identified the O-PS biosynthetic gene cluster from B. mallei ATCC 23344 and subsequently characterized the molecular structure of the O-PS produced by this organism.

Burkholderia mallei Cluster 1 Type VI Secretion Mutants Exhibit Growth and Actin Polymerization Defects in RAW 264.7 Murine Macrophages

ABSTRACT Burkholderia mallei is a facultative intracellular pathogen that causes severe disease in animals and humans. Recent studies have shown that the cluster 1 type VI secretion system (T6SS-1) expressed by this organism is essential for survival in a hamster model of glanders. To better understand the role of T6SS-1 in the pathogenesis of disease, studies were initiated to examine the interactions of B. mallei tssE mutants with RAW 264.7 murine macrophages. Results obtained by utilizing modified gentamicin protection assays indicated that although the tssE mutants were able to survive within RAW 264.7 cells, significant growth defects were observed in comparison to controls. In addition, analysis of infected monolayers by differential interference contrast and fluorescence microscopy demonstrated that the tssE mutants lacked the ability to induce multinucleated giant cell formation. Via the use of fluorescence microscopy, tssE mutants were shown to undergo escape from lysosome-associated membrane protein 1-positive vacuoles. Curiously, however, following entry into the cytosol, the mutants exhibited actin polymerization defects resulting in inefficient intra- and intercellular spread characteristics. Importantly, all mutant phenotypes observed in this study could be restored by complementation. Based upon these findings, it appears that T6SS-1 plays a critical role in growth and actin-based motility following uptake of B. mallei by RAW 264.7 cells.

Burkholderia pseudomallei Type III Secretion System Mutants Exhibit Delayed Vacuolar Escape Phenotypes in RAW 264.7 Murine Macrophages

ABSTRACT Burkholderia pseudomallei is a facultative intracellular pathogen capable of surviving and replicating within eukaryotic cells. Recent studies have shown that B. pseudomallei Bsa type III secretion system 3 (T3SS-3) mutants exhibit vacuolar escape and replication defects in J774.2 murine macrophages. In the present study, we characterized the interactions of a B. pseudomallei bsaZ mutant with RAW 264.7 murine macrophages. Following uptake, the mutant was found to survive and replicate within infected RAW 264.7 cells over an 18-h period. In addition, high levels of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), granulocyte-macrophage colony-stimulating factor (GM-CSF), and RANTES, but not IL-1α and IL-1β, were detected in culture supernatants harvested from infected monolayers. The subcellular location of B. pseudomallei within infected RAW 264.7 cells was determined, and as expected, the bsaZ mutant demonstrated early-vacuolar-escape defects. Interestingly, however, experiments also indicated that this mutant was capable of delayed vacuolar escape. Consistent with this finding, evidence of actin-based motility and multinucleated giant cell formation were observed between 12 and 18 h postinfection. Further studies demonstrated that a triple mutant defective in all three B. pseudomallei T3SSs exhibited the same phenotype as the bsaZ mutant, indicating that functional T3SS-1 and T3SS-2 did not appear to be responsible for the delayed escape phenotype in RAW 264.7 cells. Based upon these findings, it appears that B. pseudomallei may not require T3SS-1, -2, and -3 to facilitate survival, delayed vacuolar escape, and actin-based motility in activated RAW 264.7 macrophages.

Development of a Prototype Lateral Flow Immunoassay (LFI) for the Rapid Diagnosis of Melioidosis

Burkholderia pseudomallei is a soil-dwelling bacterium and the causative agent of melioidosis. Isolation of B. pseudomallei from clinical samples is the “gold standard” for the diagnosis of melioidosis; results can take 3–7 days to produce. Alternatively, antibody-based tests have low specificity due to a high percentage of seropositive individuals in endemic areas. There is a clear need to develop a rapid point-of-care antigen detection assay for the diagnosis of melioidosis. Previously, we employed In vivo Microbial Antigen Discovery (InMAD) to identify potential B. pseudomallei diagnostic biomarkers. The B. pseudomallei capsular polysaccharide (CPS) and numerous protein antigens were identified as potential candidates. Here, we describe the development of a diagnostic immunoassay based on the detection of CPS. Following production of a CPS-specific monoclonal antibody (mAb), an antigen-capture immunoassay was developed to determine the concentration of CPS within a panel of melioidosis patient serum and urine samples. The same mAb was used to produce a prototype Active Melioidosis Detect Lateral Flow Immunoassay (AMD LFI); the limit of detection of the LFI for CPS is comparable to the antigen-capture immunoassay (∼0.2 ng/ml). The analytical reactivity (inclusivity) of the AMD LFI was 98.7% (76/77) when tested against a large panel of B. pseudomallei isolates. Analytical specificity (cross-reactivity) testing determined that 97.2% of B. pseudomallei near neighbor species (35/36) were not reactive. The non-reactive B. pseudomallei strain and the reactive near neighbor strain can be explained through genetic sequence analysis. Importantly, we show the AMD LFI is capable of detecting CPS in a variety of patient samples. The LFI is currently being evaluated in Thailand and Australia; the focus is to optimize and validate testing procedures on melioidosis patient samples prior to initiation of a large, multisite pre-clinical evaluation.

iNOS activity is critical for the clearance of Burkholderia mallei from infected RAW 264.7 murine macrophages

Burkholderia mallei is a facultative intracellular pathogen that can cause fatal disease in animals and humans. To better understand the role of phagocytic cells in the control of infections caused by this organism, studies were initiated to examine the interactions of B. mallei with RAW 264.7 murine macrophages. Utilizing modified kanamycin‐protection assays, B. mallei was shown to survive and replicate in RAW 264.7 cells infected at multiplicities of infection (moi) of ≤ 1. In contrast, the organism was efficiently cleared by the macrophages when infected at an moi of 10. Interestingly, studies demonstrated that the monolayers only produced high levels of TNF‐α, IL‐6, IL‐10, GM‐CSF, RANTES and IFN‐β when infected at an moi of 10. In addition, nitric oxide assays and inducible nitric oxide synthase (iNOS) immunoblot analyses revealed a strong correlation between iNOS activity and clearance of B. mallei from RAW 264.7 cells. Furthermore, treatment of activated macrophages with the iNOS inhibitor, aminoguanidine, inhibited clearance of B. mallei from infected monolayers. Based upon these results, it appears that moi significantly influence the outcome of interactions between B. mallei and murine macrophages and that iNOS activity is critical for the clearance of B. mallei from activated RAW 264.7 cells.

Antilipopolysaccharide II: An Antibody Protective Against Fatal Melioidosis

This was a study of IgG antibody responses to two S-type lipopolysaccharides (LPS I and LPS II) and flagellin of Burkholderia pseudomallei in patients with melioidosis. The specificity of these antibodies was 91.7%, 90.3%, and 93.8%, respectively, when compared to responses in a population where the organism is not endemic. Only the level of antibody to LPS II (anti-LPS II) was significantly higher in patients who survived than in those who died, as well as in patients with nonsepticemic vs. septicemic melioidosis. Results of logistic regression analysis, controlled for confounding factors such as duration of illness before treatment and bacteremic status, confirmed that a high level of anti-LPS II was a significant factor protective against fatal melioidosis. Thus, LPS II of B. pseudomallei would be a potentially useful component of a vaccine developed against fatal melioidosis. Further studies are in progress to determine the level of this antibody among those with asymptomatic infection in areas where melioidosis is endemic.

Melioidosis Diagnostic Workshop, 20131

Melioidosis is a severe disease that can be difficult to diagnose because of its diverse clinical manifestations and a lack of adequate diagnostic capabilities for suspected cases. There is broad interest in improving detection and diagnosis of this disease not only in melioidosis-endemic regions but also outside these regions because melioidosis may be underreported and poses a potential bioterrorism challenge for public health authorities. Therefore, a workshop of academic, government, and private sector personnel from around the world was convened to discuss the current state of melioidosis diagnostics, diagnostic needs, and future directions.

Burkholderia mallei tssM Encodes a Putative Deubiquitinase That Is Secreted and Expressed inside Infected RAW 264.7 Murine Macrophages

ABSTRACT Burkholderia mallei, a category B biothreat agent, is a facultative intracellular pathogen that causes the zoonotic disease glanders. The B. mallei VirAG two-component regulatory system activates the transcription of ∼60 genes, including a large virulence gene cluster encoding a type VI secretion system (T6SS). The B. mallei tssM gene encodes a putative ubiquitin-specific protease that is physically linked to, and transcriptionally coregulated with, the T6SS gene cluster. Mass spectrometry and immunoblot analysis demonstrated that TssM was secreted in a virAG-dependent manner in vitro. Surprisingly, the T6SS was found to be dispensable for the secretion of TssM. The C-terminal half of TssM, which contains Cys and His box motifs conserved in eukaryotic deubiquitinases, was purified and biochemically characterized. Recombinant TssM hydrolyzed multiple ubiquitinated substrates and the cysteine at position 102 was critical for enzymatic activity. The tssM gene was expressed within 1 h after uptake of B. mallei into RAW 264.7 murine macrophages, suggesting that the TssM deubiquitinase is produced in this intracellular niche. Although the physiological substrate(s) is currently unknown, the TssM deubiquitinase may provide B. mallei a selective advantage in the intracellular environment during infection.