The molecular and cellular basis of pathogenesis in melioidosis: how does Burkholderia pseudomallei cause disease?

Genome sequence of two novel virulent clinical strains of Burkholderia pseudomallei isolated from acute melioidosis cases imported to Israel from India and Thailand

OBJECTIVE

Burkholderia pseudomallei, the etiological cause of melioidosis, is a soil saprophyte endemic in South-East Asia, where it constitutes a public health concern of high-priority. Melioidosis cases are sporadically identified in nonendemic areas, usually associated with travelers or import of goods from endemic regions. Due to extensive intercontinental traveling and the anticipated climate change-associated alterations of the soil bacterial flora, there is an increasing concern for inadvertent establishment of novel endemic areas, which may expand the global burden of melioidosis. Rapid diagnosis, isolation and characterization of B. pseudomallei isolates is therefore of utmost importance particularly in non-endemic locations.

DATA DESCRIPTION

We report the genome sequences of two novel clinical isolates (MWH2021 and MST2022) of B. pseudomallei identified in distinct acute cases of melioidosis diagnosed in two individuals arriving to Israel from India and Thailand, respectively. The data includes preliminary genetic analysis of the genomes determining their phylogenetic classification in rapport to the genomes of 131 B. pseudomallei strains documented in the NCBI database. Inspection of the genomic data revealed the presence or absence of loci encoding for several documented virulence determinants involved in the molecular pathogenesis of melioidosis. Virulence analysis in murine models of acute or chronic melioidosis established that both strains belong to the highly virulent class of B. pseudomalleii.

Function of Burkholderia pseudomallei RpoS and RpoN2 in bacterial invasion, intracellular survival, and multinucleated giant cell formation in mouse macrophage cell line

Melioidosis, a human infectious disease with a high mortality rate in many tropical countries, is caused by the pathogen Burkholderia pseudomallei (B. pseudomallei). The function of the B. pseudomallei sigma S (RpoS) transcription factor in survival during the stationary growth phase and conditions of oxidative stress is well documented. Besides the rpoS, bioinformatics analysis of B. pseudomallei genome showed the existence of two rpoN genes, named rpoN1 and rpoN2. In this study, by using the mouse macrophage cell line RAW264.7 as a model of infection, the involvement of B. pseudomallei RpoS and RpoN2 in the invasion, intracellular survival leading to the reduction in multinucleated giant cell (MNGC) formation of RAW264.7 cell line were illustrated. We have demonstrated that the MNGC formation of RAW264.7 cell was dependent on a certain number of intracellular bacteria (at least 5 × 104). In addition, the same MNGC formation (15%) observed in RAW264.7 cells infected with either B. pseudomallei wild type with multiplicity of infection (MOI) 2 or RpoN2 mutant (∆rpoN2) with MOI 10 or RpoS mutant (∆rpoS) with MOI 100. The role of B. pseudomallei RpoS and RpoN2 in the regulation of type III secretion system on bipB-bipC gene expression was also illustrated in this study.

Assessment of CD200R Activation in Combination with Doxycycline in a Model of Melioidosis

Antimicrobial resistance continues to be a global issue. Pathogens, such as Burkholderia pseudomallei, have evolved mechanisms to efflux certain antibiotics and manipulate the host response. New treatment strategies are therefore required, such as a layered defense approach. Here, we demonstrate, using biosafety level 2 (BSL-2) and BSL-3 in vivo murine models, that combining the antibiotic doxycycline with an immunomodulatory drug that targets the CD200 axis is superior to antibiotic treatment in combination with an isotype control. CD200-Fc treatment alone significantly reduces bacterial burden in lung tissue in both the BSL-2 and BSL-3 models. When CD200-Fc treatment is combined with doxycycline to treat the acute BSL-3 model of melioidosis, there is a 50% increase in survival compared with relevant controls. This benefit is not due to increasing the area under the concentration-time curve (AUC) of the antibiotic, suggesting the immunomodulatory nature of CD200-Fc treatment is playing an important role by potentially controlling the overactive immune response seen with many lethal bacterial infections. IMPORTANCE Traditional treatments for infectious disease have focused on the use of antimicrobial compounds (e.g. antibiotics) that target the infecting organism. However, timely diagnosis and administration of antibiotics remain crucial to ensure efficacy of these treatments especially for the highly virulent biothreat organisms. The need for early antibiotic treatment, combined with the increasing emergence of antibiotic resistant bacteria, means that new therapeutic strategies are required for organisms that cause rapid, acute infections. Here, we show that a layered defense approach, where an immunomodulatory compound is combined with an antibiotic, is better than an antibiotic combined with a relevant isotype control following infection with the biothreat agent Burkholderia pseudomallei. This approach has the potential to be truly broad spectrum and since the strategy includes manipulation of the host response it's application could be used in the treatment of a wide range of diseases.

Early Activation of iNKT Cells Increased Survival Time of BALB/c Mice in a Murine Model of Melioidosis

Melioidosis is an infectious disease caused by Burkholderia pseudomallei. High interferon gamma (IFN-γ) levels in naive mice were reported to mediate protection against B. pseudomallei infection. Invariant natural killer T (iNKT) cells can produce and secrete several cytokines, including IFN-γ. When iNKT cell-knockout (KO) BALB/c mice were infected with B. pseudomallei, their survival time was significantly shorter than wild-type mice. Naive BALB/c mice pretreated intraperitoneally with α-galactosylceramide (α-GalCer), an iNKT cell activator, 24 h before infection demonstrated 62.5% survival at the early stage, with prolonged survival time compared to nonpretreated infected control mice (14 ± 1 days versus 6 ± 1 days, respectively). At 4 h after injection with α-GalCer, treated mice showed significantly higher levels of serum IFN-γ, interleukin-4 (IL-4), IL-10, and IL-12 than control mice. Interestingly, the IFN-γ levels in the α-GalCer-pretreated group were decreased at 4, 24, and 48 h after infection, while they were highly increased in the control group. At 24 h postinfection in the α-GalCer group, bacterial loads were significantly lower in blood (no growth and 1,780.00 ± 51.21, P < 0.0001), spleens (no growth and 34,300 ± 1,106.04, P < 0.0001), and livers (1,550 ± 68.72 and 13,400 ± 1,066.67, P < 0.0001) than in the control group, but not in the lungs (15,300 ± 761.10 and 1,320 ± 41.63, P < 0.0001), and almost all were negative at 48 h postinfection. This study for the first time shows that early activation of iNKT cells by α-GalCer helps clearance of B. pseudomallei and prolongs mouse survival.

Burkholderia pseudomallei biofilm phenotypes confined but surviving in neutrophil extracellular traps of varying appearance

Melioidosis is a fatal infectious disease caused by Burkholderia pseudomallei. Complications following treatment are usually due to antibiotic resistance and relapse is mainly caused by B. pseudomallei biofilm. Although the release of neutrophil extracellular traps (NETs) is crucial to capture and eliminate bacterial pathogens, to date response of NETs to B. pseudomallei biofilm is poorly understood. Here we compare the NETs produced by neutrophils in response to B. pseudomallei H777 (a biofilm-producing strain containing the bpsl0618 gene), a biofilm-defect strain lacking this gene (B. pseudomallei M10) and a bpsl0618 biofilm-complemented strain, B. pseudomallei C17, in which function of bpsl0618 was restored. Co-cultivation of these strains with healthy human neutrophils at MOI 10 with or without cytochalasin D demonstrated that H777 significantly resisted neutrophil-mediated killing and non-phagocytotic mechanisms compared to M10 (p < 0.0001). Three distinct morphotypes of NETs were seen: “aggregated”, “spiky” and “cloudy”. These were induced in different proportions by the different bacterial strains. All types of NETs were shown to confine all B. pseudomallei strains. Strains H777 and C17 could stimulate production of twice as much extracellular DNA (234.62 ng/mL and 205.43 ng/mL, respectively) as did M10 (111.87 ng/mL). Cells of H777 and C17 were better able to survive in the presence of neutrophil killing mechanisms relative to M10 (p < 0.0001) and NET formation (p < 0.0001 and 0.05). These findings suggest that NET stimulation was insufficient to eradicate B. pseudomallei H777 and C17 despite their possession of bpsl0618, a sugar-transferase gene associated with biofilm formation ability. Our findings demonstrate that B. pseudomallei biofilm phenotype may be a key factor in assisting pathogens to escape killing by neutrophils. This work provides a better understanding of how B. pseudomallei biofilm-associated infections induce and survive NET formation, resulting in bacterial persistence and increased severity of disease.

Andrographolide stabilized-silver nanoparticles overcome ceftazidime-resistant Burkholderia pseudomallei: study of antimicrobial activity and mode of action

Burkholderia pseudomallei (B. pseudomallei) is a Gram-negative pathogen that causes melioidosis, a deadly but neglected tropical disease. B. pseudomallei is intrinsically resistant to a growing list of antibiotics, and alternative antimicrobial agents are being sought with urgency. In this study, we synthesize andrographolide-stabilized silver nanoparticles (andro-AgNPs, spherically shaped with 16 nm average diameter) that show excellent antimicrobial activity against B. pseudomallei, including ceftazidime-resistant strains, being 1-3 orders of magnitude more effective than ceftazidime and 1-2 orders of magnitude more effective than other green-synthesized AgNPs. The andro-AgNPs are meanwhile non-toxic to mammalian cell lines. The mode of action of Andro-AgNPs toward B. pseudomallei is unraveled by killing kinetics, membrane neutralization, silver ions (Ag+) release, reactive oxygen species (ROS) induction, membrane integrity, and cell morphology change studies. The antimicrobial activity and mode of action of andro-AgNPs against B. pseudomallei reported here may pave the way to alternative treatments for melioidosis.

Olmesartan Attenuates Single-Lung Ventilation Induced Lung Injury via Regulating Pulmonary Microbiota

Single-lung ventilation (SLV) associated acute lung injury is similar to ischemia reperfusion (IR) injury which is usually occurred during lung surgery. Olmesartan (Olm), a novel angiotensin receptor blocker (ARB), has been reported to ameliorate organ IR injury. Several recent studies have shown that lung microbiota may be involved in pulmonary diseases, but the effect of pulmonary microbiota in SLV-induced lung injury has not been reported. This study aims to determine the mechanism of how Olm attenuates SLV induced lung injury. Our data showed that 7 days Olm treatment before modeling markedly alleviated SLV-induced lung injury by suppressing inflammation and reactive oxygen species. Bronchoalveolar lavage fluid samples from the injured side were collected for 16S rRNA gene-based sequencing analysis and 53 different bacteria at the genus and species levels were identified. Furthermore, the injured lung samples were collected for metabolomics analysis using liquid chromatography-mass spectrometry analyses to explore differential metabolites. The Kyoto Encyclopedia of Genes and Genomes (KEGG) was applied to analyze the correlation between differential metabolites and lung microbiota. A total of 38 pathways were identified according to differential metabolites and 275 relevant pathways were enriched via analyzing the microbial community, 24 pathways were both identified by analyzing either metabolites or microbiota, including pyrimidine metabolism, purine metabolism, aminoacyl-tRNA biosynthesis and ATP-binding cassette transporter. Besides classical blockage of the renin-angiotensin II system, Olm could also alleviate SLV-induced lung injury by rewiring the interaction between pulmonary microbiota and metabolites.

Dual RNA-seq reveals a type 6 secretion system-dependent blockage of TNF-α signaling and BicA as a Burkholderia pseudomallei virulence factor important during gastrointestinal infection

Melioidosis is a disease caused by the Gram-negative bacillus Burkholderia pseudomallei (Bpm), commonly found in soil and water of endemic areas. Naturally acquired human melioidosis infections can result from either exposure through percutaneous inoculation, inhalation, or ingestion of soil-contaminated food or water. Our prior studies recognized Bpm as an effective enteric pathogen, capable of establishing acute or chronic gastrointestinal infections following oral inoculation. However, the specific mechanisms and virulence factors involved in the pathogenesis of Bpm during intestinal infection are unknown. In our current study, we standardized an in vitro intestinal infection model using primary intestinal epithelial cells (IECs) and demonstrated that Bpm requires a functional T6SS for full virulence. Further, we performed dual RNA-seq analysis on Bpm-infected IECs to evaluate differentially expressed host and bacterial genes in the presence or absence of a T6SS. Our results showed a dysregulation in the TNF-α signaling via NF-κB pathway in the absence of the T6SS, with some of the genes involved in inflammatory processes and cell death also affected. Analysis of the bacterial transcriptome identified virulence factors and regulatory proteins playing a role during infection, with association to the T6SS. By using a Bpm transposon mutant library and isogenic mutants, we showed that deletion of the bicA gene, encoding a putative T3SS/T6SS regulator, ablated intracellular survival and plaque formation by Bpm and impacted survival and virulence when using murine models of acute and chronic gastrointestinal infection. Overall, these results highlight the importance of the type 6 secretion system in the gastrointestinal pathogenesis of Bpm.

MicroRNA-146a inhibits autophagy to maintain the intracellular survival of Burkholderia pseudomallei by targeting LIPA

Burkholderia pseudomallei is the etiological agent of melioidosis, which is an emerging infectious disease endemic to many tropical regions. Autophagy is an intrinsic cellular process that degrades cytoplasmic components and plays an important role in protecting the host against pathogens. Like many intracellular pathogens, B. pseudomallei can evade the autophagy-dependent cellular clearance. However, the underlying mechanism remains unclear. In this study, we applied a combination of multiple assays to monitor autophagy processes and found that B. pseudomallei induced an incomplete autophagic flux and eliminate autophagy clearance in macrophages by blocking autophagosome-lysosome fusion. Based on a high-throughput microarray screening, we found that LIPA (lysosomal acid LIPAse A) was downregulated during B. pseudomallei infection. MiR-146a was then identified to be specifically upregulated upon infection with B. pseudomallei and further regulated LIPA expression by interacting with 3'UTR of LIPA. Furthermore, overexpression of miR-146a contributed to the defect of autophagic flux caused by B. pseudomallei and was beneficial for the survival of B. pseudomallei in macrophages. Therefore, our findings suggest that miR-146a inhibits autophagy via posttranscriptional suppression of LIPA expression to maintain B. pseudomallei survival in macrophages.

Transcriptome Analysis Reveals Unfolded Protein Response Was Induced During the Early Stage of Burkholderia pseudomallei Infection in A549 Cells

Burkholderia pseudomallei is a zoonotic pathogen that usually affects patients' lungs and causes serious melioidosis. The interaction of B. pseudomallei with its hosts is complex, and cellular response to B. pseudomallei infection in humans still remains to be elucidated. In this study, transcriptomic profiling of B. pseudomallei-infected human lung epithelial A549 cells was performed to characterize the cellular response dynamics during the early infection (EI) stage. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed by using the online databases DAVID 6.8 and KOBAS 3.0. Real-time quantitative PCR and western blot were used for validation experiments. Compared with the negative control group (NC), a set of 36 common genes varied over time with a cut-off level of 1.5-fold change, and a P-value < 0.05 was identified. Bioinformatics analysis indicated that the PERK-mediated unfolded protein response (UPR) was enriched as the most noteworthy biological process category, which was enriched as a branch of UPR in the signaling pathway of protein processing in the endoplasmic reticulum. Other categories, such as inflammatory responses, cell migration, and apoptosis, were also focused. The molecular chaperone Bip (GRP78), PERK, and PERK sensor-dependent phosphorylation of eIF2α (p-eIF2α) and ATF4 were verified to be increasing over time during the EI stage, suggesting that B. pseudomallei infection activated the PERK-mediated UPR in A549 cells. Collectively, these results provide important initial insights into the intimate interaction between B. pseudomallei and lung epithelial cells, which can be further explored toward the elucidation of the cellular mechanisms of B. pseudomallei infections in humans.

An increase in p62/NBR1 levels in melioidosis patients of Sri Lanka exhibit a characteristic of potential host biomarker

Introduction. Melioidosis, caused by Burkholderia pseudomallei, in endemic areas, poses a challenge for treating the diseased populations without accurate diagnosis, and the disease-specific biomarkers linked with the infection have yet to be reported. Due to the invasive nature of the causative agent, Burkholderia pseudomallei, host innate effector mechanisms, including autophagy are known to be activated, resulting in differential expression of cellular proteins and immune markers. Identification of a disease-specific biomarker associated with B. pseudomallei infection will be helpful to facilitate rapid confirmation of melioidosis, which would enable early treatment and therapeutic success.Aim. We aimed to assess the levels of a host autophagy component, p62/NBR1, which function as a cargo-receptor in the process of autophagy activation leading to the degradation of ubiquitin-coated intracellular bacteria in which p62/NBR1 itself is degraded in the clearance of the pathogen. We further probed the extent of intracellular p62/NBR1 degradation and assessed its potential as a melioidosis biomarker.Methodology. We analysed peripheral blood mononuclear cell (PBMC) lysates using an ELISA-based assay for detecting cytosolic autophagy-related proteins p62/NBR1. We measured p62/NBR1 levels in diseased (confirmed B. pseudomallei infection) and non -diseased populations and utilized receiver operating characteristic (ROC) curve and max Youden index analysis for evaluating potential disease biomarker characteristics.Results. Our results revealed a three to fivefold increase in p62/NBR1 levels confirmed melioidosis cases compared to uninfected healthy donors. Comparable to p62/NBR1, levels of cytosolic LC3-I levels also increased, whereas the levels of degraded membrane bound form LC3-II was low, suggesting autophagy deficiency. Proinflammatory serum cytokine response, particularly IL-6, was consistently higher alongside B. pseudomallei infection in comparison to healthy controls.Conclusions. ROC curve and max Youden index analysis suggest that increased p62/NBR1 levels in diseased populations display characteristics of a potential disease biomarker in melioidosis and illustrates that an elevated p62/NBR1 level, in conjunction with B. pseudomallei infection associated with autophagy deficiency.

Human Melioidosis

The causative agent of melioidosis, Burkholderia pseudomallei, a tier 1 select agent, is endemic in Southeast Asia and northern Australia, with increased incidence associated with high levels of rainfall. Increasing reports of this condition have occurred worldwide, with estimates of up to 165,000 cases and 89,000 deaths per year. The ecological niche of the organism has yet to be clearly defined, although the organism is associated with soil and water. The culture of appropriate clinical material remains the mainstay of laboratory diagnosis. Identification is best done by phenotypic methods, although mass spectrometric methods have been described. Serology has a limited diagnostic role. Direct molecular and antigen detection methods have limited availability and sensitivity. Clinical presentations of melioidosis range from acute bacteremic pneumonia to disseminated visceral abscesses and localized infections. Transmission is by direct inoculation, inhalation, or ingestion. Risk factors for melioidosis include male sex, diabetes mellitus, alcohol abuse, and immunosuppression. The organism is well adapted to intracellular survival, with numerous virulence mechanisms. Immunity likely requires innate and adaptive responses. The principles of management of this condition are drainage and debridement of infected material and appropriate antimicrobial therapy. Global mortality rates vary between 9% and 70%. Research into vaccine development is ongoing.

Immunogenicity and protective efficacy of Burkholderia pseudomallei BLF1-N and BLF1-C terminal domains against BLF1 toxin

Burkholderia lethal factor 1 (BLF1), a glutamine deamidase, is a key virulence factor that plays significant role in B. pseudomallei pathogenesis. To elucidate the BLF1 immunological responses, two truncated BLF1 structural units, BLF1-C (90-211 amino acids) with structural similarity to T. maritima Chemoreceptor glutamine deamidase (CheD) protein, and BLF1-N (1-89 amino acids) disparate to CheD were identified from the 23 kDa BLF1 protein. Both the components were devoid of toxicity in mice and elicited an antibody titer of 1:16,000 that reacted with the respective truncated proteins and BLF1. A549 cell lines supplemented with anti BLF1-N and BLF1-C antibodies exhibited 73.47% and 83.24% survival when treated with BLF1 toxin. Passive i.p. transfer with antibodies elicited by BLF1-C that contained LSGC active site resulted in 80% protection while anti BLF1-N (devoid of LSGC) antibodies provided 51.4% protection, establishing the role of BLF1-N terminal also in deamidase action. The truncated proteins also elicited cell mediated immune responses through proliferation of CD4+ T cells, IFN-γ and IL-4 cytokines but with bias towards Th2 subsets. BLF1-C and BLF1-N immunization resulted in 80% and 60% active protection when challenged with BLF1 toxin while the sham immunized mice exhibited severe histopathological changes like necrosis in liver, lung, spleen and kidney similar to that observed in melioidosis and were killed within 7 days post challenge. The higher level of active and passive protection by BLF1-C protein could be attributed to the comparatively higher level of immune responses and inclusion of LSGC residues.

GC-072: A Novel Therapeutic Candidate for Oral Treatment of Melioidosis and Infections Caused by Select Biothreat Pathogens

Burkholderia pseudomallei (B. pseudomallei), the etiological agent of melioidosis, is a Gram-negative bacterium with additional concern as a biothreat pathogen. The mortality rate from B. pseudomallei varies depending on the type of infection and extent of available health care, but in the case of septicemia left untreated it can range from 50 - 90%. Current therapy for melioidosis is biphasic, consisting of parenteral acute-phase treatment for two weeks or longer, followed by oral eradication-phase treatment lasting several months. An effective oral therapeutic for outpatient treatment of acute-phase melioidosis is needed. GC-072 is a potent, 4-oxoquinolizine antibiotic with selective inhibitory activity against bacterial topoisomerases. GC-072 has demonstrated in vitro potency against susceptible and drug-resistant strains of B. pseudomallei and is also active against Burkholderia mallei, Bacillus anthracis, Yersinia pestis, and Francisella tularensis GC-072 is bactericidal both extra- and intracellularly, with rapid killing noted within a few hours and reduced development of resistance compared to ceftazidime. GC-072, delivered intragastrically to mimic oral administration, promoted dose-dependent survival in mice using lethal inhalational models of B. pseudomallei infection following exposure to a 24 or 339 LD50 challenge with B. pseudomallei strain 1026b. Overall, GC-072 appears to be a strong candidate for first-line, oral treatment of melioidosis.

Evaluating the role of Burkholderia pseudomallei K96243 toxins BPSS0390, BPSS0395, and BPSS1584 in persistent infection

Burkholderia pseudomallei is the causative agent of melioidosis, a disease with a mortality rate of up to 40% even with treatment. Despite the ability of certain antibiotics to control initial infection, relapse occurs in treated patients. The inability of antibiotics to clear this bacterial infection is in part due to persistence, an evasion mechanism against antibiotics and the effect of host defenses. Evaluation of antibiotic efficacy against B. pseudomallei revealed that up to 48% of in vitro grown populations can survive in a persister state. Toxin-antitoxin (TA) systems have been previously implicated in modulating bacterial persistence. We generated three isogenic TA mutants and found that loss of each toxin gene did not alter antibiotic persistence or macrophage survival. In response to macrophage-induced persistence, all three toxin mutants demonstrated increased intracellular susceptibility to levofloxacin which in part was due to the inability of the mutants to induce persistence after nitric oxide or nutrient starvation. In an inhalational model of murine melioidosis, both ΔBPSS0395 and ΔBPSS1584 strains were attenuated, and treatment with levofloxacin led to significant reduction in lung colonisation and reduced splenic colonisation by ΔBPSS0395. Based on our findings, these toxins deserve additional evaluation as putative therapeutic targets.

Molecular epidemiology and antibiotic resistance of Burkholderia pseudomallei isolates from Hainan, China: A STROBE compliant observational study

Burkholderia pseudomallei is the causative agent of meliodosis, and the cases in China are gradually increasing. The present retrospective study aimed to surveil the molecular epidemiological characteristics and antibiotic resistance of B pseudomallei isolates. B pseudomallei strains were isolated and verified from meliodosis patients with relevant epidemiological information from 2004 to 2016 in Hainan, China. Pulsed-field gel electrophoresis based on Spe I digestion was carried out, and antimicrobial resistance of B pseudomallei strains was observed against 9 frequently-used antimicrobials. A total of 164 B pseudomallei isolates were successfully divided into 60 pulsed-field gel electrophoresis (PFGE) patterns, including 33 clusters and 27 single types, at an 85% similarity level. The isolates also exhibited a high level of ceftazidime resistance rate (12.8%, 21/164). B pseudomallei strains were mainly heterogenous with no predominant type, but there were some clonal populations, dominate clusters prevalent and the resistance rates of cephems antimicrobial increased significantly between 2004 and 2016 along with the number of melioidosis cases collected in Hainan (cefoperazone-sulbactam [SCF], rs = 0.96, P = .04; ceftazidime [CAZ], rs = 0.98, P = .01). In conclusion, this study will help to enhance our understanding of molecular characteristics and antibiotic resistance of B pseudomallei.

Living dangerously: Burkholderia pseudomallei modulates phagocyte cell death to survive

Melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, is a major cause of sepsis and mortality in endemic regions of Southeast Asia and Northern Australia. As a facultative intracellular pathogen, B. pseudomallei produces virulence factors to evade innate host response and survive within host cells. Neutrophils and macrophages are phagocytes that play critical roles in host defense against pathogens by their ability to detect and eliminate microbes. Host defense processes against B. pseudomallei including phagocytosis, oxidative burst, autophagy, apoptosis, and proinflammatory cytokine release are all initiated by these two phagocytes in the fight against this bacterium. In vitro studies with mouse macrophage cell lines revealed multiple evasion strategies used by B. pseudomallei to counteract these innate processes. B. pseudomallei invades and replicates in neutrophils but little is known regarding its evasion mechanisms. The bidirectional interaction of neutrophils and macrophages in controlling B. pseudomallei infection has also been overlooked. Here the hypothesis that B. pseudomallei hijacks neutrophils and uses them to transport and infect new phagocytes is proposed as an evasion strategy to survive and persist in host phagocytes. This two-pronged approach by B. pseudomallei to replicate in two different types of phagocytes and to modulate their cell death modes is effective in promoting persistence and survival of the bacterium.

Biofilm and motility in response to environmental and host-related signals in Gram negative opportunistic pathogens

Most bacteria can switch between a planktonic, sometimes motile, form and a biofilm mode, in which bacterial cells can aggregate and attach to a solid surface. The transition between these two forms represents an example of bacterial adaptation to environmental signals and stresses. In 'environmental pathogens', namely, environmental bacteria that are also able to cause disease in animals and humans, signals associated either with the host or with the external environment, such as temperature, oxygen availability, nutrient concentrations etc., play a major role in triggering the switch between the motile and the biofilm mode, via complex regulatory mechanisms that control flagellar synthesis and motility, and production of adhesion factors. In this review article, we present examples of how environmental signals can impact biofilm formation and cell motility in the Gram negative bacteria Pseudomonas aeruginosa, Escherichia coli and in the Burkholderia genus, and how the switch between motile and biofilm mode can be an essential part of a more general process of adaptation either to the host or to the external environment.

Upregulation of Immune Process-Associated Genes in RAW264.7 Macrophage Cells in Response to Burkholderia pseudomallei Infection

Melioidosis is a severe and fatal tropical zoonosis, which is triggered by Burkholderia pseudomallei. To better understand the host's response to infection of B. pseudomallei, an RNA-Seq technology was used to confirm differentially expressed genes (DEGs) in RAW264.7 cells infected with B. pseudomallei. In total, 4668 DEGs were identified across three time points (4, 8, and 11 hours after infection). Short Time-Series Expression Miner (STEM) analysis revealed the temporal gene expression profiles and identified seven significant patterns in a total of 26 profiles. Kyoto Encyclopedia of Genes and Genomes (KEGG) was utilized to confirm significantly enriched immune process-associated pathways, and 10 DEGs, including Ccl9, Ifnb1, Tnfα, Ptgs2, Tnfaip3, Zbp1, Ccl5, Ifi202b, Nfkbia, and Nfkbie, were mapped to eight immune process-associated pathways. Subsequent quantitative real-time PCR assays confirmed that the 10 DEGs were all upregulated during infection. Overall, the results showed that B. pseudomallei infection can initiate a time-series upregulation of immune process-associated DEGs in RAW264.7 macrophage cells. The discovery of this article helps us better understand the biological function of the immune process-associated genes during B. pseudomallei infection and may aid in the development of prophylaxis and treatment protocols for melioidosis.

Caspase-11-dependent pyroptosis of lung epithelial cells protects from melioidosis while caspase-1 mediates macrophage pyroptosis and production of IL-18

Infection with Burkholderia pseudomallei or B. thailandensis triggers activation of the NLRP3 and NLRC4 inflammasomes leading to release of IL-1β and IL-18 and death of infected macrophages by pyroptosis, respectively. The non-canonical inflammasome composed of caspase-11 is also activated by these bacteria and provides protection through induction of pyroptosis. The recent generation of bona fide caspase-1-deficient mice allowed us to reexamine in a mouse model of pneumonic melioidosis the role of caspase-1 independently of caspase-11 (that was also absent in previously generated Casp1^-/- mice). Mice lacking either caspase-1 or caspase-11 were significantly more susceptible than wild type mice to intranasal infection with B. thailandensis. Absence of caspase-1 completely abolished production of IL-1β and IL-18 as well as pyroptosis of infected macrophages. In contrast, in mice lacking caspase-11 IL-1β and IL-18 were produced at normal level and macrophages pyroptosis was only marginally affected. Adoptive transfer of bone marrow indicated that caspase-11 exerted its protective action both in myeloid cells and in radio-resistant cell types. B. thailandensis was shown to readily infect mouse lung epithelial cells triggering pyroptosis in a caspase-11-dependent way in vitro and in vivo. Importantly, we show that lung epithelial cells do not express inflammasomes components or caspase-1 suggesting that this cell type relies exclusively on caspase-11 for undergoing cell death in response to bacterial infection. Finally, we show that IL-18’s protective action in melioidosis was completely dependent on its ability to induce IFNγ production. In turn, protection conferred by IFNγ against melioidosis was dependent on generation of ROS through the NADPH oxidase but independent of induction of caspase-11. Altogether, our results identify two non-redundant protective roles for caspase-1 and caspase-11 in melioidosis: Caspase-1 primarily controls pyroptosis of infected macrophages and production of IL-18. In contrast, caspase-11 mediates pyroptosis of infected lung epithelial cells.

Burkholderia pseudomallei is a bacterium that infect macrophages and other cell types and causes a diseases called melioidosis. Inflammasomes are multiprotein complexes that control activation of the proteases caspase-1 and caspase-11 resulting in production of the inflammatory mediators IL-1β and IL-18 and death of infected cells. Mice deficient of caspase-1 or caspase-11 are more susceptible to infection with B. pseudomallei or the closely related B. thailandensis. Here we show that absence of caspase-1 completely abolished production of IL-1β and IL-18 as well as death of macrophages infected with B. thailandensis. In contrast, in the highly susceptible caspase-11-deficient mice, IL-1β and IL-18 production and macrophages death were not significantly affected. Rather, absence of caspase-11 abolished death of infected lung epithelial cells. Taken together, our results show that caspase-1 and caspase-11 have non-redundant protective roles in melioidosis: Caspase-1 primarily controls cell death of infected macrophages and production of IL-18. In contrast, caspase-11 mediates cell death of infected lung epithelial cells.

Melioidosis

Burkholderia pseudomallei is a Gram-negative environmental bacterium and the aetiological agent of melioidosis, a life-threatening infection that is estimated to account for ∼89,000 deaths per year worldwide. Diabetes mellitus is a major risk factor for melioidosis, and the global diabetes pandemic could increase the number of fatalities caused by melioidosis. Melioidosis is endemic across tropical areas, especially in southeast Asia and northern Australia. Disease manifestations can range from acute septicaemia to chronic infection, as the facultative intracellular lifestyle and virulence factors of B. pseudomallei promote survival and persistence of the pathogen within a broad range of cells, and the bacteria can manipulate the host's immune responses and signalling pathways to escape surveillance. The majority of patients present with sepsis, but specific clinical presentations and their severity vary depending on the route of bacterial entry (skin penetration, inhalation or ingestion), host immune function and bacterial strain and load. Diagnosis is based on clinical and epidemiological features as well as bacterial culture. Treatment requires long-term intravenous and oral antibiotic courses. Delays in treatment due to difficulties in clinical recognition and laboratory diagnosis often lead to poor outcomes and mortality can exceed 40% in some regions. Research into B. pseudomallei is increasing, owing to the biothreat potential of this pathogen and increasing awareness of the disease and its burden; however, better diagnostic tests are needed to improve early confirmation of diagnosis, which would enable better therapeutic efficacy and survival.

Checks and Balances between Autophagy and Inflammasomes during Infection

Autophagy and inflammasome complex assembly are physiological processes that control homeostasis, inflammation, and immunity. Autophagy is a ubiquitous pathway that degrades cytosolic macromolecules or organelles, as well as intracellular pathogens. Inflammasomes are multi-protein complexes that assemble in the cytosol of cells upon detection of pathogen- or danger-associated molecular patterns. A critical outcome of inflammasome assembly is the activation of the cysteine protease caspase-1, which activates the pro-inflammatory cytokine precursors pro-IL-1β and pro-IL-18. Studies on chronic inflammatory diseases, heart diseases, Alzheimer's disease, and multiple sclerosis revealed that autophagy and inflammasomes intersect and regulate each other. In the context of infectious diseases, however, less is known about the interplay between autophagy and inflammasome assembly, although it is becoming evident that pathogens have evolved multiple strategies to inhibit and/or subvert these pathways and to take advantage of their intricate crosstalk. An improved appreciation of these pathways and their subversion by diverse pathogens is expected to help in the design of anti-infective therapeutic interventions.

Burkholderia pseudomallei Evades Nramp1 (Slc11a1)- and NADPH Oxidase-Mediated Killing in Macrophages and Exhibits Nramp1-Dependent Virulence Gene Expression

Bacterial survival in macrophages can be affected by the natural resistance-associated macrophage protein 1 (Nramp1; also known as solute carrier family 11 member a1 or Slc11a1) which localizes to phagosome membranes and transports divalent cations, including iron. Little is known about the role of Nramp1 in Burkholderia infection, in particular whether this differs for pathogenic species like Burkholderia pseudomallei causing melioidosis or non-pathogenic species like Burkholderia thailandensis. Here we show that transfected macrophages stably expressing wild-type Nramp1 (Nramp1⁺) control the net replication of B. thailandensis, but not B. pseudomallei. Control of B. thailandensis was associated with increased cytokine responses, and could be abrogated by blocking NADPH oxidase-mediated production of reactive oxygen species but not by blocking generation of reactive nitrogen species. The inability of Nramp1⁺ macrophages to control B. pseudomallei was associated with rapid escape of bacteria from phagosomes, as indicated by decreased co-localization with LAMP1 compared to B. thailandensis. A B. pseudomallei bipB mutant impaired in escape from phagosomes was controlled to a greater extent than the parent strain in Nramp1⁺ macrophages, but was also attenuated in Nramp1⁻ cells. Consistent with reduced escape from phagosomes, B. thailandensis formed fewer multinucleated giant cells in Nramp1⁺ macrophages at later time points compared to B. pseudomallei. B. pseudomallei exhibited elevated transcription of virulence-associated genes of Type VI Secretion System cluster 1 (T6SS-1), the Bsa Type III Secretion System (T3SS-3) and the bimA gene required for actin-based motility in Nramp1⁺ macrophages. Nramp1⁺ macrophages were found to contain decreased iron levels that may impact on expression of such genes. Our data show that B. pseudomallei is able to evade Nramp1- and NADPH oxidase-mediated killing in macrophages and that expression of virulence-associated genes by pathogenic B pseudomallei is enhanced in macrophages expressing wild-type compared to non-functional Nramp1. B. thailandensis has been proposed as surrogate for B. pseudomallei in the study of melioidosis however our study highlights important differences in the interaction of these bacteria with macrophages.

Melioidosis: Clinical impact and public health threat in the tropics

This review briefly summarizes the geographical distribution and clinical impact of melioidosis, especially in the tropics. Burkholderia pseudomallei (a gram-negative bacterium) is the major causative agent for melioidosis, which is prevalent in Singapore, Malaysia, Thailand, Vietnam, and Northern Australia. Melioidosis patients are increasingly being recognized in other parts of the world. The bacteria are intrinsically resistant to many antimicrobial agents, but prolonged treatment, especially with combinations of antibiotics, may be effective. Despite therapy, the overall case fatality rate of septicemia in melioidosis remains significantly high. Intracellular survival of the bacteria within macrophages may progress to chronic infections, and about 10% of patients suffer relapses. In the coming decades, melioidosis will increasingly afflict travelers throughout many global regions. Clinicians managing travelers returning from the subtropics or tropics with severe pneumonia or septicemia should consider acute melioidosis as a differential diagnosis. Patients with open skin wounds, diabetes, or chronic renal disease are at higher risk for melioidosis and should avoid direct contact with soil and standing water in endemic regions. Furthermore, there are fears that B. pseudomallei may be used as a biological weapon. Technological advancements in molecular diagnostics and antibiotic therapy are improving the disease outcomes in endemic areas throughout Asia. Research and development efforts on vaccine candidates against melioidosis are ongoing.

A predicted cation transporter protein, BPSS1228, is involved in intracellular behaviour of Burkholderia pseudomallei in a human lung epithelial cell line (A549)

Burkholderia pseudomallei causes melioidosis, a potentially fatal infectious disease in tropical and subtropical countries worldwide. The intracellular behaviour of this pathogen in host cells has been reported to impact the severity of melioidosis, including the development of septicaemia, a consequence of pneumonia melioidosis. We previously identified a predicted cation transporter protein, BPSS1228, that participates in the transitional stage of this intracellular pathogen. For further analysis, in this study B. pseudomallei bpss1228 mutant and complemented strains were constructed and bacterial infectivity on human lung epithelial cells, A549, investigated in vitro Burkholderia pseudomallei bpss1228 mutant showed impaired bacterial adhesion and invasion into A549 cells compared with wild-type strain, while the deficient phenotypes were restored to wild-type levels by the complemented strain. Additionally, the inactivation of bpss1228 in the mutant strain affected flagella-based swimming on a semi-solid surface and resistance to acid stresses simulating intracellular environments. These observations of BPSS1228 relating to B. pseudomallei infection strategies shed a new light on its association with intracellular B. pseudomallei during the interaction with host cells.

Eukaryotic pathways targeted by the type III secretion system effector protein, BipC, involved in the intracellular lifecycle of Burkholderia pseudomallei

Burkholderia pseudomallei, the etiological agent for melioidosis, is known to secrete a type III secretion system (TTSS) protein into the host’s internal milieu. One of the TTSS effector protein, BipC, has been shown to play an important role in the B. pseudomallei pathogenesis. To identify the host response profile that was directly or indirectly regulated by this protein, genome-wide transcriptome approach was used to examine the gene expression profiles of infected mice. The transcriptome analysis of the liver and spleen revealed that a total of approximately 1,000 genes were transcriptionally affected by BipC. Genes involved in bacterial invasion, regulation of actin cytoskeleton, and MAPK signalling pathway were over-expressed and may be specifically regulated by BipC in vivo. These results suggest that BipC mainly targets pathways related to the cellular processes which could modulate the cellular trafficking processes. The host transcriptional response exhibited remarkable differences with and without the presence of the BipC protein. Overall, the detailed picture of this study provides new insights that BipC may have evolved to efficiently manipulate host-cell pathways which is crucial in the intracellular lifecycle of B. pseudomallei.

Unraveling the B. pseudomallei Heptokinase WcbL: From Structure to Drug Discovery

Gram-negative bacteria utilize heptoses as part of their repertoire of extracellular polysaccharide virulence determinants. Disruption of heptose biosynthesis offers an attractive target for novel antimicrobials. A critical step in the synthesis of heptoses is their 1-O phosphorylation, mediated by kinases such as HldE or WcbL. Here, we present the structure of WcbL from Burkholderia pseudomallei. We report that WcbL operates through a sequential ordered Bi-Bi mechanism, loading the heptose first and then ATP. We show that dimeric WcbL binds ATP anti-cooperatively in the absence of heptose, and cooperatively in its presence. Modeling of WcbL suggests that heptose binding causes an elegant switch in the hydrogen-bonding network, facilitating the binding of a second ATP molecule. Finally, we screened a library of drug-like fragments, identifying hits that potently inhibit WcbL. Our results provide a novel mechanism for control of substrate binding and emphasize WcbL as an attractive anti-microbial target for Gram-negative bacteria.

Functional Diversity of Cytotoxic tRNase/Immunity Protein Complexes from Burkholderia pseudomallei

Contact-dependent growth inhibition (CDI) is a widespread mechanism of inter-bacterial competition. CDI(+) bacteria deploy large CdiA effector proteins, which carry variable C-terminal toxin domains (CdiA-CT). CDI(+) cells also produce CdiI immunity proteins that specifically neutralize cognate CdiA-CT toxins to prevent auto-inhibition. Here, we present the crystal structure of the CdiA-CT/CdiI(E479) toxin/immunity protein complex from Burkholderia pseudomallei isolate E479. The CdiA-CT(E479) tRNase domain contains a core α/β-fold that is characteristic of PD(D/E)XK superfamily nucleases. Unexpectedly, the closest structural homolog of CdiA-CT(E479) is another CDI toxin domain from B. pseudomallei 1026b. Although unrelated in sequence, the two B. pseudomallei nuclease domains share similar folds and active-site architectures. By contrast, the CdiI(E479) and CdiI(1026b) immunity proteins share no significant sequence or structural homology. CdiA-CT(E479) and CdiA-CT(1026b) are both tRNases; however, each nuclease cleaves tRNA at a distinct position. We used a molecular docking approach to model each toxin bound to tRNA substrate. The resulting models fit into electron density envelopes generated by small-angle x-ray scattering analysis of catalytically inactive toxin domains bound stably to tRNA. CdiA-CT(E479) is the third CDI toxin found to have structural homology to the PD(D/E)XK superfamily. We propose that CDI systems exploit the inherent sequence variability and active-site plasticity of PD(D/E)XK nucleases to generate toxin diversity. These findings raise the possibility that many other uncharacterized CDI toxins may belong to the PD(D/E)XK superfamily.

Burkholderia pseudomallei Differentially Regulates Host Innate Immune Response Genes for Intracellular Survival in Lung Epithelial Cells

Background

Burkholderia pseudomallei, the causative agent of melioidosis poses a serious threat to humankind. B. pseudomallei secretes numerous virulence proteins that alter host cell functions to escape from intracellular immune sensors. However, the events underlying disease pathogenesis are poorly understood.

Methods

We determined the ability of B. pseudomallei to invade and survive intracellularly in A549 human lung epithelial cells, and also investigated the early transcriptional responses using an Illumina HumanHT-12 v4 microarray platform, after three hours of exposure to live B. pseudomallei (BCMS) and its secreted proteins (CCMS).

Results

We found that the ability of B. pseudomallei to invade and survive intracellularly correlated with increase of multiplicity of infection and duration of contact. Activation of host carbohydrate metabolism and apoptosis as well as suppression of amino acid metabolism and innate immune responses both by live bacteria and its secreted proteins were evident. These early events might be linked to initial activation of host genes directed towards bacterial dissemination from lungs to target organs (via proposed in vivo mechanisms) or to escape potential sensing by macrophages.

Conclusion

Understanding the early responses of A549 cells toward B. pseudomallei infection provide preliminary insights into the likely pathogenesis mechanisms underlying melioidosis, and could contribute to development of novel intervention strategies to combat B. pseudomallei infections.

Burkholderia pseudomallei, the causative agent of the fatal infectious disease melioidosis, is endemic across parts of South East Asia and Northern Australia. Melioidosis poses a serious worldwide emerging infectious disease problem and bioterrorism threat. Of the key features of B. pseudomallei, is its ability to remain latent in the host causing recrudescent disease years after initial infection. Relapses are also commonly reported despite appropriate and prolonged antibiotic therapy, suggesting the bacteria’s ability to escape the host’s front-line immune defenses and to manipulate the host’s responses to sustain survival in the host. However, the likely underlying mechanisms of bacterial persistence still remain unclear. Thus, here we proposed to study the host responses towards early interaction of the cell with live B. pseudomallei and its secretory proteins, in order to understand the potential roles of innate responses against the bacteria.

Successful treatment outcome of primary melioidosis pneumonia-a case report from Bangladesh

Background

Melioidosis is endemic in tropical Australia and Southeast Asian countries and its causative organism Burkholderia pseudomallei is a recognized cause of pneumonia in these regions. Recent isolation of the organism in the soil of Kapasia, Gazipur, Bangladesh has proven its exposure among the population residing in endemic areas of our country. Pneumonia is the most common presentation of melioidosis. Acute, subacute and chronic pneumonia due to B. pseudomallei can present as primary or secondary pneumonia. Treatment of such cases are challenging as well. Till date, few cases of acute and chronic pneumonia due to melioidosis occurring in local Bangladeshis as well as in returning travelers to Europe have been reported. To the best of our knowledge, this is the first reported case of primary melioidosis pneumonia declared cured after a 27 weeks of treatment regimen from Bangladesh.

Case presentation

A 43-year-old Bangladeshi gentleman, known diabetic, hypertensive, smoker, presented with the complaints of recurrent episodes of low to high grade intermittent fever, productive cough with occasional haemoptysis and 10 kg weight loss over one and half months. Poorly responding to conventional antibiotics, he was suspected as a case of pulmonary tuberculosis. Examination and investigations revealed left sided consolidation with cavitary lesion, hepato-splenomegaly and sputum analysis confirmed growth of Burkholderia pseudomallei. The patient was successfully treated as a case of primary melioidosis pneumonia.

Conclusion

Often misdiagnosed and empirically treated as tuberculosis, untreated melioidosis pneumonia may even lead to death. Therefore, melioidosis should be suspected in appropriate clinical scenario in patients with a history of residing in or traveling to endemic areas. In Bangladesh, time has come to explore whether melioidosis should be considered as an emerging infectious disease.

Role of Canonical and Non-canonical Inflammasomes During Burkholderia Infection

Burkholderia pseudomallei is a Gram-negative flagellate bacterium that causes melioidosis, a disease endemic to Southeast Asia and other tropical regions. Following infection of macrophages and other non-phagocytic cell types, B. pseudomallei or B. thailandensis (a related species that causes disease in mice but not humans) are able to escape the phagosome and replicate in the host cell cytoplasm. Resistance to infection with Burkholderia is dependent on the Nlrp3 and Nlrc4 inflammasomes and the non-canonical caspase-11 inflammasome. Nlrc4 mediates protection through induction of pyroptosis in the early phase of infection. As the infection progresses and as IL-18-dependent IFNγ production increases, caspase-11-dependent pyroptosis acquires a preponderant protective role. Production of IL-1β and IL-18 during infection is primarily mediated by Nlrp3. IL-18 is essential for survival because of its ability to induce IFNγ production, which in turn activates macrophage microbicidal functions and primes for caspase-11 expression. In contrast, during melioidosis, IL-1β has deleterious effects due to excessive recruitment of neutrophils to the lung and consequent tissue damage.

Utilization of Whole-Cell MALDI-TOF Mass Spectrometry to Differentiate Burkholderia pseudomallei Wild-Type and Constructed Mutants

Whole-cell matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (whole-cell MALDI-TOF MS) has been widely adopted as a useful technology in the identification and typing of microorganisms. This study employed the whole-cell MALDI-TOF MS to identify and differentiate wild-type and mutants containing constructed single gene mutations of Burkholderia pseudomallei, a pathogenic bacterium causing melioidosis disease in both humans and animals. Candidate biomarkers for the B. pseudomallei mutants, including rpoS, ppk, and bpsI isolates, were determined. Taxon-specific and clinical isolate-specific biomarkers of B. pseudomallei were consistently found and conserved across all average mass spectra. Cluster analysis of MALDI spectra of all isolates exhibited separate distribution. A total of twelve potential mass peaks discriminating between wild-type and mutant isolates were identified using ClinProTools analysis. Two peaks (m/z 2721 and 2748 Da) were specific for the rpoS isolate, three (m/z 3150, 3378, and 7994 Da) for ppk, and seven (m/z 3420, 3520, 3587, 3688, 4623, 4708, and 5450 Da) for bpsI. Our findings demonstrated that the rapid, accurate, and reproducible mass profiling technology could have new implications in laboratory-based rapid differentiation of extensive libraries of genetically altered bacteria.

The Burkholderia pseudomallei Proteins BapA and BapC Are Secreted TTSS3 Effectors and BapB Levels Modulate Expression of BopE

Many Gram-negative pathogens use a type III secretion system (TTSS) for the injection of bacterial effector proteins into host cells. The injected effector proteins play direct roles in modulation of host cell pathways for bacterial benefit. Burkholderia pseudomallei, the causative agent of melioidosis, expresses three different TTSSs. One of these systems, the TTSS3, is essential for escape from host endosomes and therefore intracellular survival and replication. Here we have characterized three putative TTSS3 proteins; namely BapA, BapB and BapC. By employing a tetracysteine (TC)-FlAsH^™ labelling technique to monitor the secretion of TC-tagged fusion proteins, BapA and BapC were shown to be secreted during in vitro growth in a TTSS3-dependant manner, suggesting a role as TTSS3 effectors. Furthermore, we constructed B. pseudomallei bapA, bapB and bapC mutants and used the well-characterized TTSS3 effector BopE as a marker of secretion to show that BapA, BapB and BapC are not essential for the secretion process. However, BopE transcription and secretion were significantly increased in the bapB mutant, suggesting that BapB levels modulate BopE expression. In a BALB/c mouse model of acute melioidosis, the bapA, bapB and bapC mutants showed a minor reduction of in vivo fitness. Thus, this study defines BapA and BapC as novel TTSS3 effectors, BapB as a regulator of BopE production, and all three as necessary for full B. pseudomallei in vivo fitness.

Diverse Burkholderia Species Isolated from Soils in the Southern United States with No Evidence of B. pseudomallei

The global distribution of the soil-dwelling bacterium Burkholderia pseudomallei, causative agent of melioidosis, is poorly understood. We used established culturing methods developed for B. pseudomallei to isolate Burkholderia species from soil collected at 18 sampling sites in three states in the southern United States (Arizona (n = 4), Florida (n = 7), and Louisiana (n = 7)). Using multi-locus sequence typing (MLST) of seven genes, we identified 35 Burkholderia isolates from these soil samples. All species belonged to the B. cepacia complex (Bcc), including B. cenocepacia, B. cepacia, B. contaminans, B. diffusa, B. metallica, B. seminalis, B. vietnamiensis and two unnamed members of the Bcc. The MLST analysis provided a high level of resolution among and within these species. Despite previous clinical cases within the U.S. involving B. pseudomallei and its close phylogenetic relatives, we did not isolate any of these taxa. The Bcc contains a number of opportunistic pathogens that cause infections in cystic fibrosis patients. Interestingly, we found that B. vietnamiensis was present in soil from all three states, suggesting it may be a common component in southern U.S. soils. Most of the Burkholderia isolates collected in this study were from Florida (30/35; 86%), which may be due to the combination of relatively moist, sandy, and acidic soils found there compared to the other two states. We also investigated one MLST gene, recA, for its ability to identify species within Burkholderia. A 365bp fragment of recA recovered nearly the same species-level identification as MLST, thus demonstrating its cost effective utility when conducting environmental surveys for Burkholderia. Although we did not find B. pseudomallei, our findings document that other diverse Burkholderia species are present in soils in the southern United States.

Comprehensive identification of virulence factors required for respiratory melioidosis using Tn-seq mutagenesis

Respiratory melioidosis is a disease presentation of the biodefense pathogen, Burkholderia pseudomallei, which is frequently associated with a lethal septicemic spread of the bacteria. We have recently developed an improved respiratory melioidosis model to study the pathogenesis of Burkholderia pseudomallei in the lung (intubation-mediated intratracheal [IMIT] inoculation), which more closely models descriptions of human melioidosis, including prominent septicemic spread from the lung and reduced involvement of the upper respiratory tract. We previously demonstrated that the Type 3 Secretion System cluster 3 (T3SS3) is a critical virulence determinant for B. pseudomallei when delivered directly into the lung. We decided to comprehensively identify all virulence determinants required for respiratory melioidosis using the Tn-seq phenotypic screen, as well as to investigate which virulence determinants are required for dissemination to the liver and spleen. While previous studies have used Tn-seq to identify essential genes for in vitro cultured B. pseudomallei, this represents the first study to use Tn-seq to identify genes required for in vivo fitness. Consistent with our previous findings, we identified T3SS3 as the largest genetic cluster required for fitness in the lung. Furthermore, we identified capsular polysaccharide and Type 6 Secretion System cluster 5 (T6SS5) as the two additional major genetic clusters facilitating respiratory melioidosis. Importantly, Tn-seq did not identify additional, novel large genetic systems supporting respiratory melioidosis, although these studies identified additional small gene clusters that may also play crucial roles in lung fitness. Interestingly, other previously identified virulence determinants do not appear to be required for lung fitness, such as lipopolysaccharide. The role of T3SS3, capsule, and T6SS5 in lung fitness was validated by competition studies, but only T3SS3 was found to be important for respiratory melioidosis when delivered as a single strain challenge, suggesting that competition studies may provide a higher resolution analysis of fitness factors in the lung. The use of Tn-seq phenotypic screening also provided key insights into the selective pressure encountered in the liver.

Applications of in vivo imaging in the evaluation of the pathophysiology of viral and bacterial infections and in development of countermeasures to BSL3/4 pathogens

While preclinical and clinical imaging have been applied to drug discovery/development and characterization of disease pathology, few examples exist where imaging has been used to evaluate infectious agents or countermeasures to biosafety level (BSL)3/4 threat agents. Viruses engineered with reporter constructs, i.e., enzymes and receptors, which are amenable to detection by positron emission tomography (PET), single photon emission tomography (SPECT), or magnetic resonance imaging (MRI) have been used to evaluate the biodistribution of viruses containing specific therapeutic or gene transfer payloads. Bioluminescence and nuclear approaches involving engineered reporters, direct labeling of bacteria with radiotracers, or tracking bacteria through their constitutively expressed thymidine kinase have been utilized to characterize viral and bacterial pathogens post-infection. Most PET, SPECT, CT, or MRI approaches have focused on evaluating host responses to the pathogens such as inflammation, brain neurochemistry, and structural changes and on assessing the biodistribution of radiolabeled drugs. Imaging has the potential when applied preclinically to the development of countermeasures against BSL3/4 threat agents to address the following: (1) presence, biodistribution, and time course of infection in the presence or absence of drug; (2) binding of the therapeutic to the target; and (3) expression of a pharmacologic effect either related to drug mechanism, efficacy, or safety. Preclinical imaging could potentially provide real-time dynamic tools to characterize the pathogen and animal model and for developing countermeasures under the U.S. FDA Animal Rule provision with high confidence of success and clinical benefit.

A Unique Set of the Burkholderia Collagen-Like Proteins Provides Insight into Pathogenesis, Genome Evolution and Niche Adaptation, and Infection Detection

Burkholderia pseudomallei and Burkholderia mallei, classified as category B priority pathogens, are significant human and animal pathogens that are highly infectious and broad-spectrum antibiotic resistant. Currently, the pathogenicity mechanisms utilized by Burkholderia are not fully understood, and correct diagnosis of B. pseudomallei and B. mallei infection remains a challenge due to limited detection methods. Here, we provide a comprehensive analysis of a set of 13 novel Burkholderia collagen-like proteins (Bucl) that were identified among B. pseudomallei and B. mallei select agents. We infer that several Bucl proteins participate in pathogenesis based on their noncollagenous domains that are associated with the components of a type III secretion apparatus and membrane transport systems. Homology modeling of the outer membrane efflux domain of Bucl8 points to a role in multi-drug resistance. We determined that bucl genes are widespread in B. pseudomallei and B. mallei; Fischer’s exact test and Cramer’s V² values indicate that the majority of bucl genes are highly associated with these pathogenic species versus nonpathogenic B. thailandensis. We designed a bucl-based quantitative PCR assay which was able to detect B. pseudomallei infection in a mouse with a detection limit of 50 CFU. Finally, chromosomal mapping and phylogenetic analysis of bucl loci revealed considerable genomic plasticity and adaptation of Burkholderia spp. to host and environmental niches. In this study, we identified a large set of phylogenetically unrelated bucl genes commonly found in Burkholderia select agents, encoding predicted pathogenicity factors, detection targets, and vaccine candidates.

Complete genome sequences for 59 burkholderia isolates, both pathogenic and near neighbor

The genus Burkholderia encompasses both pathogenic (including Burkholderia mallei and Burkholderia pseudomallei, U.S. Centers for Disease Control and Prevention Category B listed), and nonpathogenic Gram-negative bacilli. Here we present full genome sequences for a panel of 59 Burkholderia strains, selected to aid in detection assay development.

Burkholderia pseudomallei type III secretion system cluster 3 ATPase BsaS, a chemotherapeutic target for small-molecule ATPase inhibitors

Melioidosis is an infectious disease of high mortality for humans and other animal species; it is prevalent in tropical regions worldwide. The pathogenesis of melioidosis depends on the ability of its causative agent, the Gram-negative bacterium Burkholderia pseudomallei, to enter and survive in host cells. B. pseudomallei can escape from the phagosome into the cytosol of phagocytic cells where it replicates and acquires actin-mediated motility, avoiding killing by the autophagy-dependent process, LC3 (microtubule-associated protein light chain 3)-associated phagocytosis (LAP). The type III secretion system cluster 3 (TTSS3) facilitates bacterial escape from phagosomes, although the mechanism has not been fully elucidated. Given the recent identification of small-molecule inhibitors of the TTSS ATPase, we sought to determine the potential of the predicted TTSS3 ATPase, encoded by bsaS, as a target for chemotherapeutic treatment of infection. A B. pseudomallei bsaS deletion mutant was generated and used as a control against which to assess the effect of inhibitor treatment. Infection of RAW 264.7 cells with wild-type bacteria and subsequent treatment with the ATPase inhibitor compound 939 resulted in reduced intracellular bacterial survival, reduced escape from phagosomes, and increased colocalization with both LC3 and the lysosomal marker LAMP1 (lysosome-associated membrane protein 1). These changes were similar to those observed for infection of RAW 264.7 cells with the bsaS deletion mutant. We propose that treatment with the ATPase inhibitor compound 939 decreased intracellular bacterial survival through a reduced ability of bacteria to escape from phagosomes and increased killing via LAP. Therefore, small-molecule inhibitors of the TTSS3 ATPase have potential as therapeutic treatments against melioidosis.

Systematic mutagenesis of genes encoding predicted autotransported proteins of Burkholderia pseudomallei identifies factors mediating virulence in mice, net intracellular replication and a novel protein conferring serum resistance

Burkholderia pseudomallei is the causative agent of the severe tropical disease melioidosis, which commonly presents as sepsis. The B. pseudomallei K96243 genome encodes eleven predicted autotransporters, a diverse family of secreted and outer membrane proteins often associated with virulence. In a systematic study of these autotransporters, we constructed insertion mutants in each gene predicted to encode an autotransporter and assessed them for three pathogenesis-associated phenotypes: virulence in the BALB/c intra-peritoneal mouse melioidosis model, net intracellular replication in J774.2 murine macrophage-like cells and survival in 45% (v/v) normal human serum. From the complete repertoire of eleven autotransporter mutants, we identified eight mutants which exhibited an increase in median lethal dose of 1 to 2-log10 compared to the isogenic parent strain (bcaA, boaA, boaB, bpaA, bpaC, bpaE, bpaF and bimA). Four mutants, all demonstrating attenuation for virulence, exhibited reduced net intracellular replication in J774.2 macrophage-like cells (bimA, boaB, bpaC and bpaE). A single mutant (bpaC) was identified that exhibited significantly reduced serum survival compared to wild-type. The bpaC mutant, which demonstrated attenuation for virulence and net intracellular replication, was sensitive to complement-mediated killing via the classical and/or lectin pathway. Serum resistance was rescued by in trans complementation. Subsequently, we expressed recombinant proteins of the passenger domain of four predicted autotransporters representing each of the phenotypic groups identified: those attenuated for virulence (BcaA), those attenuated for virulence and net intracellular replication (BpaE), the BpaC mutant with defects in virulence, net intracellular replication and serum resistance and those displaying wild-type phenotypes (BatA). Only BcaA and BpaE elicited a strong IFN-γ response in a restimulation assay using whole blood from seropositive donors and were recognised by seropositive human sera from the endemic area. To conclude, several predicted autotransporters contribute to B. pseudomallei virulence and BpaC may do so by conferring resistance against complement-mediated killing.

Genomic characterization of Burkholderia pseudomallei isolates selected for medical countermeasures testing: comparative genomics associated with differential virulence

Burkholderia pseudomallei is the causative agent of melioidosis and a potential bioterrorism agent. In the development of medical countermeasures against B. pseudomallei infection, the US Food and Drug Administration (FDA) animal Rule recommends using well-characterized strains in animal challenge studies. In this study, whole genome sequence data were generated for 6 B. pseudomallei isolates previously identified as candidates for animal challenge studies; an additional 5 isolates were sequenced that were associated with human inhalational melioidosis. A core genome single nucleotide polymorphism (SNP) phylogeny inferred from a concatenated SNP alignment from the 11 isolates sequenced in this study and a diverse global collection of isolates demonstrated the diversity of the proposed Animal Rule isolates. To understand the genomic composition of each isolate, a large-scale blast score ratio (LS-BSR) analysis was performed on the entire pan-genome; this demonstrated the variable composition of genes across the panel and also helped to identify genes unique to individual isolates. In addition, a set of ~550 genes associated with pathogenesis in B. pseudomallei were screened against the 11 sequenced genomes with LS-BSR. Differential gene distribution for 54 virulence-associated genes was observed between genomes and three of these genes were correlated with differential virulence observed in animal challenge studies using BALB/c mice. Differentially conserved genes and SNPs associated with disease severity were identified and could be the basis for future studies investigating the pathogenesis of B. pseudomallei. Overall, the genetic characterization of the 11 proposed Animal Rule isolates provides context for future studies involving B. pseudomallei pathogenesis, differential virulence, and efficacy to therapeutics.

Transcription level analysis of intracellular Burkholderia pseudomallei illustrates the role of BPSL1502 during bacterial interaction with human lung epithelial cells

Melioidosis caused by Burkholderia pseudomallei is a globally important disease of increasing concern according to high case-fatality rate and epidemic spreading. The ability of B. pseudomallei to attach and invade host cells and subsequently survive intracellularly has stimulated many questions concerning the comprehension of bacterial pathogenesis progression. Transcription levels of intracellular B. pseudomallei genes in human lung epithelial cells were therefore analyzed using bioinformatic tools, RT-PCR and real time RT-PCR. Here, it is reported that the identification of bpsl1502, encoding B. pseudomallei SurE (stationary phase survival protein E) located in a global transcriptional regulation operon was accomplished. The up-regulation of B. pseudomallei SurE was demonstrated during intracellular survival of A549 cells at 12, 18, and 24 h post-infection. To investigate the role of this protein, a B. pseudomallei SurE defective mutant was constructed. The invasion and initial survival of the SurE mutants within the A549 cells were impaired. There was no difference, however, between the growth of B. pseudomallei SurE mutant as compared to the wild type in Luria-Bertani culture. These data suggest that SurE may assist B. pseudomallei host cells invade and facilitate early intracellular infection but is not crucial during the stationary growth phase. The identification of B. pseudomallei SurE provides more information of bacterial strategy during an early step of the pathogenesis process of melioidosis.

CD4+ T cell epitopes of FliC conserved between strains of Burkholderia: implications for vaccines against melioidosis and cepacia complex in cystic fibrosis

Burkholderia pseudomallei is the causative agent of melioidosis characterized by pneumonia and fatal septicemia and prevalent in Southeast Asia. Related Burkholderia species are strong risk factors of mortality in cystic fibrosis (CF). The B. pseudomallei flagellar protein FliC is strongly seroreactive and vaccination protects challenged mice. We assessed B. pseudomallei FliC peptide binding affinity to multiple HLA class II alleles and then assessed CD4 T cell immunity in HLA class II transgenic mice and in seropositive individuals in Thailand. T cell hybridomas were generated to investigate cross-reactivity between B. pseudomallei and the related Burkholderia species associated with Cepacia Complex CF. B. pseudomallei FliC contained several peptide sequences with ability to bind multiple HLA class II alleles. Several peptides were shown to encompass strong CD4 T cell epitopes in B. pseudomallei-exposed individuals and in HLA transgenic mice. In particular, the p38 epitope is robustly recognized by CD4 T cells of seropositive donors across diverse HLA haplotypes. T cell hybridomas against an immunogenic B. pseudomallei FliC epitope also cross-reacted with orthologous FliC sequences from Burkholderia multivorans and Burkholderia cenocepacia, important pathogens in CF. Epitopes within FliC were accessible for processing and presentation from live or heat-killed bacteria, demonstrating that flagellin enters the HLA class II Ag presentation pathway during infection of macrophages with B. cenocepacia. Collectively, the data support the possibility of incorporating FliC T cell epitopes into vaccination programs targeting both at-risk individuals in B. pseudomallei endemic regions as well as CF patients.

A polar-localized iron-binding protein determines the polar targeting of Burkholderia BimA autotransporter and actin tail formation

Intracellular bacterial pathogens including Shigella, Listeria, Mycobacteria, Rickettsia and Burkholderia spp. deploy a specialized surface protein onto one pole of the bacteria to induce filamentous actin tail formation for directional movement within host cytosol. The mechanism underlying polar targeting of the actin tail proteins is unknown. Here we perform a transposon screen in Burkholderia thailandensis and identify a conserved bimC that is required for actin tail formation mediated by BimA from B. thailandensis and its closely related pathogenic species B. pseudomallei and B. mallei. bimC is located upstream of bimA in the same operon. Loss of bimC results in even distribution of BimA on the outer membrane surface, where actin polymerization still occurs. BimC is targeted to the same bacterial pole independently of BimA. BimC confers polar targeting of BimA prior to BimA translocation across bacterial inner membrane. BimC is an iron-binding protein, requiring a four-cysteine cluster at the carboxyl terminus. Mutation of the cysteine cluster disrupts BimC polar localization. Truncation analyses identify the transmembrane domain in BimA being responsible for its polar targeting. Consistently, BimC can interact with BimA transmembrane domain in an iron binding-dependent manner. Our study uncovers a new mechanism that determines the polar distribution of bacteria-induced actin tail in infected host cells.

Antimicrobial susceptibility and genetic characterisation of Burkholderia pseudomallei isolated from Malaysian patients

Burkholderia pseudomallei, the causative agent of melioidosis, is intrinsically resistant to many antibiotics. Ceftazidime (CAZ), the synthetic β-lactam, is normally used as the first-line antibiotic therapy for treatment of melioidosis. However, acquired CAZ resistance can develop in vivo during treatment with CAZ, leading to mortality if therapy is not switched to a different antibiotic(s) in a timely manner. In this study, susceptibilities of 81 B. pseudomallei isolates to nine different antimicrobial agents were determined using the disk diffusion method, broth microdilution test and Etest. Highest percentage of susceptibility was demonstrated to CAZ, amoxicillin/clavulanic acid, meropenem, imipenem, and trimethoprim/sulfamethoxazole. Although these drugs demonstrated the highest percentage of susceptibility in B. pseudomallei, the overall results underline the importance of the emergence of resistance in this organism. PCR results showed that, of the 81 B. pseudomallei, six multidrug resistant (MDR) isolates carried bpeB, amrB, and BPSS1119 and penA genes. Genotyping of the isolates using random amplified polymorphic DNA analysis showed six different PCR fingerprinting patterns generated from the six MDR isolates clusters (A) and eight PCR fingerprinting patterns generated for the remaining 75 non-MDR isolates clusters (B).

Characterization of BPSS1521 (bprD), a regulator of Burkholderia pseudomallei virulence gene expression in the mouse model

The Gram-negative saprophytic bacterium Burkholderia pseudomallei is the causative agent of melioidosis, a severe infectious disease of both humans and animals. Severity of the disease is thought to be dependent on both the health status of the host, including diabetes mellitus and kidney disease, and bacterial-derived factors. To identify the bacterial factors important during an acute infection, gene expression profiles in the spleen, lung, and liver of BALB/c (Th2 prototype) and C57BL/6 mice (Th1 prototype) were determined using DNA microarrays. This analysis identified BPSS1521 (bprD), a predicted transcriptional regulator located in the type III secretion system (T3SS-3) operon, to be up regulated, specifically in C57BL/6 mice. BALB/c mice infected with a bprD mutant showed a shorter time to death and increased inflammation, as determined by histopathological analysis and enumeration of bacteria in the spleen. Elevated numbers of multinucleated giant cells (MNGCs), which is the hallmark of melioidosis, were detected in both the wild-type and the bprD mutants; a similar elevation occurs in melioidosis patients. One striking observation was the increased expression of BPSS1520 (bprC), located downstream of bprD, in the bprD mutant. BprC is a regulator of T6SS-1 that is required for the virulence of B. pseudomallei in murine infection models. Deletion of bprD led to the overexpression of bprC and a decreased time to death. bprD expression was elevated in C57BL/6--as compared to BALB/c--mice, suggesting a role for BprD in the natural resistance of C57BL/6 mice to B. pseudomallei. Ultimately, this analysis using mice with different immune backgrounds may enhance our understanding of the outcomes of infection in a variety of models.