Structural and biological diversity of lipopolysaccharides from Burkholderia pseudomallei and Burkholderia thailandensis

Evaluation of a potent LpxC inhibitor for post-exposure prophylaxis treatment of antibiotic-resistant Burkholderia pseudomallei in a murine infection model

LPC-233 (a.k.a. VB-233) is a potent antibiotic targeting the essential enzyme LpxC in Gram-negative bacteria. We present herein the pharmacokinetics and pharmacodynamics data of LPC-233 for treating murine infections caused by Burkholderia pseudomallei, a potential biodefense pathogen. A range of doses was evaluated in a post-aerosol exposure model of B. pseudomallei-infected mice. After the aerosol challenge with the B. pseudomallei strain K96243, treatment was initiated with 10, 30, or 90 mg/kg of LPC-233 orally every 12 h (q12h) or 90 mg/kg intraperitoneally q12h for 14 days. A vehicle-control arm and a positive-control arm consisting of one of the recommended standards of care, ceftazidime (150 mg/kg, q6h) injected subcutaneously, were included. LPC-233 significantly and dose-dependently rescued mice from B. pseudomallei infection in comparison with the vehicle (P < 0.0001). At dose levels of 30 mg/kg or higher, the survival rate with LPC-233 was significantly higher than that from the ceftazidime arm (P range: 0.001-0.05). LPC-233 reversed the murine body weight loss caused by the B. pseudomallei infection more rapidly than ceftazidime did, suggesting that it is a faster-acting antibiotic in this dosing regimen. Despite the outstanding survival advantage of LPC-233 over ceftazidime, no significant differences in tissue burdens (liver, lung, spleen, and blood) were observed among any of the treatment groups surviving to the termination of the experiment, suggesting that similar to commercial antibiotics, LPC-233 treatment for lethal B. pseudomallei infection may likely require both an acute phase of intensive treatment and an eradication phase of prolonged treatment.

Differentiation in pyroptosis induction by Burkholderia pseudomallei and Burkholderia thailandensis in primary human monocytes, a possible cause of sepsis in acute melioidosis patients

Melioidosis caused by Burkholderia pseudomallei is an infectious disease with a high mortality rate. In acute melioidosis, sepsis is a major cause of death among patients. Once the bacterium enters the bloodstream, immune system dysregulation ensues, leading to cytokine storms. In contrast to B. pseudomallei, a closely related but non-virulent strain B. thailandensis has rarely been reported to cause cytokine storms or death in patients. However, the mechanisms in which the virulent B. pseudomallei causes sepsis are not fully elucidated. It is well-documented that monocytes play an essential role in cytokine production in the bloodstream. The present study, therefore, determined whether there is a difference in the innate immune response to B. pseudomallei and B. thailandensis during infection of primary human monocytes and THP-1 monocytic cells by investigating pyroptosis, an inflammatory death pathway known to play a pivotal role in sepsis. Our results showed that although both bacterial species exhibited a similar ability to invade human monocytes, only B. pseudomallei can significantly increase the release of cytosolic enzyme lactate dehydrogenase (LDH) as well as the increases in caspase-1 and gasdermin D activations in both cell types. The results were consistent with the significant increase in IL-1β and IL-18 production, key cytokines involved in pyroptosis. Interestingly, there was no significant difference in other cytokine secretion, such as IL-1RA, IL-10, IL-12p70, IL-15, IL-8, and IL-23 in cells infected by both bacterial species. Furthermore, we also demonstrated that ROS production played a crucial role in controlling pyroptosis activation during B. pseudomallei infection in primary human monocytes. These findings suggested that pyroptosis induced by B. pseudomallei in the human monocytes may contribute to the pathogenesis of sepsis in acute melioidosis patients.

Diversity, Complexity, and Specificity of Bacterial Lipopolysaccharide (LPS) Structures Impacting Their Detection and Quantification

Endotoxins are toxic lipopolysaccharides (LPSs), extending from the outer membrane of Gram-negative bacteria and notorious for their toxicity and deleterious effects. The comparison of different LPSs, isolated from various Gram-negative bacteria, shows a global similar architecture corresponding to a glycolipid lipid A moiety, a core oligosaccharide, and outermost long O-chain polysaccharides with molecular weights from 2 to 20 kDa. LPSs display high diversity and specificity among genera and species, and each bacterium contains a unique set of LPS structures, constituting its protective external barrier. Some LPSs are not toxic due to their particular structures. Different, well-characterized, and highly purified LPSs were used in this work to determine endotoxin detection rules and identify their impact on the host. Endotoxin detection is a major task to ensure the safety of human health, especially in the pharma and food sectors. Here, we describe the impact of different LPS structures obtained under different bacterial growth conditions on selective LPS detection methods such as LAL, HEK-blue TLR-4, LC-MS2, and MALDI-MS. In these various assays, LPSs were shown to respond differently, mainly attributable to their lipid A structures, their fatty acid numbers and chain lengths, the presence of phosphate groups, and their possible substitutions.

Structural Elucidation of Intact Rough-type Lipopolysaccharides Using Field Asymmetric Ion Mobility Spectrometry and Kendrick Mass Defect Plots

Lipopolysaccharides (LPSs) are a hallmark virulence factor of Gram-negative bacteria. They are complex, structurally heterogeneous mixtures due to variations in number, type, and position of their simplest units: fatty acids and monosaccharides. Thus, LPS structural characterization by traditional mass spectrometry (MS) methods is challenging. Here, we describe the benefits of field asymmetric ion mobility spectrometry (FAIMS) for analysis of an intact R-type lipopolysaccharide complex mixture (lipooligosaccharide; LOS). Structural characterization was performed using Escherichia coli J5 (Rc mutant) LOS, a TLR4 agonist widely used in glycoconjugate vaccine research. FAIMS gas-phase fractionation improved the (S/N) ratio and number of detected LOS species. Additionally, FAIMS allowed the separation of overlapping isobars facilitating their tandem MS characterization and unequivocal structural assignments. In addition to FAIMS gas-phase fractionation benefits, extra sorting of the structurally related LOS molecules was further accomplished using Kendrick mass defect (KMD) plots. Notably, a custom KMD base unit of [Na-H] created a highly organized KMD plot that allowed identification of interesting and novel structural differences across the different LOS ion families, i.e., ions with different acylation degrees, oligosaccharides composition, and chemical modifications. Defining the composition of a single LOS ion by tandem MS along with the organized KMD plot structural network was sufficient to deduce the composition of 181 LOS species out of 321 species present in the mixture. The combination of FAIMS and KMD plots allowed in-depth characterization of the complex LOS mixture and uncovered a wealth of novel information about its structural variations.

Collab or Cancel? Bacterial Influencers of Inflammasome Signaling

The immune system of multicellular organisms protects them from harmful microbes. To establish an infection in the face of host immune responses, pathogens must evolve specific strategies to target immune defense mechanisms. One such defense is the formation of intracellular protein complexes, termed inflammasomes, that are triggered by the detection of microbial components and the disruption of homeostatic processes that occur during bacterial infection. Formation of active inflammasomes initiates programmed cell death pathways via activation of inflammatory caspases and cleavage of target proteins. Inflammasome-activated cell death pathways such as pyroptosis lead to proinflammatory responses that protect the host. Bacterial infection has the capacity to influence inflammasomes in two distinct ways: activation and perturbation. In this review, we discuss how bacterial activities influence inflammasomes, and we discuss the consequences of inflammasome activation or evasion for both the host and pathogen.

NLRP11 is a pattern recognition receptor for bacterial lipopolysaccharide in the cytosol of human macrophages

Endotoxin-bacterial lipopolysaccharide (LPS)-is a driver of lethal infection sepsis through excessive activation of innate immune responses. When delivered to the cytosol of macrophages, cytosolic LPS (cLPS) induces the assembly of an inflammasome that contains caspases-4/5 in humans or caspase-11 in mice. Whereas activation of all other inflammasomes is triggered by sensing of pathogen products by a specific host cytosolic pattern recognition receptor protein, whether pattern recognition receptors for cLPS exist has remained unclear, because caspase-4, caspase-5, and caspase-11 bind and activate LPS directly in vitro. Here, we show that the primate-specific protein NLRP11 is a pattern recognition receptor for cLPS that is required for efficient activation of the caspase-4 inflammasome in human macrophages. In human macrophages, NLRP11 is required for efficient activation of caspase-4 during infection with intracellular Gram-negative bacteria or upon electroporation of LPS. NLRP11 could bind LPS and separately caspase-4, forming a high-molecular weight complex with caspase-4 in HEK293T cells. NLRP11 is present in humans and other primates but absent in mice, likely explaining why it has been overlooked in screens looking for innate immune signaling molecules, most of which have been carried out in mice. Our results demonstrate that NLRP11 is a component of the caspase-4 inflammasome activation pathway in human macrophages.

Structural Elucidation of Intact Rough-Type Lipopolysaccharides using Field Asymmetric Ion Mobility Spectrometry and Kendrick Mass Defect Plots

Lipopolysaccharide (LPS) is a hallmark virulence factor of Gram-negative bacteria. It is a complex, structurally heterogeneous mixture due to variations in number, type, and position of its simplest units: fatty acids and monosaccharides. Thus, LPS structural characterization by traditional mass spectrometry (MS) methods is challenging. Here, we describe the benefits of field asymmetric ion mobility spectrometry (FAIMS) for analysis of intact R-type lipopolysaccharide complex mixture (lipooligosaccharide; LOS). Structural characterization was performed using Escherichia coli J5 (Rc mutant) LOS, a TLR4 agonist widely used in glycoconjugate vaccine research. FAIMS gas phase fractionation improved the (S/N) ratio and number of detected LOS species. Additionally, FAIMS allowed the separation of overlapping isobars facilitating their tandem MS characterization and unequivocal structural assignments. In addition to FAIMS gas phase fractionation benefits, extra sorting of the structurally related LOS molecules was further accomplished using Kendrick mass defect (KMD) plots. Notably, a custom KMD base unit of [NaH] created a highly organized KMD plot that allowed identification of interesting and novel structural differences across the different LOS ion families; i.e., ions with different acylation degrees, oligosaccharides composition, and chemical modifications. Defining the composition of a single LOS ion by tandem MS along with the organized KMD plot structural network was sufficient to deduce the composition of 179 LOS species out of 321 species present in the mixture. The combination of FAIMS and KMD plots allowed in-depth characterization of the complex LOS mixture and uncovered a wealth of novel information about its structural variations.

Characterization of Burkholderia pseudomallei O antigens in different clinical strains

O antigen is the major component of lipopolysaccharide LPS. The chemical structure of the O antigen determines the LPS serospecificity of the bacteria, and the diversity of O antigen is the basis for serotyping Burkholderia pseudomallei. In this study, structural elucidation of type B O antigen obtained from a clinical B. pseudomallei strain was conducted, and the effects of different types of LPS on macrophage differentiation were investigated. The O antigen was found to be composed of repeating units of [→4)-α-L-Rhap(1 → 4)-α-L-Rhap(1→2)-α-L-Rhap(1 → 2)-α-L-Rhap(1 → 3)-α-L-Rhap(1 → 3)-α-L-Rhap(1 → 4)-α-L-Rhap(1 → 6)-α-D-Galp(1→]_n, where some of the →4)-α-L-Rhap(1 → units were substituted at O-3 by β-D-Xylp(1 → residues, and minor →3)-α-L-Rhap(1 → units were substituted at O-2 by β-D-Xylp(1 → residues. Meahwhile, the →6)-α-D-Galp(1 → units were substituted at O-3 by α-D-Galp(1 → residues. Furthermore, both type A and type B O antigens of B. pseudomallei could polarize macrophages toward the M1 phenotype, but the core oligosaccharides had no such activity. Therefore, we deduced that this polarization relies on the O antigen of LPS and might be related to the ability of B. pseudomallei to survive and replicate within macrophages. Thus, the characterization of different types of O antigen structural motifs is essential for further clarifying the persistence/survival mechanisms and inflammatory potential of B. pseudomallei.

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.

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

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

Inhibition of Streptococcus mutans biofilms with bacterial-derived outer membrane vesicles

BACKGROUND

Biofilms are microbial communities surrounded by a self-produced extracellular matrix which protects them from environmental stress. Bacteria within biofilms are 10- to 1000-fold more resistant to antibiotics, making it challenging but imperative to develop new therapeutics that can disperse biofilms and eradicate infection. Gram-negative bacteria produce outer membrane vesicles (OMV) that play critical roles in communication, genetic exchange, cargo delivery, and pathogenesis. We have previously shown that OMVs derived from Burkholderia thailandensis inhibit the growth of drug-sensitive and drug-resistant bacteria and fungi.

RESULTS

Here, we examine the antibiofilm activity of Burkholderia thailandensis OMVs against the oral biofilm-forming pathogen Streptococcus mutans. We demonstrate that OMV treatment reduces biofilm biomass, biofilm integrity, and bacterial cell viability. Both heat-labile and heat-stable components, including 4-hydroxy-3-methyl-2-(2-non-enyl)-quinoline and long-chain rhamnolipid, contribute to the antibiofilm activity of OMVs. When OMVs are co-administered with gentamicin, the efficacy of the antibiotic against S. mutans biofilms is enhanced.

CONCLUSION

These studies indicate that bacterial-derived OMVs are highly effective biological nanoparticles that can inhibit and potentially eradicate biofilms.

Shigella OspC3 suppresses murine cytosolic LPS sensing

Shigella flexneri, a cytosol-invasive gram-negative pathogen, deploys an array of type III-secreted effector proteins to evade host cell defenses. Caspase-11 and its human ortholog caspase-4 detect cytosolic lipopolysaccharide (LPS) and trigger gasdermin D-mediated pyroptosis to eliminate intra-cytoplasmic bacterial threats. However, the role of caspase-11 in combating S. flexneri is unclear. The Shigella T3SS effector OspC3 reportedly suppresses cytosolic LPS sensing by inhibiting caspase-4 but not caspase-11 activity. Surprisingly, we found that S. flexneri also uses OspC3 to inhibit murine caspase-11 activity. Mechanistically, we found that OspC3 binds only to primed caspase-11. Importantly, we demonstrate that S. flexneri employs OspC3 to prevent caspase-11-mediated pyroptosis in neutrophils, enabling bacteria to disseminate and evade clearance following intraperitoneal challenge. In contrast, S. flexneri lacking OspC3 is attenuated in a caspase-11- and gasdermin D-dependent fashion. Overall, our study reveals that OspC3 suppresses cytosolic LPS detection in a broad array of mammals.

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S. flexneri T3SS-secreted OspC3 suppresses cytosolic LPS sensing by caspase-11

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OspC3 binds to caspase-11 in a priming-dependent manner

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S. flexneri employs OspC3 to prevent caspase-11-mediated pyroptosis in neutrophils

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Neutrophil caspase-11 is essential in defense against S. flexneri ΔOspC3 in vivo

An Evolutionary Arms Race Between Burkholderia pseudomallei and Host Immune System: What Do We Know?

A better understanding of co-evolution between pathogens and hosts holds promise for better prevention and control strategies. This review will explore the interactions between Burkholderia pseudomallei, an environmental and opportunistic pathogen, and the human host immune system. B. pseudomallei causes "Melioidosis," a rapidly fatal tropical infectious disease predicted to affect 165,000 cases annually worldwide, of which 89,000 are fatal. Genetic heterogeneities were reported in both B. pseudomallei and human host population, some of which may, at least in part, contribute to inter-individual differences in disease susceptibility. Here, we review (i) a multi-host-pathogen characteristic of the interaction; (ii) selection pressures acting on B. pseudomallei and human genomes with the former being driven by bacterial adaptation across ranges of ecological niches while the latter are driven by human encounter of broad ranges of pathogens; (iii) the mechanisms that generate genetic diversity in bacterial and host population particularly in sequences encoding proteins functioning in host-pathogen interaction; (iv) reported genetic and structural variations of proteins or molecules observed in B. pseudomallei-human host interactions and their implications in infection outcomes. Together, these predict bacterial and host evolutionary trajectory which continues to generate genetic diversity in bacterium and operates host immune selection at the molecular level.

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

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

Colistin and its role in the Era of antibiotic resistance: an extended review (2000-2019)

Increasing antibiotic resistance in multidrug-resistant (MDR) Gram-negative bacteria (MDR-GNB) presents significant health problems worldwide, since the vital available and effective antibiotics, including; broad-spectrum penicillins, fluoroquinolones, aminoglycosides, and β-lactams, such as; carbapenems, monobactam, and cephalosporins; often fail to fight MDR Gram-negative pathogens as well as the absence of new antibiotics that can defeat these "superbugs". All of these has prompted the reconsideration of old drugs such as polymyxins that were reckoned too toxic for clinical use. Only two polymyxins, polymyxin E (colistin) and polymyxin B, are currently commercially available. Colistin has re-emerged as a last-hope treatment in the mid-1990s against MDR Gram-negative pathogens due to the development of extensively drug-resistant GNB. Unfortunately, rapid global resistance towards colistin has emerged following its resurgence. Different mechanisms of colistin resistance have been characterized, including intrinsic, mutational, and transferable mechanisms.In this review, we intend to discuss the progress over the last two decades in understanding the alternative colistin mechanisms of action and different strategies used by bacteria to develop resistance against colistin, besides providing an update about what is previously recognized and what is novel concerning colistin resistance.

Lipopolysaccharide Recognition in the Crossroads of TLR4 and Caspase-4/11 Mediated Inflammatory Pathways

The innate immune response to lipopolysaccharide is essential for host defense against Gram-negative bacteria. In response to bacterial infection, the TLR4/MD-2 complex that is expressed on the surface of macrophages, monocytes, dendritic, and epithelial cells senses picomolar concentrations of endotoxic LPS and triggers the production of various pro-inflammatory mediators. In addition, LPS from extracellular bacteria which is either endocytosed or transfected into the cytosol of host cells or cytosolic LPS produced by intracellular bacteria is recognized by cytosolic proteases caspase-4/11 and hosts guanylate binding proteins that are involved in the assembly and activation of the NLRP3 inflammasome. All these events result in the initiation of pro-inflammatory signaling cascades directed at bacterial eradication. However, TLR4-mediated signaling and caspase-4/11-induced pyroptosis are largely involved in the pathogenesis of chronic and acute inflammation. Both extra- and intracellular LPS receptors-TLR4/MD-2 complex and caspase-4/11, respectively-are able to directly bind the lipid A motif of LPS. Whereas the structural basis of lipid A recognition by the TLR4 complex is profoundly studied and well understood, the atomic mechanism of LPS/lipid A interaction with caspase-4/11 is largely unknown. Here we describe the LPS-induced TLR4 and caspase-4/11 mediated signaling pathways and their cross-talk and scrutinize specific structural features of the lipid A motif of diverse LPS variants that have been reported to activate caspase-4/11 or to induce caspase-4/11 mediated activation of NLRP3 inflammasome (either upon transfection of LPS in vitro or upon infection of cell cultures with intracellular bacteria or by LPS as a component of the outer membrane vesicles). Generally, inflammatory caspases show rather similar structural requirements as the TLR4/MD-2 complex, so that a "basic" hexaacylated bisphosphorylated lipid A architecture is sufficient for activation. However, caspase-4/11 can sense and respond to much broader variety of lipid A variants compared to the very "narrow" specificity of TLR4/MD-2 complex as far as the number and the length of lipid chains attached at the diglucosamine backbone of lipid A is concerned. Besides, modification of the lipid A phosphate groups with positively charged appendages such as phosphoethanolamine or aminoarabinose could be essential for the interaction of lipid A/LPS with inflammatory caspases and related proteins.

Recent advances in the understanding of trimeric autotransporter adhesins

Adhesion is the initial step in the infection process of gram-negative bacteria. It is usually followed by the formation of biofilms that serve as a hub for further spread of the infection. Type V secretion systems engage in this process by binding to components of the extracellular matrix, which is the first step in the infection process. At the same time they provide protection from the immune system by either binding components of the innate immune system or by establishing a physical layer against aggressors. Trimeric autotransporter adhesins (TAAs) are of particular interest in this family of proteins as they possess a unique structural composition which arises from constraints during translocation. The sequence of individual domains can vary dramatically while the overall structure can be very similar to one another. This patchwork approach allows researchers to draw conclusions of the underlying function of a specific domain in a structure-based approach which underscores the importance of solving structures of yet uncharacterized TAAs and their individual domains to estimate the full extent of functions of the protein a priori. Here, we describe recent advances in understanding the translocation process of TAAs and give an overview of structural motifs that are unique to this class of proteins. The role of BpaC in the infection process of Burkholderia pseudomallei is highlighted as an exceptional example of a TAA being at the centre of infection initiation.

Distinct Pathogenic Patterns of Burkholderia pseudomallei Isolates Selected from Caenorhabditis elegans and Dictyostelium discoideum Models

Burkholderia pseudomallei is a selective agent that causes septic melioidosis and exhibits a broad range of lethal doses in animals. Host cellular virulence and phagocytic resistance are pathologic keys of B. pseudomallei. We first proposed Caenorhabditis elegans as the host cellular virulence model to mimic bacterial virulence against mammals and second established the resistance of B. pseudomallei to predation by Dictyostelium discoideum as the phagocytosis model. The saprophytic sepsis-causing Burkholderia sp. (B. pseudomallei, Burkholderia thailandensis, Burkholderia cenocepacia, and Burkholderia multivorans) exhibited different virulence patterns in both simple models, but B. pseudomallei was the most toxic. Using both models, attenuated isolates of B. pseudomallei were selected from a transposon-mutant library and a panel of environmental isolates and reconfirmed by in vitro mouse peritoneal exudate cell association and invasion assays. The distinct pathological patterns of melioidosis were inducted by different selected B. pseudomallei isolates. Fatal melioidosis was induced by the isolates with high virulence in both simple models within 4-5 day, whereas the low-virulence isolates resulted in prolonged survival greater than 30 day. Infection with the isolates having high resistance to D. discoideum predation but a low C. elegans killing effect led to 83% of mice with neurologic melioidosis. By contrast, infection with the isolates having low resistance to D. discoideum predation but high C. elegans killing effect led to 20% cases with inflammation in the salivary glands. Our results indicated that individual B. pseudomallei isolates selected from simple biological models contribute differently to disease progression and/or tissue tropism.

Lipopolysaccharides from Different Burkholderia Species with Different Lipid A Structures Induce Toll-Like Receptor 4 Activation and React with Melioidosis Patient Sera

Lipopolysaccharides (LPSs) of Gram-negative bacteria comprise lipid A, core, and O-polysaccharide (OPS) components. Studies have demonstrated that LPSs isolated from the pathogenic species Burkholderia pseudomallei and Burkholderia mallei and from less-pathogenic species, such as Burkholderia thailandensis, are potent immune stimulators.

Lipopolysaccharides (LPSs) of Gram-negative bacteria comprise lipid A, core, and O-polysaccharide (OPS) components. Studies have demonstrated that LPSs isolated from the pathogenic species Burkholderia pseudomallei and Burkholderia mallei and from less-pathogenic species, such as Burkholderia thailandensis, are potent immune stimulators. The LPS structure of B. pseudomallei, the causative agent of melioidosis, is highly conserved in isolates from Thailand; however, the LPSs isolated from other, related species have not been characterized to enable understanding of their immune recognition and antigenicities. Here, we describe the structural and immunological characteristics of the LPSs isolated from eight Burkholderia species and compare those for B. pseudomallei to those for the other seven species. Matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS), gas chromatography (GC), SDS-PAGE, Toll-like receptor 4 (TLR4) stimulation, and immunoblot analysis were performed on these Burkholderia species. MALDI-TOF profiles demonstrated that Burkholderia lipid A contains predominantly penta-acylated species modified with 4-amino-4-deoxy-arabinose residues at both terminal phosphate groups. The lipid A could be differentiated based on mass differences at m/z 1,511, 1,642, 1,773, and 1,926 and on fatty acid composition. LPSs of all species induced TLR4-dependent NF-κB responses; however, while SDS-PAGE analysis showed similar LPS ladder patterns for B. pseudomallei, B. thailandensis, and B. mallei, these patterns differed from those of other Burkholderia species. Interestingly, immunoblot analysis demonstrated that melioidosis patient sera cross-reacted with OPSs of other Burkholderia species. These findings can be used to better understand the characteristics of LPS in Burkholderia species, and they have implications for serological diagnostics based on the detection of antibodies to OPS.

Polysaccharides from Burkholderia species as targets for vaccine development, immunomodulation and chemical synthesis

Covering: up to 2018 Burkholderia species are a vast group of human pathogenic, phytopathogenic, and plant- or environment-associated bacteria. B. pseudomallei, B. mallei, and B. cepacia complex are the causative agents of melioidosis, glanders, and cystic fibrosis-related infections, respectively, which are fatal diseases in humans and animals. Due to their high resistance to antibiotics, high mortality rates, and increased infectivity via the respiratory tract, B. pseudomallei and B. mallei have been listed as potential bioterrorism agents by the Centers for Disease Control and Prevention. Burkholderia species are able to produce a large network of surface-exposed polysaccharides, i.e., lipopolysaccharides, capsular polysaccharides, and exopolysaccharides, which are virulence factors, immunomodulators, major biofilm components, and protective antigens, and have crucial implications in the pathogenicity of Burkholderia-associated diseases. This review provides a comprehensive and up-to-date account regarding the structural elucidation and biological activities of surface polysaccharides produced by Burkholderia species. The chemical synthesis of oligosaccharides mimicking Burkholderia polysaccharides is described in detail. Emphasis is placed on the recent research efforts toward the development of glycoconjugate vaccines against melioidosis and glanders based on synthetic or native Burkholderia oligo/polysaccharides.

Burkholderia pseudomallei Lipopolysaccharide Genotype Does Not Correlate With Severity or Outcome in Melioidosis: Host Risk Factors Remain the Critical Determinant

Background

The causative agent of melioidosis is the Gram-negative bacterium Burkholderia pseudomallei. Clinical presentations of melioidosis are notably diverse, with host risk factors considered central to progression from infection to disease and clinical outcome. Ubiquitous and variably present virulence determinants have been described for B pseudomallei, with several variably present minority genotypes associated with specific disease presentations. The lipopolysaccharide (LPS) O-antigen of B pseudomallei is highly diverse with 3 types described. In vitro data suggest differential virulence between LPS types, but it remains unclear whether this LPS O-antigen diversity influences clinical presentation, severity, and outcomes in patients with melioidosis.

Methods

Whole-genome sequencing was performed to assign an LPS type to 1005 consecutive B pseudomallei strains, each corresponding to a melioidosis patient enrolled in the 28-year Darwin Prospective Melioidosis study. Correlations of LPS genotype with clinical parameters was then undertaken.

Results

Bivariate analysis demonstrated that mortality and the rates of bacteremia and septic shock were the same for patients with the 2 predominant B pseudomallei LPS genotypes A (87% of cases) and B (12% of all cases). Mortality was 12% and 12%, bacteremia was 57% and 53%, and septic shock was 22% and 18% for LPS A and LPS B, respectively.

Conclusions

Lipopolysaccharide genotype was not associated with melioidosis severity or outcome. These findings suggest that in vitro differential virulence between B pseudomallei LPS genotypes does not translate to clinical significance, and this supports the primary role of host risk factors in determining disease severity and outcomes in melioidosis.

Lipid A Remodeling Is a Pathoadaptive Mechanism That Impacts Lipopolysaccharide Recognition and Intracellular Survival of Burkholderia pseudomallei

Burkholderia pseudomallei causes the severe disease melioidosis. The bacterium subverts the host immune system and replicates inside cells, and host mortality results primarily from sepsis-related complications.

Burkholderia pseudomallei causes the severe disease melioidosis. The bacterium subverts the host immune system and replicates inside cells, and host mortality results primarily from sepsis-related complications. Lipopolysaccharide (LPS) is a major virulence factor and mediator of sepsis that many pathogens capable of intracellular growth modify to reduce their immunological “footprint.” The binding strength of B. pseudomallei LPS for human LPS binding protein (hLBP) was measured using surface plasmon resonance. The structures of lipid A isolated from B. pseudomallei under different temperatures were analyzed by matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS), and the gene expression of two lipid A remodeling genes, lpxO and pagL, was investigated. The LPS was characterized for its ability to trigger tumor necrosis factor alpha (TNF-α) release and to activate caspase-11-triggered pyroptosis by introduction of LPS into the cytosol. Lipid A from long-term chronic-infection isolates was isolated and characterized by MALDI-TOF MS and also by the ability to trigger caspase-11-mediated cell death. Lipid A from B. pseudomallei 1026b lpxO and pagL mutants were characterized by positive- and negative-mode MALDI-TOF MS to ultimately identify their role in lipid A structural modifications. Replication of lpxO and pagL mutants and their complements within macrophages showed that lipid A remodeling can effect growth in host cells and activation of caspase-11-mediated cytotoxicity.

Burkholderia pseudomallei-absent soil bacterial community results in secondary metabolites that kill this pathogen

Burkholderia pseudomallei is a Gram-negative bacterium found in soil and the causative agent of a severe disease in humans and animals known as melioidosis. It is intrinsically resistant to many antibiotics and has been reported resistant to the drugs of choice; ceftazidime. Microbial communities in soil in the presence and absence of B. pseudomallei were investigated using metagenomics approach. The variation in bacterial species diversity was significantly higher in soil samples without B. pseudomallei. Abundances of phyla Actinobacteria and Firmicutes were found significantly higher in B. pseudomallei-negative soils. Bacillus amyloliquefaciens KKU1 in phylum Firmicutes was discovered from negative soil and its secondary metabolites could inhibit clinical, environmental and drug resistant isolates of B. pseudomallei, together with some pathogenic Gram-negative but not Gram-positive bacteria. The antimicrobial activity from KKU 1 against B. pseudomallei was abolished when treated with proteinase K, stable in a wide range of pH and remained active after heating at 100 °C for 15 min. Precipitated proteins from KKU1 were demonstrated to cause lysis and corrugated surfaces of B. pseudomallei. The minimum inhibitory concentrations and minimum bactericidal concentrations of the precipitated proteins from KKU1 against B. pseudomallei were 0.97 μg/ml and 3.9 μg/ml. Interestingly, Native SDS-PAGE showed small active compounds of less than 6 kDa, along with other information collectively suggesting the properties of antimicrobial peptides. For the first time, culture-independent information in melioidosis endemic area could lead to a suspected source of metabolites that may help defense against B. pseudomallei and other pathogenic Gram-negative bacteria.

Comprehensive analysis of clinical Burkholderia pseudomallei isolates demonstrates conservation of unique lipid A structure and TLR4-dependent innate immune activation

Burkholderia pseudomallei is an environmental bacterium that causes melioidosis, a major community-acquired infection in tropical regions. Melioidosis presents with a range of clinical symptoms, is often characterized by a robust inflammatory response, may relapse after treatment, and results in high mortality rates. Lipopolysaccharide (LPS) of B. pseudomallei is a potent immunostimulatory molecule comprised of lipid A, core, and O-polysaccharide (OPS) components. Four B. pseudomallei LPS types have been described based on SDS-PAGE patterns that represent the difference of OPS-type A, type B, type B2 and rough LPS. The majority of B. pseudomallei isolates are type A. We used matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) followed by electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-QqTOF MS) and gas chromatography to characterize the lipid A of B. pseudomallei within LPS type A isolates. We determined that B. pseudomallei lipid A is represented by penta- and tetra-acylated species modified with 4-amino-4-deoxy-arabinose (Ara4N). The MALDI-TOF profiles from 171 clinical B. pseudomallei isolates, including 68 paired primary and relapse isolates and 35 within-host isolates were similar. We did not observe lipid A structural changes when the bacteria were cultured in different growth conditions. Dose-dependent NF-κB activation in HEK cells expressing TLR4 was observed using multiple heat-killed B. pseudomallei isolates and corresponding purified LPS. We demonstrated that TLR4-dependent NF-κB activation induced by heat-killed bacteria or LPS prepared from OPS deficient mutant was significantly greater than those induced by wild type B. pseudomallei. These findings suggest that the structure of B. pseudomallei lipid A is highly conserved in a wide variety of clinical and environmental circumstances but that the presence of OPS may modulate LPS-driven innate immune responses in melioidosis.

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.

Outer Membrane Vesicle Vaccines from Biosafe Surrogates Prevent Acute Lethal Glanders in Mice

Burkholderia mallei is a host-adapted Gram-negative mammalian pathogen that causes the severe disease glanders. Glanders can manifest as a rapid acute progression or a chronic debilitating syndrome primarily affecting solipeds and humans in close association with infected animals. In USA, B. mallei is classified as one of the most important bacterial biothreat agents. Presently, there is no licensed glanders vaccine available for humans or animals. In this work, outer membrane vesicles (OMVs) were isolated from three attenuated biosafe bacterial strains, Burkholderia pseudomallei Bp82, B. thailandensis E555, and B. thailandensis TxDOH and used to vaccinate mice. B. thailandensis OMVs induced significantly higher antibody responses that were investigated. B. mallei specific serum antibody responses were of higher magnitude in mice vaccinated with B. thailandensis OMVs compared to levels in mice vaccinated with B. pseudomallei OMVs. OMVs derived from biosafe strains protected mice from acute lethal glanders with vesicles from the two B. thailandensis strains affording significant protection (>90%) up to 35 days post-infection with some up to 60 days. Organ loads from 35-day survivors indicated bacteria colonization of the lungs, liver, and spleen while those from 60 days had high CFUs in the spleens. The highest antibody producing vaccine (B. thailandensis E555 OMVs) also protected C57BL/6 mice from acute inhalational glanders with evidence of full protection.

Burkholderia thailandensis as a microbial cell factory for the bioconversion of used cooking oil to polyhydroxyalkanoates and rhamnolipids

The present work assessed the feasibility of used cooking oil as a low cost carbon source for rhamnolipid biosurfactant production employing the strain Burkholderia thailandensis. According to the results, B. thailandensis was able to produce rhamnolipids up to 2.2 g/L, with the dominant congener being the di-rhamnolipid Rha-Rha-C₁₄-C₁₄. Rhamnolipids had the ability to reduce the surface tension to 37.7 mN/m and the interfacial tension against benzene and oleic acid to 4.2 and 1.5 mN/m, while emulsification index against kerosene reached up to 64%. The ability of B. thailandensis to accumulate intracellular biopolymers, in the form of polyhydroxyalkanoates (PHA), was also monitored. Polyhydroxybutyrate (PHB) was accumulated simultaneously and consisted of up to 60% of the cell dry weight. PHB was further characterized in terms of its molecular weight and thermal properties. This is the first study reporting the simultaneous production of polyhydroxyalkanoates and rhamnolipids by the non-pathogen rhamnolipid producer B. thailandensis.

Synergy between Active Efflux and Outer Membrane Diffusion Defines Rules of Antibiotic Permeation into Gram-Negative Bacteria

Gram-negative bacteria are notoriously resistant to antibiotics, but the extent of the resistance varies broadly between species. We report that in significant human pathogens Acinetobacter baumannii, Pseudomonas aeruginosa, and Burkholderia spp., the differences in antibiotic resistance are largely defined by their penetration into the cell. For all tested antibiotics, the intracellular penetration was determined by a synergistic relationship between active efflux and the permeability barrier. We found that the outer membrane (OM) and efflux pumps select compounds on the basis of distinct properties and together universally protect bacteria from structurally diverse antibiotics. On the basis of their interactions with the permeability barriers, antibiotics can be divided into four clusters that occupy defined physicochemical spaces. Our results suggest that rules of intracellular penetration are intrinsic to these clusters. The identified specificities in the permeability barriers should help in the designing of successful therapeutic strategies against antibiotic-resistant pathogens.IMPORTANCE Multidrug-resistant strains of Gram-negative pathogens rapidly spread in clinics. Significant efforts worldwide are currently directed to finding the rules of permeation of antibiotics across two membrane envelopes of these bacteria. This study created the tools for analysis of and identified the major differences in antibacterial activities that distinguish the permeability barriers of P. aeruginosa, A. baumannii, Burkholderia thailandensis, and B. cepacia We conclude that synergy between active efflux and the outer membrane barrier universally protects Gram-negative bacteria from antibiotics. We also found that the diversity of antibiotics affected by active efflux and outer membrane barriers is broader than previously thought and that antibiotics cluster according to specific biological determinants such as the requirement of specific porins in the OM, targeting of the OM, or specific recognition by efflux pumps. No universal rules of antibiotic permeation into Gram-negative bacteria apparently exist. Our results suggest that antibiotic clusters are defined by specific rules of permeation and that further studies could lead to their discovery.

Deciphering minimal antigenic epitopes associated with Burkholderia pseudomallei and Burkholderia mallei lipopolysaccharide O-antigens

Burkholderia pseudomallei (Bp) and Burkholderia mallei (Bm), the etiologic agents of melioidosis and glanders, respectively, cause severe disease in both humans and animals. Studies have highlighted the importance of Bp and Bm lipopolysaccharides (LPS) as vaccine candidates. Here we describe the synthesis of seven oligosaccharides as the minimal structures featuring all of the reported acetylation/methylation patterns associated with Bp and Bm LPS O-antigens (OAgs). Our approach is based on the conversion of an L-rhamnose into a 6-deoxy-L-talose residue at a late stage of the synthetic sequence. Using biochemical and biophysical methods, we demonstrate the binding of several Bp and Bm LPS-specific monoclonal antibodies with terminal OAg residues. Mice immunized with terminal disaccharide-CRM197 constructs produced high-titer antibody responses that crossreacted with Bm-like OAgs. Collectively, these studies serve as foundation for the development of novel therapeutics, diagnostics, and vaccine candidates to combat diseases caused by Bp and Bm.Melioidosis and glanders are multifaceted infections caused by gram-negative bacteria. Here, the authors synthesize a series of oligosaccharides that mimic the lipopolysaccharides present on the pathogens' surface and use them to develop novel glycoconjugates for vaccine development.

Bilayer Properties of Lipid A from Various Gram-Negative Bacteria

Lipid A is the lipid anchor of a lipopolysaccharide in the outer leaflet of the outer membrane of Gram-negative bacteria. In general, lipid A consists of two phosphorylated N-acetyl glucosamine and several acyl chains that are directly linked to the two sugars. Depending on the bacterial species and environments, the acyl chain number and length vary, and lipid A can be chemically modified with phosphoethanolamine, aminoarabinose, or glycine residues, which are key to bacterial pathogenesis. In this work, homogeneous lipid bilayers of 21 distinct lipid A types from 12 bacterial species are modeled and simulated to investigate the differences and similarities of their membrane properties. In addition, different neutralizing ion types (Ca²⁺, K⁺, and Na⁺) are considered to examine the ion's influence on the membrane properties. The trajectory analysis shows that (1) the area per lipid is mostly correlated to the acyl chain number, and the area per lipid increases as a function of the acyl chain number; (2) the hydrophobic thickness is mainly determined by the average acyl chain length with slight dependence on the acyl chain number, and the hydrophobic thickness generally increases with the average acyl chain length; (3) a good correlation is observed among the area per lipid, hydrophobic thickness, and acyl chain order; and (4) although the influence of neutralizing ion types on the area per lipid and hydrophobic thickness is minimal, Ca²⁺ stays longer on the membrane surface than K⁺ or Na⁺, consequently leading to lower lateral diffusion and a higher compressibility modulus, which agrees well with available experiments.

A comparison of the immunological potency of Burkholderia lipopolysaccharides in endotoxemic BALB/c mice

Lipopolysaccharide is one of the virulence factors of the soil-borne pathogens Burkholderia pseudomallei, B. thailandensis, B. cenocepacia and B. multivorans, which cause septic melioidosis (often in B. pseudomallei infections but rarely in B. thailandensis infections) or cepacia syndromes (commonly in B. cenocepacia infections but rarely in B. multivorans infections). The inflammatory responses in Burkholderia LPS-induced endotoxemia were evaluated in this study. Prior to induction, the conserved structures and functions of each purified LPS were determined using electrophoretic phenotypes, the ratios of 3-hydroxytetradecanoic to 3-hydroxyhexadecanoic acid and endotoxin units. In an in vitro assay, cytokine expression of myeloid differentiation primary response gene 88 and Toll/IL-1 receptor domain containing adapter-inducing INF-β-dependent signaling-dependent signaling differed when stimulated by different LPS. Endotoxemia was induced in mice by s.c. injection as evidenced by increasing serum concentrations of 3-hydroxytetradecanoic acid and the septic prognostic markers CD62E and ICAM-1. During endotoxemia, splenic CD11b⁺ I-A⁺ , CD11b⁺ CD80⁺ , CD11b⁺ CD86⁺ and CD11b⁺ CD11c⁺ subpopulations increased. After induction with B. pseudomallei LPS, there were significant increases in splenic CD49b NK cells and CD14 macrophages. The inflamed CD11b⁺ CCR2⁺ , CD11b⁺ CD31⁺ , CD11b⁺ CD14⁺ , resident CD11b⁺ CX₃ CR1⁺ and progenitor CD11b⁺ CD34⁺ cells showed delayed increases in bone marrow. B. multivorans LPS was the most potent inducer of serum cytokines and chemokines, whereas B. cenocepacia LPS induced relatively low concentrations of the chemokines MIP-1α and MIP-1β. Endotoxin activities did not correlate with the virulence of Burkholderia strains. Thus factors other than LPS and/or other mechanisms of low activity LPS must mediate the pathogenicity of highly virulent Burkholderia strains.

Structural diversity of Burkholderia pseudomallei lipopolysaccharides affects innate immune signaling

Burkholderia pseudomallei (Bp) causes the disease melioidosis. The main cause of mortality in this disease is septic shock triggered by the host responding to lipopolysaccharide (LPS) components of the Gram-negative outer membrane. Bp LPS is thought to be a weak inducer of the host immune system. LPS from several strains of Bp were purified and their ability to induce the inflammatory mediators TNF-α and iNOS in murine macrophages at low concentrations was investigated. Innate and adaptive immunity qPCR arrays were used to profile expression patterns of 84 gene targets in response to the different LPS types. Additional qPCR validation confirmed large differences in macrophage response. LPS from a high-virulence serotype B strain 576a and a virulent rough central nervous system tropic strain MSHR435 greatly induced the innate immune response indicating that the immunopathogenesis of these strains is different than in infections with strains similar to the prototype strain 1026b. The accumulation of autophagic vesicles was also increased in macrophages challenged with highly immunogenic Bp LPS. Gene induction and concomitant cytokine secretion profiles of human PBMCs in response to the various LPS were also investigated. MALDI-TOF/TOF was used to probe the lipid A portions of the LPS, indicating substantial structural differences that likely play a role in host response to LPS. These findings add to the evolving knowledge of host-response to bacterial LPS, which can be used to better understand septic shock in melioidosis patients and in the rational design of vaccines.

Lipidomics, en route to accurate quantitation

Accurate quantitation is prerequisite for the sustainable development of lipidomics via enabling its applications in various biological and biomedical settings. In this review, the technical considerations and limitations of existent lipidomics technologies, particularly in terms of accurate quantitation; as well as the potential sources of errors along a typical lipidomic workflow that could ultimately give rise to quantitative inaccuracies will be addressed. Furthermore, the pressing need to exercise stricter definitions of terms and protocol standardization pertaining to quantitative lipidomics will be critically discussed, as quantitative accuracy may substantially impact upon the persevering development of lipidomics in the long run. This article is part of a Special Issue entitled: BBALIP_Lipidomics Opinion Articles edited by Sepp Kohlwein.

Development of Immunoassays for Burkholderia pseudomallei Typical and Atypical Lipopolysaccharide Strain Typing

Burkholderia pseudomallei is the causative agent of melioidosis, a severe infection endemic to many tropical regions. Lipopolysaccharide (LPS) is recognized as an important virulence factor used by B. pseudomallei. Isolates of B. pseudomallei have been shown to express one of four different types of LPS (typical LPS, atypical LPS types B and B2, and rough LPS) and in vitro studies have demonstrated that LPS types may impact disease severity. The association between LPS types and clinical manifestations, however, is still unknown, in part because an effective method for LPS type identification is not available. Thus, we developed antigen capture immunoassays capable of distinguishing between the LPS types. Mice were injected with B or B2 LPS for atypical LPS–specific monoclonal antibody (mAb) isolation; only two mAbs (3A2 and 5B4) were isolated from mice immunized with B2 LPS. Immunoblot analysis and surface plasmon resonance demonstrated that 3A2 and 5B4 are reactive with both B2 and B LPS where 3A2 was shown to possess higher affinity. Assays were then developed using capsular polysaccharide–specific mAb 4C4 for bacterial capture and 4C7 (previously shown to bind typical LPS) or 3A2 mAbs for typical or atypical LPS strain detection, respectively. The evaluations performed with 197 strains of Burkholderia and non-Burkholderia species showed that the assays are reactive to B. pseudomallei and Burkholderia mallei strains and have an accuracy of 98.8% (zero false positives and two false negatives) for LPS typing. The results suggest that the assays are effective and applicable for B. pseudomallei LPS typing.

Two-Phase Bactericidal Mechanism of Silver Nanoparticles against Burkholderia pseudomallei

Silver nanoparticles (AgNPs) have a strong antimicrobial activity against a variety of pathogenic bacteria. The killing mechanism of AgNPs involves direct physical membrane destruction and subsequent molecular damage from both AgNPs and released Ag⁺. Burkholderia pseudomallei is the causative agent of melioidosis, an endemic infectious disease primarily found in northern Australia and Southeast Asia. B. pseudomallei is intrinsically resistant to most common antibiotics. In this study, the antimicrobial activity and mechanism of AgNPs (10–20 nm) against B. pseudomallei were investigated. The MIC and MBC for nine B. pseudomallei strains ranged from 32–48 μg/mL and 96–128 μg/mL, respectively. Concentrations of AgNPs less than 256 μg/mL were not toxic to human red blood cells. AgNPs exhibited a two-phase mechanism: cell death induction and ROS induction. The first phase was a rapid killing step within 5 min, causing the direct damage of the cytoplasmic membrane of the bacterial cells, as observed by a time-kill assay and fluorescence microscopy. During the period of 5–30 min, the cell surface charge was rapidly neutralized from -8.73 and -7.74 to 2.85 and 2.94 mV in two isolates of B. pseudomallei, as revealed by zeta potential measurement. Energy-dispersive X-ray (EDX) spectroscopy showed the silver element deposited on the bacterial membrane, and TEM micrographs of the AgNP-treated B. pseudomallei cells showed severe membrane damage and cytosolic leakage at 1/5 MIC and cell bursting at MBC. During the killing effect the released Ag⁺ from AgNPs was only 3.9% from the starting AgNPs concentration as observed with ICP-OES experiment. In the second phase, the ROS induction occurred 1–4 hr after the AgNP treatment. Altogether, we provide direct kinetic evidence of the AgNPs killing mechanism, by which cell death is separable from the ROS induction and AgNPs mainly contributes in the killing action. AgNPs may be considered a potential candidate to develop a novel alternative agent for melioidosis treatment with fast action.

Antibiotic resistance in Burkholderia species

The genus Burkholderia comprises metabolically diverse and adaptable Gram-negative bacteria, which thrive in often adversarial environments. A few members of the genus are prominent opportunistic pathogens. These include Burkholderia mallei and Burkholderia pseudomallei of the B. pseudomallei complex, which cause glanders and melioidosis, respectively. Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia vietnamiensis belong to the Burkholderia cepacia complex and affect mostly cystic fibrosis patients. Infections caused by these bacteria are difficult to treat because of significant antibiotic resistance. The first line of defense against antimicrobials in Burkholderia species is the outer membrane penetration barrier. Most Burkholderia contain a modified lipopolysaccharide that causes intrinsic polymyxin resistance. Contributing to reduced drug penetration are restrictive porin proteins. Efflux pumps of the resistance nodulation cell division family are major players in Burkholderia multidrug resistance. Third and fourth generation β-lactam antibiotics are seminal for treatment of Burkholderia infections, but therapeutic efficacy is compromised by expression of several β-lactamases and ceftazidime target mutations. Altered DNA gyrase and dihydrofolate reductase targets cause fluoroquinolone and trimethoprim resistance, respectively. Although antibiotic resistance hampers therapy of Burkholderia infections, the characterization of resistance mechanisms lags behind other non-enteric Gram-negative pathogens, especially ESKAPE bacteria such as Acinetobacter baumannii, Klebsiella pneumoniae and Pseudomonas aeruginosa.

Melioidosis and glanders modulation of the innate immune system: barriers to current and future vaccine approaches

Burkholderia pseudomallei and Burkholderia mallei are pathogenic bacteria causing fatal infections in animals and humans. Both organisms are classified as Tier 1 Select Agents owing to their highly fatal nature, potential/prior use as bioweapons, severity of disease via respiratory exposure, intrinsic resistance to antibiotics, and lack of a current vaccine. Disease manifestations range from acute septicemia to chronic infection, wherein the facultative intracellular lifestyle of these organisms promotes persistence within a broad range of hosts. This ability to thrive intracellularly is thought to be related to exploitation of host immune response signaling pathways. There are currently considerable gaps in our understanding of the molecular strategies employed by these pathogens to modulate these pathways and evade intracellular killing. A better understanding of the specific molecular basis for dysregulation of host immune responses by these organisms will provide a stronger platform to identify novel vaccine targets and develop effective countermeasures.

Differential Toll-Like Receptor-Signalling of Burkholderia pseudomallei Lipopolysaccharide in Murine and Human Models

The Gram-negative bacterium Burkholderia pseudomallei causes melioidosis and is a CDC category B bioterrorism agent. Toll-like receptor (TLR)-2 impairs host defense during pulmonary B.pseudomallei infection while TLR4 only has limited impact. We investigated the role of TLRs in B.pseudomallei-lipopolysaccharide (LPS) induced inflammation. Purified B.pseudomallei-LPS activated only TLR2-transfected-HEK-cells during short stimulation but both HEK-TLR2 and HEK-TLR4-cells after 24 h. In human blood, an additive effect of TLR2 on TLR4-mediated signalling induced by B.pseudomallei-LPS was observed. In contrast, murine peritoneal macrophages recognized B.pseudomallei-LPS solely through TLR4. Intranasal inoculation of B.pseudomallei-LPS showed that both TLR4-knockout(-/-) and TLR2x4-/-, but not TLR2-/- mice, displayed diminished cytokine responses and neutrophil influx compared to wild-type controls. These data suggest that B.pseudomallei-LPS signalling occurs solely through murine TLR4, while in human models TLR2 plays an additional role, highlighting important differences between specificity of human and murine models that may have important consequences for B.pseudomallei-LPS sensing by TLRs and subsequent susceptibility to melioidosis.

Canonical Inflammasomes Drive IFN-γ to Prime Caspase-11 in Defense against a Cytosol-Invasive Bacterium

The inflammatory caspases 1 and 11 are activated in response to different agonists and act independently to induce pyroptosis. In the context of IL-1β/IL-18 secretion, however, in vitro studies indicate that caspase-11 acts upstream of NLRP3 and caspase-1. By contrast, studying infection in vivo by the cytosol-invasive bacterium Burkholderia thailandensis, we find that caspase-1 activity is required upstream of caspase-11 to control infection. Caspase-1-activated IL-18 induces IFN-γ to prime caspase-11 and rapidly clear B. thailandensis infection. In the absence of IL-18, bacterial burdens persist, eventually triggering other signals that induce IFN-γ. Whereas IFN-γ was essential, endogenous type I interferons were insufficient to prime caspase-11. Although mice transgenic for caspase-4, the human ortholog of caspase-11, cleared B. thailandensis in vivo, they did not strictly require IFN-γ priming. Thus, caspase-1 provides priming signals upstream of caspase-11 but not caspase-4 during murine defense against a cytosol-invasive bacterium.

Chemical synthesis of Burkholderia Lipid A modified with glycosyl phosphodiester-linked 4-amino-4-deoxy-β-L-arabinose and its immunomodulatory potential

Modification of the Lipid A phosphates by positively charged appendages is a part of the survival strategy of numerous opportunistic Gram-negative bacteria. The phosphate groups of the cystic fibrosis adapted Burkholderia Lipid A are abundantly esterified by 4-amino-4-deoxy-β-l-arabinose (β-l-Ara4N), which imposes resistance to antibiotic treatment and contributes to bacterial virulence. To establish structural features accounting for the unique pro-inflammatory activity of Burkholderia LPS we have synthesised Lipid A substituted by β-l-Ara4N at the anomeric phosphate and its Ara4N-free counterpart. The double glycosyl phosphodiester was assembled by triazolyl-tris-(pyrrolidinyl)phosphonium-assisted coupling of the β-l-Ara4N H-phosphonate to α-lactol of β(1→6) diglucosamine, pentaacylated with (R)-(3)-acyloxyacyl- and Alloc-protected (R)-(3)-hydroxyacyl residues. The intermediate 1,1′-glycosyl-H-phosphonate diester was oxidised in anhydrous conditions to provide, after total deprotection, β-l-Ara4N-substituted Burkholderia Lipid A. The β-l-Ara4N modification significantly enhanced the pro-inflammatory innate immune signaling of otherwise non-endotoxic Burkholderia Lipid A.

Mechanisms of polymyxin resistance: acquired and intrinsic resistance in bacteria

Polymyxins are polycationic antimicrobial peptides that are currently the last-resort antibiotics for the treatment of multidrug-resistant, Gram-negative bacterial infections. The reintroduction of polymyxins for antimicrobial therapy has been followed by an increase in reports of resistance among Gram-negative bacteria. Some bacteria, such as Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii, develop resistance to polymyxins in a process referred to as acquired resistance, whereas other bacteria, such as Proteus spp., Serratia spp., and Burkholderia spp., are naturally resistant to these drugs. Reports of polymyxin resistance in clinical isolates have recently increased, including acquired and intrinsically resistant pathogens. This increase is considered a serious issue, prompting concern due to the low number of currently available effective antibiotics. This review summarizes current knowledge concerning the different strategies bacteria employ to resist the activities of polymyxins. Gram-negative bacteria employ several strategies to protect themselves from polymyxin antibiotics (polymyxin B and colistin), including a variety of lipopolysaccharide (LPS) modifications, such as modifications of lipid A with phosphoethanolamine and 4-amino-4-deoxy-L-arabinose, in addition to the use of efflux pumps, the formation of capsules and overexpression of the outer membrane protein OprH, which are all effectively regulated at the molecular level. The increased understanding of these mechanisms is extremely vital and timely to facilitate studies of antimicrobial peptides and find new potential drugs targeting clinically relevant Gram-negative bacteria.

The molecular mechanism of species-specific recognition of lipopolysaccharides by the MD-2/TLR4 receptor complex

Lipid A, a component of bacterial lipopolysaccharide, is a conserved microbe-associated molecular pattern that activates the MD-2/TLR4 receptor complex. Nevertheless, bacteria produce lipid A molecules of considerable structural diversity. The human MD-2/TLR4 receptor most efficiently recognizes hexaacylated bisphosphorylated lipid A produced by enterobacteria, but in some animal species the immune response can be elicited also by alternative lipid A varieties, such as tetraacylated lipid IVa or pentaacylated lipid A of Rhodobacter spheroides. Several crystal structures revealed that hexaacylated lipid A and tetraacylated lipid IVa activate the murine MD-2/TLR4 in a similar manner, but failed to explain the antagonistic vs. agonistic activity of lipid IVa in the human vs. equine receptor, respectively. Targeted mutagenesis studies of the receptor complex revealed intricate combination of electrostatic and hydrophobic interactions primarily within the MD-2 co-receptor, but with a contribution of TLR4 as well, that contribute to species-specific recognition of lipid A. We will review current knowledge regarding lipid A diversity and species-specific activation of the MD-2/TLR4 receptor complex in different species (e.g. human, mouse or equine) by lipid A varieties.

Membrane-active mechanism of LFchimera against Burkholderia pseudomallei and Burkholderia thailandensis

LFchimera, a construct combining two antimicrobial domains of bovine lactoferrin, lactoferrampin265-284 and lactoferricin17-30, possesses strong bactericidal activity. As yet, no experimental evidence was presented to evaluate the mechanisms of LFchimera against Burkholderia isolates. In this study we analyzed the killing activity of LFchimera on the category B pathogen Burkholderia pseudomallei in comparison to the lesser virulent Burkholderia thailandensis often used as a model for the highly virulent B. pseudomallei. Killing kinetics showed that B. thailandensis E264 was more susceptible for LFchimera than B. pseudomallei 1026b. Interestingly the bactericidal activity of LFchimera appeared highly pH dependent; B. thailandensis killing was completely abolished at and below pH 6.4. FITC-labeled LFchimera caused a rapid accumulation within 15 min in the cytoplasm of both bacterial species. Moreover, freeze-fracture electron microscopy demonstrated extreme effects on the membrane morphology of both bacterial species within 1 h of incubation, accompanied by altered membrane permeability monitored as leakage of nucleotides. These data indicate that the mechanism of action of LFchimera is similar for both species and encompasses disruption of the plasma membrane and subsequently leakage of intracellular nucleotides leading to cell dead.

Alteration of the phenotypic and pathogenic patterns of Burkholderia pseudomallei that persist in a soil environment

Melioidosis is caused by the soil-borne pathogen Burkholderia pseudomallei. To investigate whether the distinct phenotypic and virulent characteristics result from environmental adaptations in the soil or from the host body, two pairs of isogenic strains were generated by passages in soil or mice. After cultivation in soil, the levels of 3-hydroxytetradecanoic acid, biofilm formation, flagellar expression, and ultrastructure were altered in the bacteria. Uniformly fatal melioidosis developed as a result of infection with mouse-derived strains; however, the survival rates of mice infected with soil-derived strains prolonged. After primary infection or reinfection with soil-derived strains, the mice developed a low degree of bacterial hepatitis and bacterial colonization in the liver and bone marrow compared with mice that were infected with isogenic or heterogenic mouse-derived strains. We suggest that specific phenotypic and pathogenic patterns can be induced through infection with B. pseudomallei that has been cultured in different (soil versus mouse) environments.

Survey of innate immune responses to Burkholderia pseudomallei in human blood identifies a central role for lipopolysaccharide

B. pseudomallei is a gram-negative bacterium that causes the tropical infection melioidosis. In northeast Thailand, mortality from melioidosis approaches 40%. As exemplified by the lipopolysaccharide-Toll-like receptor 4 interaction, innate immune responses to invading bacteria are precipitated by activation of host pathogen recognition receptors by pathogen associated molecular patterns. Human melioidosis is characterized by up-regulation of pathogen recognition receptors and pro-inflammatory cytokine release. In contrast to many gram-negative pathogens, however, the lipopolysaccharide of B. pseudomallei is considered only weakly inflammatory. We conducted a study in 300 healthy Thai subjects to investigate the ex vivo human blood response to various bacterial pathogen associated molecular patterns, including lipopolysaccharide from several bacteria, and to two heat-killed B. pseudomallei isolates. We measured cytokine levels after stimulation of fresh whole blood with a panel of stimuli. We found that age, sex, and white blood cell count modulate the innate immune response to B. pseudomallei. We further observed that, in comparison to other stimuli, the innate immune response to B. pseudomallei is most highly correlated with the response to lipopolysaccharide. The magnitude of cytokine responses induced by B. pseudomallei lipopolysaccharide was significantly greater than those induced by lipopolysaccharide from Escherichia coli and comparable to many responses induced by lipopolysaccharide from Salmonella minnesota despite lower amounts of lipid A in the B. pseudomallei lipopolysaccharide preparation. In human monocytes stimulated with B. pseudomallei, addition of polymyxin B or a TLR4/MD-2 neutralizing antibody inhibited the majority of TNF-α production. Challenging existing views, our data indicate that the innate immune response to B. pseudomallei in human blood is largely driven by lipopolysaccharide, and that the response to B. pseudomallei lipopolysaccharide in blood is greater than the response to other lipopolysaccharide expressing isolates. Our findings suggest that B. pseudomallei lipopolysaccharide may play a central role in stimulating the host response in melioidosis.

Complete Bordetella avium, Bordetella hinzii and Bordetella trematum lipid A structures and genomic sequence analyses of the loci involved in their modifications

Endotoxin is recognized as one of the virulence factors of the Bordetella avium bird pathogen, and characterization of its structure and corresponding genomic features are important for an understanding of its role in pathogenicity and for an improved general knowledge of Bordetella spp virulence factors. The structure of the biologically active part of B. avium LPS, lipid A, is described and compared to those of another bird pathogen, opportunistic in humans, Bordetella hinzii, and to that of Bordetella trematum, a human pathogen. Sequence analyses showed that the three strains have homologues of acyl-chain modifying enzymes PagL, PagP and LpxO, of the 1-phosphatase LpxE, in addition to LgmA, LgmB and LgmC, which are required for the glucosamine modification. MALDI mass spectrometry identified a high amount of glucosamine substituting the phosphate groups of B. avium lipid A; this modification was absent from B. hinzii and B. trematum. The acylation patterns of the three lipid As were similar, but they differed from those of Bordetella pertussis and Bordetella parapertussis. They were also found to be close to the lipid A structure of Bordetella bronchiseptica, a mammalian pathogen, only differing from the latter by the degree of hydroxylation of the branched fatty acid.

Mechanisms of antibiotic resistance in Burkholderia pseudomallei: implications for treatment of melioidosis

Burkholderia pseudomallei is the etiologic agent of melioidosis. This multifaceted disease is difficult to treat, resulting in high morbidity and mortality. Treatment of B. pseudomallei infections is lengthy and necessitates an intensive phase (parenteral ceftazidime, amoxicillin-clavulanic acid or meropenem) and an eradication phase (oral trimethoprim-sulfamethoxazole). The main resistance mechanisms affecting these antibiotics include enzymatic inactivation, target deletion and efflux from the cell, and are mediated by chromosomally encoded genes. Overproduction and mutations in the class A PenA β-lactamase cause ceftazidime and amoxicillin-clavulanic acid resistance. Deletion of the penicillin binding protein 3 results in ceftazidime resistance. BpeEF-OprC efflux pump expression causes trimethoprim and trimethoprim-sulfamethoxazole resistance. Although resistance is still relatively rare, therapeutic efficacies may be compromised by resistance emergence due to increased use of antibiotics in endemic regions. Novel agents and therapeutic strategies are being tested and, in some instances, show promise as anti-B. pseudomallei infectives.