Exploitation of host cells by Burkholderia pseudomallei

Emerging roles of low-molecular-weight thiols at the host-microbe interface

Low-molecular-weight (LMW) thiols are an abundant class of cysteine-derived small molecules found in all forms of life that maintain reducing conditions within cells. While their contributions to cellular redox homeostasis are well established, LMW thiols can also mediate other aspects of cellular physiology, including intercellular interactions between microbial and host cells. Here we discuss emerging roles for these redox-active metabolites at the host-microbe interface. We begin by providing an overview of chemical and computational approaches to LMW-thiol discovery. Next, we highlight mechanisms of virulence regulation by LMW thiols in infected cells. Finally, we describe how microbial metabolism of these compounds may influence host physiology.

Pyroptosis Mediates Neutrophil Extracellular Trap Formation during Bacterial Infection in Zebrafish

The formation of neutrophil extracellular trap (NET) is a critical host defense when neutrophils migrate to infection sites. Pyroptosis is a newly identified programmed cell death, which is tightly regulated by inflammasome activation. However, the mechanism of pyroptotic signaling participating in NET production remains to be elucidated. In this study, the zebrafish larvae otic vesicle microinjection model was used to infect larvae with hemolysin-overexpressing Edwardsiella piscicida (EthA⁺), and a rapid migration of neutrophils to infection sites was observed. Intriguingly, EthA⁺ infection effectively induced significant neutrophil membrane rupture in vivo, which was dependent on caspase-B (caspy2) and gasdermin Eb (GSDMEb) but not caspase-A or gasdermin Ea. Specifically, the EthA⁺ E. piscicida infection induced pyroptosis along with NETosis in vitro, and depletion of either caspy2 or GSDMEb impaired NET formation in vivo. Consequently, inhibition of the caspy2-GSDMEb axis-gated NETosis impaired bacterial clearance in vivo. Altogether, these data provide evidence that teleost fish innate immune cells, including neutrophils, express features of pyroptosis that are critical for NETosis in teleost innate immunity.

Influence of pH on the activity of finafloxacin against extracellular and intracellular Burkholderia thailandensis, Yersinia pseudotuberculosis and Francisella philomiragia and on its cellular pharmacokinetics in THP-1 monocytes

OBJECTIVES

Burkholderia pseudomallei, Yersinia pestis and Francisella tularensis are facultative intracellular bacteria causing life-threatening infections. We have (a) compared the activity of finafloxacin (a fluoroquinolone in development showing improved activity at acidic pH) with that of ciprofloxacin, levofloxacin and imipenem against the extracellular and intracellular (THP-1 monocytes) forms of infection by attenuated surrogates of these species (B. thailandensis, Y. pseudotuberculosis, F. philomiragia) and (b) assessed finafloxacin cellular pharmacokinetics (accumulation, distribution, efflux).

METHODS

Bacteria in broth or in infected monocytes were exposed to antibiotics at pH 7.4 or 5.5 for 24 hr. Maximal relative efficacies (E_max) and static concentrations (C_s) were calculated using the Hill equation (concentration-response curves). Finafloxacin pharmacokinetics in cells at pH 7.4 or 5.5 was investigated using ¹⁴C-labelled drug.

RESULTS

Extracellularly, all drugs sterilized the cultures, with finafloxacin being two to six times more potent at acidic pH. Intracellularly, E_max reached the limit of detection (4-5 log₁₀ cfu decrease) for finafloxacin against all species, but only against B. thailandensis and F. philomiragia for ciprofloxacin and levofloxacin, while imipenem caused less than 2 log₁₀ cfu decrease for all species. At acid pH, C_s shifted to two to five times lower values for finafloxacin and to one to four times higher values for the other drugs. Finafloxacin accumulated in THP-1 cells by approximately fivefold at pH 7.4 but up to 20-fold at pH 5.5, and distributed in the cytosol.

CONCLUSIONS

Fluoroquinolones have proven to be effective in reducing the intracellular reservoirs of B. thailandensis, Y. pseudotuberculosis and F. philomiragia, with finafloxacin demonstrating an additional advantage in acidic environments.

Neutrophil extracellular traps in immunity and disease

Neutrophils are innate immune phagocytes that have a central role in immune defence. Our understanding of the role of neutrophils in pathogen clearance, immune regulation and disease pathology has advanced dramatically in recent years. Web-like chromatin structures known as neutrophil extracellular traps (NETs) have been at the forefront of this renewed interest in neutrophil biology. The identification of molecules that modulate the release of NETs has helped to refine our view of the role of NETs in immune protection, inflammatory and autoimmune diseases and cancer. Here, I discuss the key findings and concepts that have thus far shaped the field of NET biology.

Nematode Peptides with Host-Directed Anti-inflammatory Activity Rescue Caenorhabditis elegans from a Burkholderia pseudomallei Infection

Burkholderia pseudomallei, the causative agent of melioidosis, is among a growing number of bacterial pathogens that are increasingly antibiotic resistant. Antimicrobial peptides (AMPs) have been investigated as an alternative approach to treat microbial infections, as generally, there is a lower likelihood that a pathogen will develop resistance to AMPs. In this study, 36 candidate Caenorhabditis elegans genes that encode secreted peptides of <150 amino acids and previously shown to be overexpressed during infection by B. pseudomallei were identified from the expression profile of infected nematodes. RNA interference (RNAi)-based knockdown of 12/34 peptide-encoding genes resulted in enhanced nematode susceptibility to B. pseudomallei without affecting worm fitness. A microdilution test demonstrated that two peptides, NLP-31 and Y43C5A.3, exhibited anti-B. pseudomallei activity in a dose dependent manner on different pathogens. Time kill analysis proposed that these peptides were bacteriostatic against B. pseudomallei at concentrations up to 8× MIC90. The SYTOX green assay demonstrated that NLP-31 and Y43C5A.3 did not disrupt the B. pseudomallei membrane. Instead, gel retardation assays revealed that both peptides were able to bind to DNA and interfere with bacterial viability. In parallel, microscopic examination showed induction of cellular filamentation, a hallmark of DNA synthesis inhibition, of NLP-31 and Y43C5A.3 treated cells. In addition, the peptides also regulated the expression of inflammatory cytokines in B. pseudomallei infected macrophage cells. Collectively, these findings demonstrate the potential of NLP-31 and Y43C5A.3 as anti-B. pseudomallei peptides based on their function as immune modulators.

Triggering Receptor Expressed on Myeloid Cells (TREM)-2 Impairs Host Defense in Experimental Melioidosis

Background

Triggering receptor expressed on myeloid cells (TREM) -1 and TREM-2 are key regulators of the inflammatory response that are involved in the clearance of invading pathogens. Melioidosis, caused by the "Tier 1" biothreat agent Burkholderia pseudomallei, is a common form of community-acquired sepsis in Southeast-Asia. TREM-1 has been suggested as a biomarker for sepsis and melioidosis. We aimed to characterize the expression and function of TREM-1 and TREM-2 in melioidosis.

Methodology/Principal Findings

Wild-type, TREM-1/3 (Trem-1/3^-/-) and TREM-2 (Trem-2^-/-) deficient mice were intranasally infected with live B. pseudomallei and killed after 24, and/or 72 h for the harvesting of lungs, liver, spleen, and blood. Additionally, survival studies were performed. Cellular functions were further analyzed by stimulation and/or infection of isolated cells. TREM-1 and TREM-2 expression was increased both in the lung and liver of B. pseudomallei-infected mice. Strikingly, Trem-2^-/-, but not Trem-1/3^-/-, mice displayed a markedly improved host defense as reflected by a strong survival advantage together with decreased bacterial loads, less inflammation and reduced organ injury. Cellular responsiveness of TREM-2, but not TREM-1, deficient blood and bone-marrow derived macrophages (BMDM) was diminished upon exposure to B. pseudomallei. Phagocytosis and intracellular killing of B. pseudomallei by BMDM and alveolar macrophages were TREM-1 and TREM-2-independent.

Conclusions/Significance

We found that TREM-2, and to a lesser extent TREM-1, plays a remarkable detrimental role in the host defense against a clinically relevant Gram-negative pathogen in mice: TREM-2 deficiency restricts the inflammatory response, thereby decreasing organ damage and mortality.

Triggering receptor expressed on myeloid cells (TREM)-1 and -2 are receptors on immune cells that act as mediators of the innate immune response. It is thought that TREM-1 amplifies the immune response, while TREM-2 acts as a negative regulator. Previously, we found that TREM-1 is upregulated in melioidosis patients. In contrast, nothing is known on TREM-2 expression and its role in melioidosis. In this study we examined the expression and functional role of both TREM-1 and -2 in a murine melioidosis model. We found that TREM-1 and-2 expression was upregulated during melioidosis. Using our experimental melioidosis model, we observed that Trem-2^-/- mice were protected against B.pseudomallei-induced lethality. Trem-2^-/- mice demonstrated reduced bacterial loads, inflammation and organ damage compared to wild-type mice in experimental melioidosis. Despite reduced bacterial dissemination of B.pseudomallei to distant organs in Trem-1/3^-/ mice^-, no differences in survival were found between Trem-1/3^-/- and wild-type mice during melioidosis. Lastly, we investigated cellular functions of TREM-1 and TREM-2 and found that TREM-2 deficiency led to decreased cellular responsiveness to B. pseudomallei infection. In conclusion, we found that TREM-2 plays an important role during experimental murine melioidosis. TREM-2-deficiency reduces inflammation and organ damage, thereby improving survival.

Mechanisms of Disease: Host-Pathogen Interactions between Burkholderia Species and Lung Epithelial Cells

Members of the Burkholderia species can cause a range of severe, often fatal, respiratory diseases. A variety of in vitro models of infection have been developed in an attempt to elucidate the mechanism by which Burkholderia spp. gain entry to and interact with the body. The majority of studies have tended to focus on the interaction of bacteria with phagocytic cells with a paucity of information available with regard to the lung epithelium. However, the lung epithelium is becoming more widely recognized as an important player in innate immunity and the early response to infections. Here we review the complex relationship between Burkholderia species and epithelial cells with an emphasis on the most pathogenic species, Burkholderia pseudomallei and Burkholderia mallei. The current gaps in knowledge in our understanding are highlighted along with the epithelial host-pathogen interactions that offer potential opportunities for therapeutic intervention.

Neurotropic threat characterization of Burkholderia pseudomallei strains

Unique virulence factors of certain strains facilitate central nervous system invasion, regardless of infection route.

The death rate for neurologic melioidosis is high. Whether certain Burkholderia pseudomallei strains are more likely than other strains to cause central nervous system infection and whether route of infection influences the neurotropic threat remain unclear. Therefore, we compared the virulence and dissemination of Australian clinical isolates collected during October 1989–October 2012 from patients with neurologic and nonneurologic melioidosis after intranasal and subcutaneous infection of mice in an experimental model. We did not observe neurotropism as a unique characteristic of isolates from patients with neurologic melioidosis. Rather, a distinct subset of B. pseudomallei strains appear to have heightened pathogenic potential for rapid dissemination to multiple tissues, including the central nervous system, irrespective of the infection route. This finding has valuable public health ramifications for initiating appropriate and timely therapy after exposure to systemically invasive B. pseudomallei strains. Increasing understanding of B. pseudomallei pathology and its influencing factors will further reduce illness and death from this disease.

Altered Proteome of Burkholderia pseudomallei Colony Variants Induced by Exposure to Human Lung Epithelial Cells

Burkholderia pseudomallei primary diagnostic cultures demonstrate colony morphology variation associated with expression of virulence and adaptation proteins. This study aims to examine the ability of B. pseudomallei colony variants (wild type [WT] and small colony variant [SCV]) to survive and replicate intracellularly in A549 cells and to identify the alterations in the protein expression of these variants, post-exposure to the A549 cells. Intracellular survival and cytotoxicity assays were performed followed by proteomics analysis using two-dimensional gel electrophoresis. B. pseudomallei SCV survive longer than the WT. During post-exposure, among 259 and 260 protein spots of SCV and WT, respectively, 19 were differentially expressed. Among SCV post-exposure up-regulated proteins, glyceraldehyde 3-phosphate dehydrogenase, fructose-bisphosphate aldolase (CbbA) and betaine aldehyde dehydrogenase were associated with adhesion and virulence. Among the down-regulated proteins, enolase (Eno) is implicated in adhesion and virulence. Additionally, post-exposure expression profiles of both variants were compared with pre-exposure. In WT pre- vs post-exposure, 36 proteins were differentially expressed. Of the up-regulated proteins, translocator protein, Eno, nucleoside diphosphate kinase (Ndk), ferritin Dps-family DNA binding protein and peptidyl-prolyl cis-trans isomerase B were implicated in invasion and virulence. In SCV pre- vs post-exposure, 27 proteins were differentially expressed. Among the up-regulated proteins, flagellin, Eno, CbbA, Ndk and phenylacetate-coenzyme A ligase have similarly been implicated in adhesion, invasion. Protein profiles differences post-exposure provide insights into association between morphotypic and phenotypic characteristics of colony variants, strengthening the role of B. pseudomallei morphotypes in pathogenesis of melioidosis.

Interaction of Interferon gamma-induced reactive oxygen species with ceftazidime leads to synergistic killing of intracellular Burkholderia pseudomallei

Burkholderia pseudomallei, a facultative intracellular pathogen, causes severe infections and is inherently refractory to many antibiotics. Previous studies from our group have shown that interferon gamma (IFN-γ) interacts synergistically with the antibiotic ceftazidime to kill bacteria in infected macrophages. The present study aimed to identify the underlying mechanism of that interaction. We first showed that blocking reactive oxygen species (ROS) pathways reversed IFN-γ- and ceftazidime-mediated killing, which led to our hypothesis that IFN-γ-induced ROS interacted with ceftazidime to synergistically kill Burkholderia bacteria. Consistent with this hypothesis, we also observed that buthionine sulfoximine (BSO), another inducer of ROS, could substitute for IFN-γ to similarly potentiate the effect of ceftazidime on intracellular killing. Next, we observed that IFN-γ induced ROS-mediated killing of intracellular but not extracellular bacteria. On the other hand, ceftazidime effectively reduced extracellular bacteria but was not capable of intracellular killing when applied at 10 μg/ml. We investigated the exact role of IFN-γ-induced ROS responses on intracellular bacteria and notably observed a lack of actin polymerization associated with Burkholderia bacteria in IFN-γ-treated macrophages, which led to our finding that IFN-γ-induced ROS blocks vacuolar escape. Based on these results, we propose a model in which synergistically reduced bacterial burden is achieved primarily through separate and compartmentalized killing: intracellular killing by IFN-γ-induced ROS responses and extracellular killing by ceftazidime. Our findings suggest a means of enhancing antibiotic activity against Burkholderia bacteria through combination with drugs that induce ROS pathways or otherwise target intracellular spread and/or replication of bacteria.

Novel Burkholderia mallei virulence factors linked to specific host-pathogen protein interactions

Burkholderia mallei is an infectious intracellular pathogen whose virulence and resistance to antibiotics makes it a potential bioterrorism agent. Given its genetic origin as a commensal soil organism, it is equipped with an extensive and varied set of adapted mechanisms to cope with and modulate host-cell environments. One essential virulence mechanism constitutes the specialized secretion systems that are designed to penetrate host-cell membranes and insert pathogen proteins directly into the host cell's cytosol. However, the secretion systems' proteins and, in particular, their host targets are largely uncharacterized. Here, we used a combined in silico, in vitro, and in vivo approach to identify B. mallei proteins required for pathogenicity. We used bioinformatics tools, including orthology detection and ab initio predictions of secretion system proteins, as well as published experimental Burkholderia data to initially select a small number of proteins as putative virulence factors. We then used yeast two-hybrid assays against normalized whole human and whole murine proteome libraries to detect and identify interactions among each of these bacterial proteins and host proteins. Analysis of such interactions provided both verification of known virulence factors and identification of three new putative virulence proteins. We successfully created insertion mutants for each of these three proteins using the virulent B. mallei ATCC 23344 strain. We exposed BALB/c mice to mutant strains and the wild-type strain in an aerosol challenge model using lethal B. mallei doses. In each set of experiments, mice exposed to mutant strains survived for the 21-day duration of the experiment, whereas mice exposed to the wild-type strain rapidly died. Given their in vivo role in pathogenicity, and based on the yeast two-hybrid interaction data, these results point to the importance of these pathogen proteins in modulating host ubiquitination pathways, phagosomal escape, and actin-cytoskeleton rearrangement processes.

Arp2/3-mediated actin-based motility: a tail of pathogen abuse

Intracellular pathogens have developed elaborate mechanisms to exploit the different cellular systems of their unwilling hosts to facilitate their entry, replication, and survival. In particular, a diverse range of bacteria and viruses have evolved unique strategies to harness the power of Arp2/3-mediated actin polymerization to enhance their cell-to-cell spread. In this review, we discuss how studying these pathogens has revolutionized our molecular understanding of Arp2/3-dependent actin assembly and revealed key signaling pathways regulating actin assembly in cells. Future analyses of microbe-host interactions are likely to continue uncovering new mechanisms regulating actin assembly and dynamics, as well as unexpected cellular functions for actin. Further, studies with known and newly emerging pathogens will also undoubtedly continue to enhance our understanding of the role of the actin cytoskeleton during pathogenesis and potentially highlight future therapeutic approaches.

Burkholderia mallei and Burkholderia pseudomallei stimulate differential inflammatory responses from human alveolar type II cells (ATII) and macrophages

Alveolar type II pneumocytes (ATII) and alveolar macrophages (AM) play a crucial role in the lung's innate immune response. Burkholderia pseudomallei (BP) and Burkholderia mallei (BM) are facultative Gram-negative bacilli that cause melioidosis and glanders, respectively. The inhalation of these pathogens can cause lethal disease and death in humans. We sought to compare the pathogenesis of and host responses to BP and BM through contact with human primary ATII cells and monocytes-derived macrophages (MDM). We hypothesized that because BP and BM induce different disease outcomes, each pathogen would induce distinct, unique host immune responses from resident pulmonary cells. Our findings showed that BP adhered readily to ATII cells compared to BM. BP, but not BM, was rapidly internalized by macrophages where it replicated to high numbers. Further, BP-induced significantly higher levels of pro-inflammatory cytokine secretion from ATII cells (IL-6, IL-8) and macrophages (IL-6, TNFα) at 6 h post-infection compared to BM (p < 0.05). Interestingly, BM-induced the anti-inflammatory cytokine, IL-10, in ATII cells and macrophages at 6 h post-infection, with delayed induction of inflammatory cytokines at 24 h post-infection. Because BP is flagellated and produces LPS, we confirmed that it stimulated both Toll-like receptor (TLR) 4 and TLR5 via NF-κb activation while the non-flagellated BM stimulated only TLR4. These data show the differences in BP and BM pathogenicity in the lung when infecting human ATII cells and macrophages and demonstrate the ability of these pathogens to elicit distinct immune responses from resident lung cells which may open new targets for therapeutic intervention to fight against these pathogens.

Crystallization and preliminary X-ray analysis of the receiver domain of a putative response regulator, BPSL0128, from Burkholderia pseudomallei

bpsl0128, a gene encoding a putative response regulator from Burkholderia pseudomallei strain D286, has been cloned into a pETBLUE-1 vector system, overexpressed in Escherichia coli and purified. The full-length protein is degraded during purification to leave a fragment corresponding to the putative receiver domain, and crystals of this protein that diffracted to beyond 1.75 Å resolution have been grown by the hanging-drop vapour-diffusion technique using PEG 6000 as the precipitant. The crystals belonged to one of the enantiomorphic pair of space groups P3(1)21 and P3(2)21, with unit-cell parameters a = b = 65.69, c = 105.01 Å and either one or two molecules in the asymmetric unit.

Burkholderia pseudomallei transcriptional adaptation in macrophages

Background

Burkholderia pseudomallei is a facultative intracellular pathogen of phagocytic and non-phagocytic cells. How the bacterium interacts with host macrophage cells is still not well understood and is critical to appreciate the strategies used by this bacterium to survive and how intracellular survival leads to disease manifestation.

Results

Here we report the expression profile of intracellular B. pseudomallei following infection of human macrophage-like U937 cells. During intracellular growth over the 6 h infection period, approximately 22 % of the B. pseudomallei genome showed significant transcriptional adaptation. B. pseudomallei adapted rapidly to the intracellular environment by down-regulating numerous genes involved in metabolism, cell envelope, motility, replication, amino acid and ion transport system and regulatory function pathways. Reduced expression in catabolic and housekeeping genes suggested lower energy requirement and growth arrest during macrophage infection, while expression of genes encoding anaerobic metabolism functions were up regulated. However, whilst the type VI secretion system was up regulated, expression of many known virulence factors was not significantly modulated over the 6hours of infection.

Conclusions

The transcriptome profile described here provides the first comprehensive view of how B. pseudomallei survives within host cells and will help identify potential virulence factors and proteins that are important for the survival and growth of B. pseudomallei within human cells.

Strategies for Intracellular Survival of Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, a disease with high mortality that is prevalent in tropical regions of the world. A key component of the pathogenesis of melioidosis is the ability of B. pseudomallei to enter, survive, and replicate within mammalian host cells. For non-phagocytic cells, bacterial adhesins have been identified both on the bacterial surface and associated with Type 4 pili. Cell invasion involves components of one or more of the three Type 3 Secretion System clusters, which also mediate, at least in part, the escape of bacteria from the endosome into the cytoplasm, where bacteria move by actin-based motility. The mechanism of actin-based motility is not clearly understood, but appears to differ from characterized mechanisms in other bacterial species. A small proportion of intracellular bacteria is targeted by host cell autophagy, involving direct recruitment of LC3 to endosomes rather than through uptake by canonical autophagosomes. However, the majority of bacterial cells are able to circumvent autophagy and other intracellular defense mechanisms such as the induction of inducible nitric oxide synthase, and then replicate in the cytoplasm and spread to adjacent cells via membrane fusion, resulting in the formation of multi-nucleated giant cells. A potential role for host cell ubiquitin in the autophagic response to bacterial infection has recently been proposed.

Identification of Burkholderia mallei and Burkholderia pseudomallei adhesins for human respiratory epithelial cells

Background

Burkholderia pseudomallei and Burkholderia mallei cause the diseases melioidosis and glanders, respectively. A well-studied aspect of pathogenesis by these closely-related bacteria is their ability to invade and multiply within eukaryotic cells. In contrast, the means by which B. pseudomallei and B. mallei adhere to cells are poorly defined. The purpose of this study was to identify adherence factors expressed by these organisms.

Results

Comparative sequence analyses identified a gene product in the published genome of B. mallei strain ATCC23344 (locus # BMAA0649) that resembles the well-characterized Yersinia enterocolitica autotransporter adhesin YadA. The gene encoding this B. mallei protein, designated boaA, was expressed in Escherichia coli and shown to significantly increase adherence to human epithelial cell lines, specifically HEp2 (laryngeal cells) and A549 (type II pneumocytes), as well as to cultures of normal human bronchial epithelium (NHBE). Consistent with these findings, disruption of the boaA gene in B. mallei ATCC23344 reduced adherence to all three cell types by ~50%. The genomes of the B. pseudomallei strains K96243 and DD503 were also found to contain boaA and inactivation of the gene in DD503 considerably decreased binding to monolayers of HEp2 and A549 cells and to NHBE cultures.

A second YadA-like gene product highly similar to BoaA (65% identity) was identified in the published genomic sequence of B. pseudomallei strain K96243 (locus # BPSL1705). The gene specifying this protein, termed boaB, appears to be B. pseudomallei-specific. Quantitative attachment assays demonstrated that recombinant E. coli expressing BoaB displayed greater binding to A549 pneumocytes, HEp2 cells and NHBE cultures. Moreover, a boaB mutant of B. pseudomallei DD503 showed decreased adherence to these respiratory cells. Additionally, a B. pseudomallei strain lacking expression of both boaA and boaB was impaired in its ability to thrive inside J774A.1 murine macrophages, suggesting a possible role for these proteins in survival within professional phagocytic cells.

Conclusions

The boaA and boaB genes specify adhesins that mediate adherence to epithelial cells of the human respiratory tract. The boaA gene product is shared by B. pseudomallei and B. mallei whereas BoaB appears to be a B. pseudomallei-specific adherence factor.

Burkholderia thailandensis as a model system for the study of the virulence-associated type III secretion system of Burkholderia pseudomallei

Burkholderia pseudomallei is a bacterial pathogen that causes a broad spectrum of clinical symptoms collectively known as melioidosis. Since it can be acquired by inhalation and is difficult to eradicate due to its resistance to a wide group of antibiotics and capacity for latency, work with B. pseudomallei requires a biosafety level 3 (BSL-3) containment facility. The bsa (Burkholderia secretion apparatus)-encoded type III secretion system (TTSS) has been shown to be required for its full virulence in a number of animal models. TTSSs are export devices found in a variety of gram-negative bacteria that translocate bacterial effector proteins across host cell membranes into the cytoplasm of host cells. Although the Bsa TTSS has been shown to play an important role in the ability of B. pseudomallei to survive and replicate in mammalian cells, escape from the endocytic vacuole, and spread from cell to cell, little is known about its effectors mediating these functions. Using bioinformatics, we identified homologs of several known TTSS effectors from other bacteria in the B. pseudomallei genome. In addition, we show that orthologs of these putative effectors exist in the genome of B. thailandensis, a closely related bacterium that is rarely pathogenic to humans. By generating a Bsa TTSS mutant B. thailandensis strain, we also demonstrated that the Bsa TTSS has similar functions in the two species. Therefore, we propose B. thailandensis as a useful BSL-1 model system to study the role of the Bsa TTSS during Burkholderia infection of mammalian cells and animals.

The guanine-nucleotide-exchange factor BopE from Burkholderia pseudomallei adopts a compact version of the Salmonella SopE/SopE2 fold and undergoes a closed-to-open conformational change upon interaction with Cdc42

BopE is a type III secreted protein from Burkholderia pseudomallei, the aetiological agent of melioidosis, a severe emerging infection. BopE is a GEF (guanine-nucleotide-exchange factor) for the Rho GTPases Cdc42 (cell division cycle 42) and Rac1. We have determined the structure of BopE catalytic domain (amino acids 78-261) by NMR spectroscopy and it shows that BopE(78-261) comprises two three-helix bundles (alpha1alpha4alpha5 and alpha2alpha3alpha6). This fold is similar to that adopted by the BopE homologues SopE and SopE2, which are GEFs from Salmonella. Whereas the two three-helix bundles of SopE(78-240) and SopE2(69-240) form the arms of a 'Lambda' shape, BopE(78-261) adopts a more closed conformation with substantial interactions between the two three-helix bundles. We propose that arginine and proline residues are important in the conformational differences between BopE and SopE/E2. Analysis of the molecular interface in the SopE(78-240)-Cdc42 complex crystal structure indicates that, in a BopE-Cdc42 interaction, the closed conformation of BopE(78-261) would engender steric clashes with the Cdc42 switch regions. This implies that BopE(78-261) must undergo a closed-to-open conformational change in order to catalyse guanine nucleotide exchange. In an NMR titration to investigate the BopE(78-261)-Cdc42 interaction, the appearance of additional peaks per NH for residues in hinge regions of BopE(78-261) indicates that BopE(78-261) does undergo a closed-to-open conformational change in the presence of Cdc42. The conformational change hypothesis is further supported by substantial improvement of BopE(78-261) catalytic efficiency through mutations that favour an open conformation. Requirement for closed-to-open conformational change explains the 10-40-fold lower k(cat) of BopE compared with SopE and SopE2.

Burkholderia pseudomallei animal and human isolates from Malaysia exhibit different phenotypic characteristics

Burkholderia pseudomallei is a Gram-negative saprophytic soil bacterium, which is the etiologic agent of melioidosis, a severe and fatal infectious disease occurring in human and animals. Distinct clinical and animal isolates have been shown to exhibit differences in phenotypic trait such as growth rate, colony morphology, antimicrobial resistance, and virulence. This study was carried out to gain insight into the intrinsic differences between 4 clinical and 6 animal B. pseudomallei isolates from Malaysia. The 16S rRNA-encoding genes from these 10 isolates of B. pseudomallei were sequenced to confirm the identity of these isolates along with the avirulent Burkholderia thailandensis. The nucleotide sequences indicated that the 16S rRNA-encoding genes among the 10 B. pseudomallei isolates were identical to each other. However, the nucleotide sequence differences in the 16S rRNA-encoding genes appeared to be B. pseudomallei and B. thailandensis specific. The growth rate of all B. pseudomallei isolates was determined by generating growth curves at 37 degrees C for 72 h. The isolates were found to differ in growth rates with doubling time varying from 1.5 to 2.3 h. In addition, the B. pseudomallei isolates exhibited considerable variation in colony morphology when grown on Ashdown media, brain-heart infusion agar, and Luria-Bertani agar over 9 days of observation. Antimicrobial susceptibility tests indicated that 80% of the isolates examined were Amp(R) Cb(R) Kn(R) Gm(R) Chl(S) Te(S). Virulence of the B. pseudomallei clinical and animal isolates was evaluated in B. pseudomallei-susceptible BALB/c mice. Most of the clinical isolates were highly virulent. However, virulence did not correlate with isolate origin since 2 of the animal isolates were also highly virulent.

Actin-based motility of intracellular bacteria, and polarized surface distribution of the bacterial effector molecules

Several intracellular bacterial pathogens, including species of Listeria, Rickettsia, Shigella, Mycobacteria, and Burkholderia, have evolved mechanisms to exploit the actin polymerization machinery of their hosts to induce actin-based motility, enabling these pathogens to spread between host cells without exposing themselves to the extracellular milieu. Efficient cell-to-cell spread requires directional motility, which the bacteria may achieve by concentrating the effector molecules at one pole of their cell body, thereby restricting polymerization of monomeric actin into actin tails to this pole. The study of the molecular processes involved in the initiation of actin tail formation at the bacterial surface, and subsequent actin-based motility, has provided much insight into the pathogenesis of infections caused by these bacteria and into the cell biology of actin dynamics. Concomitantly, this field of research has provided an opportunity to understand the mechanisms whereby bacteria can achieve a polarized distribution of surface proteins. This review will describe the process of actin-based motility of intracellular bacteria, and the mechanisms by which bacteria can obtain a polarized distribution of their surface proteins.

Expression, purification, crystallization and preliminary crystallographic analysis of BipD, a component of the Burkholderia pseudomallei type III secretion system

A construct consisting of residues 10-310 of BipD, a component of the Burkholderia pseudomallei type III secretion system (T3SS), has been overexpressed as a GST fusion, cleaved from the GST tag and purified. Crystals were grown of native and selenomethionine-labelled BipD. The crystals grow in two different polymorphs from the same condition. The first polymorph belongs to space group C222, with unit-cell parameters a = 103.98, b = 122.79, c = 49.17 A, a calculated Matthews coefficient of 2.4 A(3) Da(-1) (47% solvent content) and one molecule per asymmetric unit. The second polymorph belongs to space group P2(1)2(1)2, with unit-cell parameters a = 136.47, b = 89.84, c = 50.15 A, and a calculated Matthews coefficient of 2.3 A(3) Da(-1) (45% solvent content) for two molecules per asymmetric unit (analysis of the self-rotation function indicates the presence of a weak twofold non-crystallographic symmetry axis in this P2(1)2(1)2 form). The native crystals of both forms give diffraction data to 2.7 A resolution, while the SeMet-labelled P2(1)2(1)2 crystals diffract to 3.3 A resolution. A K(2)PtCl(4) derivative of the P2(1)2(1)2 form was also obtained and data were collected to 2.7 A with radiation of wavelength lambda = 0.933 A. The Pt-derivative anomalous difference Patterson map revealed two self-peaks on the Harker sections.

High resolution structure of BipD: an invasion protein associated with the type III secretion system of Burkholderia pseudomallei

Burkoldheria pseudomallei is a Gram-negative bacterium that possesses a protein secretion system similar to those found in Salmonella and Shigella. Recent work has indicated that the protein encoded by the BipD gene of B. pseudomallei is an important secreted virulence factor. BipD is similar in sequence to IpaD from Shigella and SipD from Salmonella and is therefore likely to be a translocator protein in the type-III secretion system of B. pseudomallei. The crystal structure of BipD has been solved at a resolution of 2.1 A revealing the detailed tertiary fold of the molecule. The overall structure is appreciably extended and consists of a bundle of antiparallel alpha-helical segments with two small beta-sheet regions. The longest helices of the molecule form a four-helix bundle and most of the remaining secondary structure elements (three helices and two three-stranded beta-sheets) are formed by the region linking the last two helices of the four-helix bundle. The structure suggests that the biologically active form of the molecule may be a dimer formed by contacts involving the C-terminal alpha-helix, which is the most strongly conserved part of the protein. Comparison of the structure of BipD with immunological and other data for IpaD indicates that the C-terminal alpha-helix is also involved in contacts with other proteins that form the translocon.

Identification of Burkholderia pseudomallei genes required for the intracellular life cycle and in vivo virulence

The bacterial pathogen Burkholderia pseudomallei invades host cells, escapes from endocytic vesicles, multiplies intracellularly, and induces the formation of actin tails and membrane protrusions, leading to direct cell-to-cell spreading. This study was aimed at the identification of B. pseudomallei genes responsible for the different steps of this intracellular life cycle. B. pseudomallei transposon mutants were screened for a reduced ability to form plaques on PtK2 cell monolayers as a result of reduced intercellular spreading. Nine plaque assay mutants with insertions in different open reading frames were selected for further studies. One mutant defective in a hypothetical protein encoded within the Bsa type III secretion system gene cluster was found to be unable to escape from endocytic vesicles after invasion but still multiplied within the vacuoles. Another mutant with a defect in a putative exported protein reached the cytoplasm but exhibited impaired actin tail formation in addition to a severe intracellular growth defect. In four mutants, the transposon had inserted into genes involved in either purine, histidine, or p-aminobenzoate biosynthesis, suggesting that these pathways are essential for intracellular growth. Three mutants with reduced plaque formation were shown to have gene defects in a putative cytidyltransferase, a putative lipoate-protein ligase B, and a hypothetical protein. All nine mutants proved to be significantly attenuated in a murine model of infection, with some mutants being essentially avirulent. In conclusion, we have identified a number of novel major B. pseudomallei virulence genes which are essential for the intracellular life cycle of this pathogen.

Melioidosis: insights into the pathogenicity of Burkholderia pseudomallei

Burkholderia pseudomallei is a potential bioterror agent and the causative agent of melioidosis, a severe disease that is endemic in areas of Southeast Asia and Northern Australia. Infection is often associated with bacterial dissemination to distant sites, and there are many possible disease manifestations, with melioidosis septic shock being the most severe. Eradication of the organism following infection is difficult, with a slow fever-clearance time, the need for prolonged antibiotic therapy and a high rate of relapse if therapy is not completed. Mortality from melioidosis septic shock remains high despite appropriate antimicrobial therapy. Prevention of disease and a reduction in mortality and the rate of relapse are priority areas for future research efforts. Studying how the disease is acquired and the host-pathogen interactions involved will underpin these efforts; this review presents an overview of current knowledge in these areas, highlighting key topics for evaluation.

Genetic and transcriptional analysis of the siderophore malleobactin biosynthesis and transport genes in the human pathogen Burkholderia pseudomallei K96243

Burkholderia pseudomallei is a gram-negative facultative intracellular pathogen that causes melioidosis, an invasive disease of humans and animals. To address the response of this bacterium to iron-limiting conditions, we first performed a global transcriptional analysis of RNA extracted from bacteria grown under iron-limiting and iron-rich conditions by microarrays. We focused our study on those open reading frames (ORFs) induced under iron limitation, which encoded predicted proteins that could be involved in the biosynthesis and uptake of the siderophore malleobactin. We purified this siderophore and determined that it consisted of at least three compounds with different molecular weights. We demonstrated that ORFs BPSL1776 and BPSL1774, designated mbaA and mbaF, respectively, are involved in the biosynthesis of malleobactin, while BPSL1775, named fmtA, is involved in its transport. These genes are in an operon with two other ORFs (mbaJ and mbaI) whose transcription is under the control of MbaS, a protein that belongs to the extracytoplasmic function sigma factors. Interestingly, the transcription of the mbaA, fmtA, and mbaS genes is not controlled by the availability of the siderophore malleobactin.

Actin-dependent movement of bacterial pathogens

Listeria, Rickettsia, Burkholderia, Shigella and Mycobacterium species subvert cellular actin dynamics to facilitate their movement within the host cytosol and to infect neighbouring cells while evading host immune surveillance and promoting their intracellular survival. 'Attaching and effacing' Escherichia coli do not enter host cells but attach intimately to the cell surface, inducing motile actin-rich pedestals, the function of which is currently unclear. The molecular basis of actin-based motility of these bacterial pathogens reveals novel insights about bacterial pathogenesis and fundamental host-cell pathways.

Who's talking to whom? Epithelial-bacterial pathogen interactions

Our perception that host-bacterial interactions lead to disease comes from rare, unsuccessful interactions resulting in the development of detectable symptoms. In contrast, the majority of host-bacterial interactions go unnoticed as the host and bacteria perceive each other to be no threat. In July 2004, a focused international symposium on epithelial-bacterial pathogen interactions was held in Newcastle upon Tyne (UK). The symposium concentrated on recent advances in our understanding of bacterial interactions at respiratory and gastrointestinal mucosal epithelial layers. For the host these epithelial tissues represent a first line of defence against invading bacterial pathogens. Through the discovery that the innate immune system plays a pivotal role during host-bacterial interactions, it has become clear that epithelia are being utilized by the host to monitor or communicate with both pathogenic and commensal bacteria. Interest in understanding the bacterial perspective of these interactions has lead researchers to realize that the bacteria utilize the same factors associated with disease to establish successful long-term interactions. Here we discuss several common themes and concepts that emerged from recent studies that have allowed physiologists and microbiologists to interact at a common interface similar to their counterparts -- epithelia and bacterial pathogens. These studies highlight the need for further multidisciplinary studies into how the host differentiates between pathogenic and commensal bacteria.

Hydrodynamic stability of helical growth at low Reynolds number

A cylindrical object growing at a low Reynolds number can spontaneously develop a helical shape. We have studied this phenomenon numerically, and our results may shed some light on the spontaneous formation of helical tails of a dense protein network observed in experiments on actin based motility. We also identify an unstable critical pitch angle which separates helices that straighten into rods from helices that flatten into planar curves as they grow. At the critical angle the pitch angle remains constant, whereas both helical diameter and pitch increase with the helical contour length.

Identification of a bacterial factor required for actin-based motility of Burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative facultative intracellular pathogen that enters and escapes from eukaryotic cells using the power of actin polymerization. We have identified a bacterial protein (BimA) that is required for the ability of B. pseudomallei to induce the formation of actin tails. BimA contains proline-rich motifs and WH2-like domains and shares limited homology at the C-terminus with the Yersinia autosecreted adhesin YadA. BimA is located at the pole of the bacterial cell at which actin polymerization occurs and mutation of bimA abolished actin-based motility of the pathogen in J774.2 cells. Transient expression of BimA in HeLa cells resulted in F-actin clustering reminiscent of that seen on WASP overexpression. Antibody-mediated clustering of a CD32 chimera in which the cytoplasmic domain was replaced with BimA resulted in localization of the chimera to the tips of F-actin enriched membrane protrusions. We report that purified truncated BimA protein binds monomeric actin in a concentration-dependent manner in cosedimentation assays and that BimA stimulates actin polymerization in vitro in a manner independent of the cellular Arp2/3 complex.

Attenuated virulence and protective efficacy of a Burkholderia pseudomallei bsa type III secretion mutant in murine models of melioidosis

Melioidosis is a severe infectious disease of animals and humans caused by the Gram-negative intracellular pathogen Burkholderia pseudomallei. An Inv/Mxi-Spa-like type III protein secretion apparatus, encoded by the B. pseudomallei bsa locus, facilitates bacterial invasion of epithelial cells, escape from endocytic vesicles and intracellular survival. This study investigated the role of the Bsa type III secretion system in the pathogenesis of melioidosis in murine models. B. pseudomallei bipD mutants, lacking a component of the translocation apparatus, were found to be significantly attenuated following intraperitoneal or intranasal challenge of BALB/c mice. Furthermore, a bipD mutant was attenuated in C57BL/6 IL-12 p40(-/-) mice, which are highly susceptible to B. pseudomallei infection. Mutation of bipD impaired bacterial replication in the liver and spleen of BALB/c mice in the early stages of infection. B. pseudomallei mutants lacking either the type III secreted guanine nucleotide exchange factor BopE or the putative effectors BopA or BopB exhibited varying degrees of attenuation, with mutations in bopA and bopB causing a significant delay in median time to death. This indicates that bsa-encoded type III secreted proteins may act in concert to determine the outcome of B. pseudomallei infection in mice. Mice inoculated with the B. pseudomallei bipD mutant were partially protected against subsequent challenge with wild-type B. pseudomallei. However, immunization of mice with purified BipD protein was not protective.