Inactivation of Burkholderia pseudomallei bsaQ results in decreased invasion efficiency and delayed escape of bacteria from endocytic vesicles

A call for standardization: Evaluating different methodologies to induce in vitro foreign body giant cell formation for biomaterials research and design

Foreign body giant cells (FBGCs) are crucial in the foreign body reaction at the biomaterial-tissue interface, forming through the fusion of cells from the monocyte/macrophage lineage and performing functions such as material degradation and fibrous encapsulation. Yet, their presence and role in biomaterials research is only slowly unveiled. This review analyzed existing FBGC literature identified through a search string and sources from FBGC articles to evaluate the most commonly used methods and highlight the challenges in establishing a standardized protocol. Our findings revealed a fragmented research landscape marked by significant variability in in vitro culture conditions, i.e., cell origin and type, culture media and sera, fusion-inducing factors, seeding density, culture surface, and inconsistencies in the read-outs. This complicates efforts toward standardization and hampers cross-study comparisons. Based on these results, we highlight the need and propose guidelines for standardized culture protocols for FBGC research. Overall, this review aims to underscore the relevance of improving reproducibility and reliability in FBGC research, facilitating effective cross-study comparisons and advancing understanding of FBGC formation and function, ultimately contributing to designing more effective biomaterial-based therapies. STATEMENT OF SIGNIFICANCE: Foreign body giant cells (FBGCs) are crucial in the body's response to implanted biomaterials. Yet, current research addressing their role and impact is highly fragmented. This review comprehensively and systematically examines the diverse methodologies and definitions used in FBGC research and identifies critical gaps and inconsistencies hindering the reproducibility and comparison of findings. By advocating for standardized protocols, we aim to enhance the reliability and equivalence of research, thus providing a stronger foundation for understanding biomaterial-driven FBGC formation and function. Establishing such a framework will impact biomaterial-based therapies, supporting their effectiveness and safety in medical applications, and is thus of relevance for scientists, companies, and clinicians in the biomaterial and medical device communities.

Burkholderia pseudomallei BopE suppresses the Rab32-dependent defense pathway to promote its intracellular replication and virulence

Melioidosis is a serious infectious disease caused by the Gram-negative bacterium Burkholderia pseudomallei. Recently, Rab32-dependent immune vesicles emerge as a critical defense pathway to restrict the intracellular B. pseudomallei. However, B. pseudomallei can evade host immune vesicles and survive in the cytoplasm, although this mechanism is not well understood. In this study, we found Rab32-dependent vesicles could effectively combat B. pseudomallei infection, but not all intracellular B. pseudomallei were encapsulated in Rab32-positive vesicles. To explore how B. pseudomallei counteracted the Rab32-dependent defense pathway, transcriptomic profiling of B. pseudomallei was performed to characterize the response dynamics during infection. We found that the type III secretion system of B. pseudomallei was activated, and a variety of effector proteins were highly upregulated. Among them, BopE, BprD, and BipC were shown to interact with Rab32. Interestingly, BopE directly interacts with host Rab32, potentially suppressing Rab32 function by interfering with nucleotide exchange, which in turn restricts the recruitment of Rab32 to bacterial-containing vesicles. Knocking out of BopE can increase the proportion of Rab32-positive vesicles, suppressing the intracellular replication and virulence of B. pseudomallei. Collectively, our findings have demonstrated that BopE may be an important effector for B. pseudomallei to evade from the Rab32-dependent killing vesicles into the cytosol for survival and replication. Therefore, a deeper understanding of the interaction between BopE and the host Rab32-dependent restriction pathway may provide an effective therapeutic strategy for the elimination of intracellular B. pseudomallei.IMPORTANCEB. pseudomallei is facultative intracellular bacterium that has evolved numerous strategies to evade host immune vesicles and survive in the cytoplasm. Rab32-dependent vesicles are one of these immune vesicles, but the mechanism by which B. pseudomallei escape Rab32-dependent vesicles remains elusive. Here, we find B. pseudomallei infection leading the activation of the type III secretion system (T3SS-3) and increasing the expression of various effectors. Specifically, we identify that BopE, an effector secreted by T3SS-3, triggers vesicle escape to promote B. pseudomallei pathogenicity and survival. Mechanistically, BopE suppresses the activation of Rab32 by interfering with nucleotide exchange, ultimately triggering vesicle escape and intracellular survival. We also find knocking out the bopE gene can increase the proportion of Rab32-positive vesicles that trap B. pseudomallei, dampening the survival of B. pseudomallei both in vitro and in vivo. Taken together, our findings provide insights into the molecular mechanisms of pathogen effector-induced vesicle escape, indicating a potential melioidosis treatment via blocking B. pseudomallei BopE-host Rab32 interaction.

Transitioning from Soil to Host: Comparative Transcriptome Analysis Reveals the Burkholderia pseudomallei Response to Different Niches

Burkholderia pseudomallei, a soil and water saprophyte, is responsible for the tropical human disease melioidosis. A hundred years since its discovery, there is still much to learn about B. pseudomallei proteins that are essential for the bacterium's survival in and interaction with the infected host, as well as their roles within the bacterium's natural soil habitat. To address this gap, bacteria grown under conditions mimicking the soil environment were subjected to transcriptome sequencing (RNA-seq) analysis. A dual RNA-seq approach was used on total RNA from spleens isolated from a B. pseudomallei mouse infection model at 5 days postinfection. Under these conditions, a total of 1,434 bacterial genes were induced, with 959 induced in the soil environment and 475 induced in bacteria residing within the host. Genes encoding metabolism and transporter proteins were induced when the bacteria were present in soil, while virulence factors, metabolism, and bacterial defense mechanisms were upregulated during active infection of mice. On the other hand, capsular polysaccharide and quorum-sensing pathways were inhibited during infection. In addition to virulence factors, reactive oxygen species, heat shock proteins, siderophores, and secondary metabolites were also induced to assist bacterial adaptation and survival in the host. Overall, this study provides crucial insights into the transcriptome-level adaptations which facilitate infection by soil-dwelling B. pseudomallei. Targeting novel therapeutics toward B. pseudomallei proteins required for adaptation provides an alternative treatment strategy given its intrinsic antimicrobial resistance and the absence of a vaccine. IMPORTANCE Burkholderia pseudomallei, a soil-dwelling bacterium, is the causative agent of melioidosis, a fatal infectious disease of humans and animals. The bacterium has a large genome consisting of two chromosomes carrying genes that encode proteins with important roles for survival in diverse environments as well as in the infected host. While a general mechanism of pathogenesis has been proposed, it is not clear which proteins have major roles when the bacteria are in the soil and whether the same proteins are key to successful infection and spread. To address this question, we grew the bacteria in soil medium and then in infected mice. At 5 days postinfection, bacteria were recovered from infected mouse organs and their gene expression was compared against that of bacteria grown in soil medium. The analysis revealed a list of genes expressed under soil growth conditions and a different set of genes encoding proteins which may be important for survival, replication, and dissemination in an infected host. These proteins are a potential resource for understanding the full adaptation mechanism of this pathogen. In the absence of a vaccine for melioidosis and with treatment being reliant on combinatorial antibiotic therapy, these proteins may be ideal targets for designing antimicrobials to treat melioidosis.

Vitamin D (1α,25(OH)2D3) supplementation minimized multinucleated giant cells formation and inflammatory response during Burkholderia pseudomallei infection in human lung epithelial cells

Melioidosis is an infectious disease with high mortality rates in human, caused by the bacterium Burkholderia pseudomallei. As an intracellular pathogen, B. pseudomallei can escape from the phagosome and induce multinucleated giant cells (MNGCs) formation resulting in antibiotic resistance and immune evasion. A novel strategy to modulate host response against B. pseudomallei pathogenesis is required. In this study, an active metabolite of vitamin D3 (1α,25-dihydroxyvitamin D3 or 1α,25(OH)2D3) was selected to interrupt pathogenesis of B. pseudomallei in a human lung epithelium cell line, A549. The results demonstrated that pretreatment with 10-6 M 1α,25(OH)2D3 could reduce B. pseudomallei internalization to A549 cells at 4 h post infection (P < 0.05). Interestingly, the presence of 1α,25(OH)2D3 gradually reduced MNGC formation at 8, 10 and 12 h compared to that of the untreated cells (P < 0.05). Furthermore, pretreatment with 10-6 M 1α,25(OH)2D3 considerably increased hCAP-18/LL-37 mRNA expression (P < 0.001). Additionally, pro-inflammatory cytokines, including MIF, PAI-1, IL-18, CXCL1, CXCL12 and IL-8, were statistically decreased (P < 0.05) in 10-6 M 1α,25(OH)2D3-pretreated A549 cells by 12 h post-infection. Taken together, this study indicates that pretreatment with 10-6 M 1α,25(OH)2D3 has the potential to reduce the internalization of B. pseudomallei into host cells, decrease MNGC formation and modulate host response during B. pseudomallei infection by minimizing the excessive inflammatory response. Therefore, 1α,25(OH)2D3 supplement may provide an effective supportive treatment for melioidosis patients to combat B. pseudomallei infection and reduce inflammation in these patients.

Pathogenicity and virulence of Burkholderia pseudomallei

The soil saprophyte, Burkholderia pseudomallei, is the causative agent of melioidosis, a disease endemic in South East Asia and northern Australia. Exposure to B. pseudomallei by either inhalation or inoculation can lead to severe disease. B. pseudomallei rapidly shifts from an environmental organism to an aggressive intracellular pathogen capable of rapidly spreading around the body. The expression of multiple virulence factors at every stage of intracellular infection allows for rapid progression of infection. Following invasion or phagocytosis, B. pseudomallei resists host-cell killing mechanisms in the phagosome, followed by escape using the type III secretion system. Several secreted virulence factors manipulate the host cell, while bacterial cells undergo a shift in energy metabolism allowing for overwhelming intracellular replication. Polymerisation of host cell actin into “actin tails” propels B. pseudomallei to the membranes of host cells where the type VI secretion system fuses host cells into multinucleated giant cells (MNGCs) to facilitate cell-to-cell dissemination. This review describes the various mechanisms used by B. pseudomallei to survive within cells.

Cycle-Inhibiting Factor Is Associated with Burkholderia pseudomallei Invasion in Human Neuronal Cells

Burkholderia pseudomallei is a pathogenic bacterium that causes human melioidosis, which is associated with a high mortality rate. However, the underlying mechanisms of B. pseudomallei pathogenesis are largely unknown. In this study, we examined the infection of human neuronal SH-Sy5y cells by several clinically relevant B. pseudomallei strains. We found that all tested B. pseudomallei strains can invade SH-Sy5y cells, undergo intracellular replication, cause actin-tail formation, and form multinucleated giant cells. Additionally, a deletion mutant of B. pseudomallei cycle-inhibiting factor (cif) was constructed that exhibited reduced invasion in SH-Sy5y cells. Complementation of cif restored invasion of the B. pseudomallei cif-deleted mutant. Our findings enhance understanding of B. pseudomallei pathogenicity in terms of the virulence factor Cif and demonstrate the function of Cif in neurological melioidosis. This may eventually lead to the discovery of novel targets for treatment and a strategy to control the disease.

LC3-Associated Phagocytosis in Bacterial Infection

LC3-associated phagocytosis (LAP) is a noncanonical autophagy process reported in recent years and is one of the effective mechanisms of host defense against bacterial infection. During LAP, bacteria are recognized by pattern recognition receptors (PRRs), enter the body, and then recruit LC3 onto a single-membrane phagosome to form a LAPosome. LC3 conjugation can promote the fusion of the LAPosomes with lysosomes, resulting in their maturation into phagolysosomes, which can effectively kill the identified pathogens. However, to survive in host cells, bacteria have also evolved strategies to evade killing by LAP. In this review, we summarized the mechanism of LAP in resistance to bacterial infection and the ways in which bacteria escape LAP. We aim to provide new clues for developing novel therapeutic strategies for bacterial infectious diseases.

Burkholderia pseudomallei pathogenesis in human skin fibroblasts: A Bsa type III secretion system is involved in the invasion, multinucleated giant cell formation, and cellular damage

Burkholderia pseudomallei-a causative agent of melioidosis that is endemic in Southeast Asia and Northern Australia-is a Gram-negative bacterium transmitted to humans via inhalation, inoculation through skin abrasions, and ingestion. Melioidosis causes a range of clinical presentations including skin infection, pneumonia, and septicemia. Despite skin infection being one of the clinical symptoms of melioidosis, the pathogenesis of B. pseudomallei in skin fibroblasts has not yet been elucidated. In this study, we investigated B. pseudomallei pathogenesis in the HFF-1 human skin fibroblasts. On the basis of co-culture assays between different B. pseudomallei clinical strains and the HFF-1 human skin fibroblasts, we found that all B. pseudomallei strains have the ability to mediate invasion, intracellular replication, and multinucleated giant cell (MNGC) formation. Furthermore, all strains showed a significant increase in cytotoxicity in human fibroblasts, which coincides with the augmented expression of matrix metalloproteinase-2. Using B. pseudomallei mutants, we showed that the B. pseudomallei Bsa type III secretion system (T3SS) contributes to skin fibroblast pathogenesis, but O-polysaccharide, capsular polysaccharide, and short-chain dehydrogenase metabolism do not play a role in this process. Taken together, our findings reveal a probable connection for the Bsa T3SS in B. pseudomallei infection of skin fibroblasts, and this may be linked to the pathogenesis of cutaneous melioidosis.

Genetic Diversity of Type 3 Secretion System in Burkholderia s.l. and Links With Plant Host Adaptation

Burkholderia sensu lato species are prominent for their diversity of hosts. The type 3 secretion system (T3SS) is a major mechanism impacting the interactions between bacteria and eukaryotic hosts. Besides the human pathogenic species Burkholderia pseudomallei and closely affiliated species, the T3SS has received little attention in this genus as in taxonomically and evolutionary close genera Paraburkholderia, Caballeronia, Trinickia, and Mycetohabitans. We proceeded to identify and characterize the diversity of T3SS types using the genomic data from a subset of 145 strains representative of the species diversity found in the Burkholderia s.l. group. Through an analysis of their phylogenetic distribution, we identified two new T3SS types with an atypical chromosomal organization and which we propose to name BCI (Burkholderia cepacia complex Injectisome) and PSI (Paraburkholderia Short Injectisome). BCI is the dominant T3SS type found in Burkholderia sensu stricto (s.s.) species and PSI is mostly restricted to the Paraburkholderia genus. By correlating their distribution with the ecology of their strains of origin, we propose a role in plant interaction for these T3SS types. Experimentally, we demonstrated that a BCI deficient B. vietnamiensis LMG10929 mutant was strongly affected in its rice colonization capacity.

At the Gate of Mutualism: Identification of Genomic Traits Predisposing to Insect-Bacterial Symbiosis in Pathogenic Strains of the Aphid Symbiont Serratia symbiotica

Mutualistic associations between insects and heritable bacterial symbionts are ubiquitous in nature. The aphid symbiont Serratia symbiotica is a valuable candidate for studying the evolution of bacterial symbiosis in insects because it includes a wide diversity of strains that reflect the diverse relationships in which bacteria can be engaged with insects, from pathogenic interactions to obligate intracellular mutualism. The recent discovery of culturable strains, which are hypothesized to resemble the ancestors of intracellular strains, provide an opportunity to study the mechanisms underlying bacterial symbiosis in its early stages. In this study, we analyzed the genomes of three of these culturable strains that are pathogenic to aphid hosts, and performed comparative genomic analyses including mutualistic host-dependent strains. All three genomes are larger than those of the host-restricted S. symbiotica strains described so far, and show significant enrichment in pseudogenes and mobile elements, suggesting that these three pathogenic strains are in the early stages of the adaptation to their host. Compared to their intracellular mutualistic relatives, the three strains harbor a greater diversity of genes coding for virulence factors and metabolic pathways, suggesting that they are likely adapted to infect new hosts and are a potential source of metabolic innovation for insects. The presence in their genomes of secondary metabolism gene clusters associated with the production of antimicrobial compounds and phytotoxins supports the hypothesis that S. symbiotia symbionts evolved from plant-associated strains and that plants may serve as intermediate hosts. Mutualistic associations between insects and bacteria are the result of independent transitions to endosymbiosis initiated by the acquisition of environmental progenitors. In this context, the genomes of free-living S. symbiotica strains provide a rare opportunity to study the inventory of genes held by bacterial associates of insects that are at the gateway to a host-dependent lifestyle.

Functional redundancy of Burkholderia pseudomallei phospholipase C enzymes and their role in virulence

Phospholipase C (PLC) enzymes are key virulence factors in several pathogenic bacteria. Burkholderia pseudomallei, the causative agent of melioidosis, possesses at least three plc genes (plc1, plc2 and plc3). We found that in culture medium plc1 gene expression increased with increasing pH, whilst expression of the plc3 gene was pH (4.5 to 9.0) independent. Expression of the plc2 gene was not detected in culture medium. All three plc genes were expressed during macrophage infection by B. pseudomallei K96243. Comparing B. pseudomallei wild-type with plc mutants revealed that plc2, plc12 or plc123 mutants showed reduced intracellular survival in macrophages and reduced plaque formation in HeLa cells. However, plc1 or plc3 mutants showed no significant differences in plaque formation compared to wild-type bacteria. These findings suggest that Plc2, but not Plc1 or Plc3 are required for infection of host cells. In Galleria mellonella, plc1, plc2 or plc3 mutants were not attenuated compared to the wild-type strain, but multiple plc mutants showed reduced virulence. These findings indicate functional redundancy of the B. pseudomallei phospholipases in virulence.

In silico prediction of host-pathogen protein interactions in melioidosis pathogen Burkholderia pseudomallei and human reveals novel virulence factors and their targets

The aerobic, Gram-negative motile bacillus, Burkholderia pseudomallei is a facultative intracellular bacterium causing melioidosis, a critical disease of public health importance, which is widely endemic in the tropics and subtropical regions of the world. Melioidosis is associated with high case fatality rates in animals and humans; even with treatment, its mortality is 20-50%. It also infects plants and is designated as a biothreat agent. B. pseudomallei is pathogenic due to its ability to invade, resist factors in serum and survive intracellularly. Despite its importance, to date only a few effector proteins have been functionally characterized, and there is not much information regarding the host-pathogen protein-protein interactions (PPI) of this system, which are important to studying infection mechanisms and thereby develop prevention measures. We explored two computational approaches, the homology-based interolog and the domain-based method, to predict genome-scale host-pathogen interactions (HPIs) between two different strains of B. pseudomallei (prototypical, and highly virulent) and human. In total, 76 335 common HPIs (between the two strains) were predicted involving 8264 human and 1753 B. pseudomallei proteins. Among the unique PPIs, 14 131 non-redundant HPIs were found to be unique between the prototypical strain and human, compared to 3043 non-redundant HPIs between the highly virulent strain and human. The protein hubs analysis showed that most B. pseudomallei proteins formed a hub with human dnaK complex proteins associated with tuberculosis, a disease similar in symptoms to melioidosis. In addition, drug-binding and carbohydrate-binding mechanisms were found overrepresented within the host-pathogen network, and metabolic pathways were frequently activated according to the pathway enrichment. Subcellular localization analysis showed that most of the pathogen proteins are targeting human proteins inside cytoplasm and nucleus. We also discovered the host targets of the drug-related pathogen proteins and proteins that form T3SS and T6SS in B. pseudomallei. Additionally, a comparison between the unique PPI patterns present in the prototypical and highly virulent strains was performed. The current study is the first report on developing a genome-scale host-pathogen protein interaction networks between the human and B. pseudomallei, a critical biothreat agent. We have identified novel virulence factors and their interacting partners in the human proteome. These PPIs can be further validated by high-throughput experiments and may give new insights on how B. pseudomallei interacts with its host, which will help medical researchers in developing better prevention measures.

Burkholderia pseudomallei pathogenesis and survival in different niches

Burkholderia pseudomallei (Bp) is the causative agent of melioidosis, a disease of the tropics with high clinical mortality rates. To date, no vaccines are approved for melioidosis and current treatment relies on antibiotics. Conversely, common misdiagnosis and high pathogenicity of Bp hamper efforts to fight melioidosis. This bacterium can be isolated from a wide range of niches such as waterlogged fields, stagnant water bodies, salt water bodies and from human and animal clinical specimens. Although extensive studies have been undertaken to elucidate pathogenesis mechanisms of Bp, little is known about how a harmless soil bacterium adapts to different environmental conditions, in particular, the shift to a human host to become a highly virulent pathogen. The bacterium has a large genome encoding an armory of factors that assist the pathogen in surviving under stressful conditions and assuming its role as a deadly intracellular pathogen. This review presents an overview of what is currently known about how the pathogen adapts to different environments. With in-depth understanding of Bp adaptation and survival, more effective therapies for melioidosis can be developed by targeting related genes or proteins that play a major role in the bacteria's survival.

Burkholderia pseudomallei BimC Is Required for Actin-Based Motility, Intracellular Survival, and Virulence

The intracellular pathogen Burkholderia pseudomallei, the etiological agent of melioidosis in humans and various animals, is capable of survival and movement within the cytoplasm of host cells by a process known as actin-based motility. The bacterial factor BimA is required for actin-based motility through its direct interaction with actin, and by mediating actin polymerization at a single pole of the bacterium to promote movement both within and between cells. However, little is known about the other bacterial proteins required for this process. Here, we have investigated the role of the bimC gene (bpss1491) which lies immediately upstream of the bimA gene (bpss1492) on the B. pseudomallei chromosome 2. Conserved amongst all B. pseudomallei, B. mallei and B. thailandensis strains sequenced to date, this gene encodes an iron-binding protein with homology to a group of proteins known as the bacterial autotransporter heptosyltransferase (BAHT) family. We have constructed a B. pseudomallei bimC deletion mutant and demonstrate that it is defective in intracellular survival in HeLa cells, but not in J774.1 macrophage-like cells. The bimC mutant is defective in cell to cell spread as demonstrated by ablation of plaque formation in HeLa cells, and by the inability to form multi-nucleated giant cells in J774.1 cells. These phenotypes in intracellular survival and cell to cell spread are not due to the loss of expression and polar localization of the BimA protein on the surface of intracellular bacteria, however they do correlate with an inability of the bacteria to recruit and polymerize actin. Furthermore, we also establish a role for bimC in virulence of B. pseudomallei using a Galleria mellonella larvae model of infection. Taken together, our findings indicate that B. pseudomallei BimC plays an important role in intracellular behavior and virulence of this emerging pathogen.

The Burkholderia Type VI Secretion System 5: Composition, Regulation and Role in Virulence

The soil saprophyte and Tier I select agent Burkholderia pseudomallei can cause rapidly fatal infections in humans and animals. The capability of switching to an intracellular life cycle during infection appears to be a decisive trait of B. pseudomallei for causing disease. B. pseudomallei harbors multiple type VI secretion systems (T6SSs) orthologs of which are present in the surrogate organism Burkholderia thailandensis. Upon host cell entry and vacuolar escape into the cytoplasm, B. pseudomallei and B. thailandensis manipulate host cells by utilizing the T6SS-5 (also termed T6SS1) to form multinucleated giant cells for intercellular spread. Disruption of the T6SS-5 in B. thailandensis causes a drastic attenuation of virulence in wildtype but not in mice lacking the central innate immune adapter protein MyD88. This result suggests that the T6SS-5 is deployed by the bacteria to overcome innate immune responses. However, important questions in this field remain unsolved including the mechanism underlying T6SS-5 activity and its physiological role during infection. In this review, we summarize the current knowledge on the components and regulation of the T6SS-5 as well as its role in virulence in mammalian hosts.

Inflammasomes, Autophagy, and Cell Death: The Trinity of Innate Host Defense against Intracellular Bacteria

Inflammasome activation is an innate host defense mechanism initiated upon sensing pathogens or danger in the cytosol. Both autophagy and cell death are cell autonomous processes important in development, as well as in host defense against intracellular bacteria. Inflammasome, autophagy, and cell death pathways can be activated by pathogens, pathogen-associated molecular patterns (PAMPs), cell stress, and host-derived damage-associated molecular patterns (DAMPs). Phagocytosis and toll-like receptor (TLR) signaling induce reactive oxygen species (ROS), type I IFN, NFκB activation of proinflammatory cytokines, and the mitogen-activated protein kinase cascade. ROS and IFNγ are also prominent inducers of autophagy. Pathogens, PAMPs, and DAMPs activate TLRs and intracellular inflammasomes, inducing apoptotic and inflammatory caspases in a context-dependent manner to promote various forms of cell death to eliminate pathogens. Common downstream signaling molecules of inflammasomes, autophagy, and cell death pathways interact to initiate appropriate measures against pathogens and determine host survival as well as pathological consequences of infection. The integration of inflammasome activation, autophagy, and cell death is central to pathogen clearance. Various pathogens produce virulence factors to control inflammasomes, subvert autophagy, and modulate host cell death in order to evade host defense. This review highlights the interaction of inflammasomes, autophagy, and host cell death pathways in counteracting Burkholderia pseudomallei, the causative agent of melioidosis. Contrasting evasion strategies used by B. pseudomallei, Mycobacterium tuberculosis, and Legionella pneumophila to avoid and dampen these innate immune responses will be discussed.

Proteomic Profiling of Burkholderia thailandensis During Host Infection Using Bio-Orthogonal Noncanonical Amino Acid Tagging (BONCAT)

Burkholderia pseudomallei and B. mallei are the causative agents of melioidosis and glanders, respectively, and are often fatal to humans and animals. Owing to the high fatality rate, potential for spread by aerosolization, and the lack of efficacious therapeutics, B. pseudomallei and B. mallei are considered biothreat agents of concern. In this study, we investigate the proteome of Burkholderia thailandensis, a closely related surrogate for the two more virulent Burkholderia species, during infection of host cells, and compare to that of B. thailandensis in culture. Studying the proteome of Burkholderia spp. during infection is expected to reveal molecular mechanisms of intracellular survival and host immune evasion; but proteomic profiling of Burkholderia during host infection is challenging. Proteomic analyses of host-associated bacteria are typically hindered by the overwhelming host protein content recovered from infected cultures. To address this problem, we have applied bio-orthogonal noncanonical amino acid tagging (BONCAT) to B. thailandensis, enabling the enrichment of newly expressed bacterial proteins from virtually any growth condition, including host cell infection. In this study, we show that B. thailandensis proteins were selectively labeled and efficiently enriched from infected host cells using BONCAT. We also demonstrate that this method can be used to label bacteria in situ by fluorescent tagging. Finally, we present a global proteomic profile of B. thailandensis as it infects host cells and a list of proteins that are differentially regulated in infection conditions as compared to bacterial monoculture. Among the identified proteins are quorum sensing regulated genes as well as homologs to previously identified virulence factors. This method provides a powerful tool to study the molecular processes during Burkholderia infection, a much-needed addition to the Burkholderia molecular toolbox.

Burkholderia pseudomallei Adaptation for Survival in Stressful Conditions

Burkholderia pseudomallei is a Gram-negative bacterium that causes melioidosis, which can be fatal in humans. Melioidosis is prevalent in the tropical regions of Southeast Asia and Northern Australia. Ecological data have shown that this bacterium can survive as a free-living organism in environmental niches, such as soil and water, as well as a parasite living in host organisms, such as ameba, plants, fungi, and animals. This review provides an overview of the survival and adaptation of B. pseudomallei to stressful conditions induced by hostile environmental factors, such as salinity, oxidation, and iron levels. The adaptation of B. pseudomallei in host cells is also reviewed. The adaptive survival mechanisms of this pathogen mainly involve modulation of gene and protein expression, which could cause alterations in the bacteria's cell membrane, metabolism, and virulence. Understanding the adaptations of this organism to environmental factors provides important insights into the survival and pathogenesis of B. pseudomallei, which may lead to the development of novel strategies for the control, prevention, and treatment of melioidosis in the future.

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.

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

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

BipC, a Predicted Burkholderia pseudomallei Type 3 Secretion System Translocator Protein with Actin Binding Activity

Burkholderia pseudomallei is an intracellular bacterial pathogen and the causative agent of melioidosis, a severe disease of humans and animals. Like other clinically important Gram-negative bacteria, fundamental to B. pseudomallei pathogenesis is the Bsa Type III Secretion System. The Bsa system injects bacterial effector proteins into the cytoplasm of target host cells subverting cellular pathways for the benefit of the bacteria. It is required for invasion of non-phagocytic host cells, escape from the endocytic compartment into the host cell cytoplasm, and for virulence in murine models of melioidosis. We have recently described the repertoire of effector proteins secreted by the B. pseudomallei Bsa system, however the functions of many of these effector proteins remain an enigma. One such protein is BipC, a homolog of the translocator/effector proteins SipC and IpaC from Salmonella spp. and Shigella flexneri respectively. SipC and IpaC each have separate and distinct roles acting both as translocators, involved in creating a pore in the eukaryotic cell membrane through which effector proteins can transit, and as effectors by interacting with and polymerizing host cell actin. In this study, pull-down assays demonstrate an interaction between BipC and actin. Furthermore, we show that BipC directly interacts with actin, preferentially with actin polymers (F-actin) and has the ability to polymerize actin in a similar manner as that described for SipC. Yet unlike SipC, BipC does not stabilize F-actin filaments, indicating a functionally distinct interaction with actin. Expression of Myc-tagged BipC in HeLa cells induces the formation of pseudopodia similar to that seen for IpaC. This study explores the effector function of BipC and reveals that actin interaction is conserved within the BipC/SipC/IpaC family of translocator/effector proteins.

Effects of sodium chloride on heat resistance, oxidative susceptibility, motility, biofilm and plaque formation of Burkholderia pseudomallei

Burkholderia pseudomallei is an environmental saprophyte and the causative agent of melioidosis, a severe infectious disease prevalent in tropical areas, including southeast Asia and northern Australia. In Thailand, the highest incidence of melioidosis is in the northeast region, where saline soil and water are abundant. We hypothesized that B. pseudomallei develops an ability to thrive in saline conditions and gains a selective ecological advantage over other soil-dwelling microorganisms. However, little is known about how an elevated NaCl concentration affects survival and adaptive changes in this pathogen. In this study, we examined the adaptive changes in six isolates of B. pseudomallei after growth in Luria-Bertani medium containing different concentrations of NaCl at 37°C for 6 hr. The bacteria were then investigated for resistance to heat at 50°C and killing by hydrogen peroxide (H₂ O₂ ). In addition, flagellar production, biofilm formation, and the plaque formation efficiency of B. pseudomallei after culture in saline conditions were observed. In response to exposure to 150 and 300 mmol L^-1 NaCl, all B. pseudomallei isolates showed significantly increased thermal tolerance, oxidative resistance, and plaque-forming efficiency. However, NaCl exposure notably decreased the number of B. pseudomallei flagella. Taken together, these results provide insight into the adaptations of B. pseudomallei that might be crucial for survival and persistence in the host and/or endemic environments with high salinity.

Type III Secretion in the Melioidosis Pathogen Burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative intracellular pathogen and the causative agent of melioidosis, a severe disease of both humans and animals. Melioidosis is an emerging disease which is predicted to be vastly under-reported. Type III Secretion Systems (T3SSs) are critical virulence factors in Gram negative pathogens of plants and animals. The genome of B. pseudomallei encodes three T3SSs. T3SS-1 and -2, of which little is known, are homologous to Hrp2 secretion systems of the plant pathogens Ralstonia and Xanthomonas. T3SS-3 is better characterized and is homologous to the Inv/Mxi-Spa secretion systems of Salmonella spp. and Shigella flexneri, respectively. Upon entry into the host cell, B. pseudomallei requires T3SS-3 for efficient escape from the endosome. T3SS-3 is also required for full virulence in both hamster and murine models of infection. The regulatory cascade which controls T3SS-3 expression and the secretome of T3SS-3 have been described, as well as the effect of mutations of some of the structural proteins. Yet only a few effector proteins have been functionally characterized to date and very little work has been carried out to understand the hierarchy of assembly, secretion and temporal regulation of T3SS-3. This review aims to frame current knowledge of B. pseudomallei T3SSs in the context of other well characterized model T3SSs, particularly those of Salmonella and Shigella.

PKC-η-MARCKS Signaling Promotes Intracellular Survival of Unopsonized Burkholderia thailandensis

Pathogenic Burkholderia rely on host factors for efficient intracellular replication and are highly refractory to antibiotic treatment. To identify host genes that are required by Burkholderia spp. during infection, we performed a RNA interference (RNAi) screen of the human kinome and identified 35 host kinases that facilitated Burkholderia thailandensis intracellular survival in human monocytic THP-1 cells. We validated a selection of host kinases using imaging flow cytometry to assess efficiency of B. thailandensis survival in the host upon siRNA-mediated knockdown. We focused on the role of the novel protein kinase C isoform, PKC-η, in Burkholderia infection and characterized PKC-η/MARCKS signaling as a key event that promotes the survival of unopsonized B. thailandensis CDC2721121 within host cells. While infection of lung epithelial cells with unopsonized Gram-negative bacteria stimulated phosphorylation of Ser175/160 in the MARCKS effector domain, siRNA-mediated knockdown of PKC-η expression reduced the levels of phosphorylated MARCKS by >3-fold in response to infection with Bt CDC2721121. We compared the effect of the conventional PKC-α and novel PKC-η isoforms on the growth of B. thailandensis CDC2721121 within monocytic THP-1 cells and found that ≥75% knock-down of PRKCH transcript levels reduced intracellular bacterial load 100% more efficiently when compared to growth in cells siRNA-depleted of the classical PKC-α, suggesting that the PKC-η isoform can specifically mediate Burkholderia intracellular survival. Based on imaging studies of intracellular B. thailandensis, we found that PKC-η function stimulates phagocytic pathways that promote B. thailandensis escape into the cytoplasm leading to activation of autophagosome flux. Identification of host kinases that are targeted by Burkholderia during infection provides valuable molecular insights in understanding Burkholderia pathogenesis, and ultimately, in designing effective host-targeted therapies against infectious disease caused by intracellular pathogens.

Activation of MAPK/ERK signaling by Burkholderia pseudomallei cycle inhibiting factor (Cif)

Cycle inhibiting factors (Cifs) are virulence proteins secreted by the type III secretion system of some Gram-negative pathogenic bacteria including Burkholderia pseudomallei. Cif is known to function to deamidate Nedd8, leading to inhibition of Cullin E3 ubiquitin ligases (CRL) and consequently induction of cell cycle arrest. Here we show that Cif can function as a potent activator of MAPK/ERK signaling without significant activation of other signaling pathways downstream of receptor tyrosine kinases. Importantly, we found that the ability of Cif to activate ERK is dependent on its deamidase activity, but independent of Cullin E3 ligase inhibition. This suggests that apart from Nedd8, other cellular targets of Cif-dependent deamidation exist. We provide evidence that the mechanism involved in Cif-mediated ERK activation is dependent on recruitment of the Grb2-SOS1 complex to the plasma membrane. Further investigation revealed that Cif appears to modify the phosphorylation status of SOS1 in a region containing the CDC25-H and proline-rich domains. It is known that prolonged Cullin E3 ligase inhibition leads to cellular apoptosis. Therefore, we hypothesize that ERK activation is an important mechanism to counter the pro-apoptotic effects of Cif. Indeed, we show that Cif dependent ERK activation promotes phosphorylation of the proapoptotic protein Bim, thereby potentially conferring a pro-survival signal. In summary, we identified a novel deamidation-dependent mechanism of action of the B. pseudomallei virulence factor Cif/CHBP to activate MAPK/ERK signaling. Our study demonstrates that bacterial proteins such as Cif can serve as useful molecular tools to uncover novel aspects of mammalian signaling pathways.

Rapid and Sensitive Multiplex Detection of Burkholderia pseudomallei-Specific Antibodies in Melioidosis Patients Based on a Protein Microarray Approach

BACKGROUND

The environmental bacterium Burkholderia pseudomallei causes the infectious disease melioidosis with a high case-fatality rate in tropical and subtropical regions. Direct pathogen detection can be difficult, and therefore an indirect serological test which might aid early diagnosis is desirable. However, current tests for antibodies against B. pseudomallei, including the reference indirect haemagglutination assay (IHA), lack sensitivity, specificity and standardization. Consequently, serological tests currently do not play a role in the diagnosis of melioidosis in endemic areas. Recently, a number of promising diagnostic antigens have been identified, but a standardized, easy-to-perform clinical laboratory test for sensitive multiplex detection of antibodies against B. pseudomallei is still lacking.

METHODS AND PRINCIPAL FINDINGS

In this study, we developed and validated a protein microarray which can be used in a standard 96-well format. Our array contains 20 recombinant and purified B. pseudomallei proteins, previously identified as serodiagnostic candidates in melioidosis. In total, we analyzed 196 sera and plasmas from melioidosis patients from northeast Thailand and 210 negative controls from melioidosis-endemic and non-endemic regions. Our protein array clearly discriminated between sera from melioidosis patients and controls with a specificity of 97%. Importantly, the array showed a higher sensitivity than did the IHA in melioidosis patients upon admission (cut-off IHA titer ≥1:160: IHA 57.3%, protein array: 86.7%; p = 0.0001). Testing of sera from single patients at 0, 12 and 52 weeks post-admission revealed that protein antigens induce either a short- or long-term antibody response.

CONCLUSIONS

Our protein array provides a standardized, rapid, easy-to-perform test for the detection of B. pseudomallei-specific antibody patterns. Thus, this system has the potential to improve the serodiagnosis of melioidosis in clinical settings. Moreover, our high-throughput assay might be useful for the detection of anti-B. pseudomallei antibodies in epidemiological studies. Further studies are needed to elucidate the clinical and diagnostic significance of the different antibody kinetics observed during melioidosis.

Role of Canonical and Non-canonical Inflammasomes During Burkholderia Infection

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

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

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

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.

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

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

Melioidosis: molecular aspects of pathogenesis

Burkholderia pseudomallei is a gram-negative bacterium that causes melioidosis, a multifaceted disease that is highly endemic in southeast Asia and northern Australia. This facultative intracellular pathogen possesses a large genome that encodes a wide array of virulence factors that promote survival in vivo by manipulating host cell processes and disarming elements of the host immune system. Antigens and systems that play key roles in B. pseudomallei virulence include capsular polysaccharide, lipopolysaccharide, adhesins, specialized secretion systems, actin-based motility and various secreted factors. This review provides an overview of the current and steadily expanding knowledge regarding the molecular mechanisms used by this organism to survive within a host and their contribution to the pathogenesis of melioidosis.

Pathogens penetrating the central nervous system: infection pathways and the cellular and molecular mechanisms of invasion

The brain is well protected against microbial invasion by cellular barriers, such as the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). In addition, cells within the central nervous system (CNS) are capable of producing an immune response against invading pathogens. Nonetheless, a range of pathogenic microbes make their way to the CNS, and the resulting infections can cause significant morbidity and mortality. Bacteria, amoebae, fungi, and viruses are capable of CNS invasion, with the latter using axonal transport as a common route of infection. In this review, we compare the mechanisms by which bacterial pathogens reach the CNS and infect the brain. In particular, we focus on recent data regarding mechanisms of bacterial translocation from the nasal mucosa to the brain, which represents a little explored pathway of bacterial invasion but has been proposed as being particularly important in explaining how infection with Burkholderia pseudomallei can result in melioidosis encephalomyelitis.

Functional characterizations of effector protein BipC, a type III secretion system protein, in Burkholderia pseudomallei pathogenesis

OBJECTIVES

The bsa locus of Burkholderia pseudomallei encodes several proteins that are components of the type III secretion system (TTSS). BipC was postulated as one of the TTSS-3 effector proteins, but its role in the pathogenesis of B. pseudomallei infection is not well understood. Thus, the aim of this study was to determine its role(s) in the virulence of B. pseudomallei pathogenesis.

METHODS

A bipC TTSS-3-deficient strain of B. pseudomallei and complemented strains were generated to assess the role of BipC as a type III translocation apparatus. Human cell lines and a mouse model of melioidosis were used for in vitro and in vivo assays, respectively.

RESULTS

A significant 2-fold reduction was demonstrated in the percentage of adherence, invasion, intracellular survival, and phagosomal escape of the bipC mutant. Interestingly, microscopic studies have shown that BipC was capable of delayed B. pseudomallei actin-based motility. The virulence of the mutant strain in a murine model of melioidosis demonstrated that the bipC mutant was less virulent, compared with the wild type.

CONCLUSION

The results suggested that BipC possesses virulence determinants that play significant roles in host cell invasion and immune evasion.

Analysis of the prevalence, secretion and function of a cell cycle-inhibiting factor in the melioidosis pathogen Burkholderia pseudomallei

Enteropathogenic and enterohaemorrhagic Escherichia coli express a cell cycle-inhibiting factor (Cif), that is injected into host cells via a Type III secretion system (T3SS) leading to arrest of cell division, delayed apoptosis and cytoskeletal rearrangements. A homologue of Cif has been identified in Burkholderia pseudomallei (CHBP; Cif homologue in B. pseudomallei; BPSS1385), which shares catalytic activity, but its prevalence, secretion and function are ill-defined. Among 43 available B. pseudomallei genome sequences, 33 genomes (76.7%) harbor the gene encoding CHBP. Western blot analysis using antiserum raised to a synthetic CHBP peptide detected CHBP in 46.6% (7/15) of clinical B. pseudomallei isolates from the endemic area. Secretion of CHBP into bacterial culture supernatant could not be detected under conditions where a known effector (BopE) was secreted in a manner dependent on the Bsa T3SS. In contrast, CHBP could be detected in U937 cells infected with B. pseudomallei by immunofluorescence microscopy and Western blotting in a manner dependent on bsaQ. Unlike E. coli Cif, CHBP was localized within the cytoplasm of B. pseudomallei-infected cells. A B. pseudomallei chbP insertion mutant showed a significant reduction in cytotoxicity and plaque formation compared to the wild-type strain that could be restored by plasmid-mediated trans-complementation. However, there was no defect in actin-based motility or multinucleated giant cell formation by the chbP mutant. The data suggest that the level or timing of CHBP secretion differs from a known Bsa-secreted effector and that CHBP is required for selected virulence-associated phenotypes in vitro.

A high-content imaging assay for the quantification of the Burkholderia pseudomallei induced multinucleated giant cell (MNGC) phenotype in murine macrophages

BACKGROUND

Burkholderia pseudomallei (Bp), a Gram-negative, motile, facultative intracellular bacterium is the causative agent of melioidosis in humans and animals. The Bp genome encodes a repertoire of virulence factors, including the cluster 3 type III secretion system (T3SS-3), the cluster 1 type VI secretion system (T6SS-1), and the intracellular motility protein BimA, that enable the pathogen to invade both phagocytic and non-phagocytic cells. A unique hallmark of Bp infection both in vitro and in vivo is its ability to induce cell-to-cell fusion of macrophages to form multinucleated giant cells (MNGCs), which to date are semi-quantitatively reported following visual inspection.

RESULTS

In this study we report the development of an automated high-content image acquisition and analysis assay to quantitate the Bp induced MNGC phenotype. Validation of the assay was performed using T6SS-1 (∆hcp1) and T3SS-3 (∆bsaZ) mutants of Bp that have been previously reported to exhibit defects in their ability to induce MNGCs. Finally, screening of a focused small molecule library identified several Histone Deacetylase (HDAC) inhibitors that inhibited Bp-induced MNGC formation of macrophages.

CONCLUSIONS

We have successfully developed an automated HCI assay to quantitate MNGCs induced by Bp in macrophages. This assay was then used to characterize the phenotype of the Bp mutants for their ability to induce MNGC formation and identify small molecules that interfere with this process. Successful application of chemical genetics and functional reverse genetics siRNA approaches in the MNGC assay will help gain a better understanding of the molecular targets and cellular mechanisms responsible for the MNGC phenotype induced by Bp, by other bacteria such as Mycobacterium tuberculosis, or by exogenously added cytokines.

The role of short-chain dehydrogenase/oxidoreductase, induced by salt stress, on host interaction of B. pseudomallei

Background

Burkholderia pseudomallei is the causative agent of melioidosis, a frequently occurring disease in northeastern Thailand, where soil and water high in salt content are common. Using microarray analysis, we previously showed that B. pseudomallei up-regulated a short-chain dehydrogenase/oxidoreductase (SDO) under salt stress. However, the importance of SDO in B. pseudomallei infection is unknown. This study aimed to explore the function of B. pseudomallei SDO, and to investigate its role in interactions between B. pseudomallei and host cells.

Results

Bioinformatics analysis of B. pseudomallei SDO structure, based on homology modeling, revealed a NAD⁺ cofactor domain and a catalytic triad containing Ser149, Tyr162, and Lys166. This is similar to Bacillus megaterium glucose 1-dehydrogenase. To investigate the role of this protein, we constructed a B. pseudomallei SDO defective mutant, measured glucose dehydrogenase (GDH) activity, and tested the interactions with host cells. The B. pseudomallei K96243 wild type exhibited potent GDH activity under condition containing 300 mM NaCl, while the mutant showed activity levels 15 times lower. Both invasion into the A549 cell line and early intracellular survival within the J774A.1 macrophage cell were impaired in the mutant. Complementation of SDO was able to restore the mutant ability to produce GDH activity, invade epithelial cells, and survive in macrophages.

Conclusions

Our data suggest that induced SDO activity during salt stress may facilitate B. pseudomallei invasion and affect initiation of successful intracellular infection. Identifying the role of B. pseudomallei SDO provides a better understanding of the association between bacterial adaptation and pathogenesis in melioidosis.

Comparative assessment of the intracellular survival of the Burkholderia pseudomallei bopC mutant

Burkholderia pseudomallei, the causative agent of melioidosis, is a Gram-negative saprophytic bacterium capable of surviving within phagocytic cells. To assess the role of BopC (a type III secreted effector protein) in the pathogenesis of B. pseudomallei, a B. pseudomallei bopC mutant was used to infect J774A.1 macrophage-like cells. The bopC mutant showed significantly reduced intracellular survival in infected macrophages compared to wild-type B. pseudomallei. In addition, the bopC mutant displayed delayed escape from endocytic vesicles compared with the wild-type strain. This indicates that BopC is important, and at least in part, needed for intracellular survival of B. pseudomallei.

Functional characterization of Burkholderia pseudomallei trimeric autotransporters

Burkholderia pseudomallei is a tier 1 select agent and the causative agent of melioidosis, a severe and often fatal disease with symptoms ranging from acute pneumonia and septic shock to a chronic infection characterized by abscess formation in the lungs, liver, and spleen. Autotransporters (ATs) are exoproteins belonging to the type V secretion system family, with many playing roles in pathogenesis. The genome of B. pseudomallei strain 1026b encodes nine putative trimeric AT proteins, of which only four have been described. Using a bioinformatic approach, we annotated putative domains within each trimeric AT protein, excluding the well-studied BimA protein, and found short repeated sequences unique to Burkholderia species, as well as an unexpectedly large proportion of ATs with extended signal peptide regions (ESPRs). To characterize the role of trimeric ATs in pathogenesis, we constructed disruption or deletion mutations in each of eight AT-encoding genes and evaluated the resulting strains for adherence to, invasion of, and plaque formation in A549 cells. The majority of the ATs (and/or the proteins encoded downstream) contributed to adherence to and efficient invasion of A549 cells. Using a BALB/c mouse model of infection, we determined the contributions of each AT to bacterial burdens in the lungs, liver, and spleen. At 48 h postinoculation, only one strain, Bp340::pDbpaC, demonstrated a defect in dissemination and/or survival in the liver, indicating that BpaC is required for wild-type virulence in this model.

Neutrophil extracellular traps exhibit antibacterial activity against burkholderia pseudomallei and are influenced by bacterial and host factors

Burkholderia pseudomallei is the causative pathogen of melioidosis, of which a major predisposing factor is diabetes mellitus. Polymorphonuclear neutrophils (PMNs) kill microbes extracellularly by the release of neutrophil extracellular traps (NETs). PMNs play a key role in the control of melioidosis, but the involvement of NETs in killing of B. pseudomallei remains obscure. Here, we showed that bactericidal NETs were released from human PMNs in response to B. pseudomallei in a dose- and time-dependent manner. B. pseudomallei-induced NET formation required NADPH oxidase activation but not phosphatidylinositol-3 kinase, mitogen-activated protein kinases, or Src family kinase signaling pathways. B. pseudomallei mutants defective in the virulence-associated Bsa type III protein secretion system (T3SS) or capsular polysaccharide I (CPS-I) induced elevated levels of NETs. NET induction by such mutants was associated with increased bacterial killing, phagocytosis, and oxidative burst by PMNs. Taken together the data imply that T3SS and the capsule may play a role in evading the induction of NETs. Importantly, PMNs from diabetic subjects released NETs at a lower level than PMNs from healthy subjects. Modulation of NET formation may therefore be associated with the pathogenesis and control of melioidosis.

Defense Mechanisms of Hepatocytes Against Burkholderia pseudomallei

The Gram-negative facultative intracellular rod Burkholderia pseudomallei causes melioidosis, an infectious disease with a wide range of clinical presentations. Among the observed visceral abscesses, the liver is commonly affected. However, neither this organotropism of B. pseudomallei nor local hepatic defense mechanisms have been thoroughly investigated so far. Own previous studies using electron microscopy of the murine liver after systemic infection of mice indicated that hepatocytes might be capable of killing B. pseudomallei. Therefore, the aim of this study was to further elucidate the interaction of B. pseudomallei with these cells and to analyze the role of hepatocytes in anti-B. pseudomallei host defense. In vitro studies using the human hepatocyte cell line HepG2 revealed that B. pseudomallei can invade these cells. Subsequently, B. pseudomallei is able to escape from the vacuole, to replicate within the cytosol of HepG2 cells involving its type 3 and type 6 secretion systems, and to induce actin tail formation. Furthermore, stimulation of HepG2 cells showed that IFNγ can restrict growth of B. pseudomallei in the early and late phase of infection whereas the combination of IFNγ, IL-1β, and TNFα is required for the maximal antibacterial activity. This anti-B. pseudomallei defense of HepG2 cells did not seem to be mediated by inducible nitric oxide synthase-derived nitric oxide or NADPH oxidase-derived superoxide. In summary, this is the first study describing B. pseudomallei intracellular life cycle characteristics in hepatocytes and showing that IFNγ-mediated, but nitric oxide- and reactive oxygen species-independent, effector mechanisms are important in anti-B. pseudomallei host defense of hepatocytes.

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.

BopC is a type III secreted effector protein of Burkholderia pseudomallei

Burkholderia pseudomallei, the causative agent of melioidosis, exploits the Bsa type III secretion system (T3SS) to deliver effector proteins into host cells. These effectors manipulate host cell functions; thus, contributing to the ability of the bacteria to evade the immune response and cause disease. Only two Bsa-secreted effectors have been conclusively identified to date. Here, we report the identification of the third B. pseudomallei type III secreted effector protein, designated BopC. BopC is encoded by the bpss1516 gene abutting bpss1517, which encodes its putative chaperone. The genes are located in the close proximity to the bsa T3SS gene cluster of B. pseudomallei K96243 (Fig. 1). BopC was secreted into culture supernatant by the wild-type B. pseudomallei strain, but its secretion was abolished in the bsaZ T3SS mutant. Using pull down and co-purification assays, we confirmed that BopC interacts with its putative chaperone, BPSS1517, in vitro. Furthermore, the first 20 N-terminal amino acids of BopC were found to be sufficient to mediate the T3SS-dependent translocation of a reporter protein from a heterologous enteropathogenic Escherichia coli host into mammalian cells. Finally, bopC mutant was found to be less invasive than the wild-type strain in the epithelial cells.

Regulation of type VI secretion system during Burkholderia pseudomallei infection

Type III and type VI secretion systems (T3SSs and T6SSs, respectively) are critical virulence determinants in several Gram-negative pathogens. In Burkholderia pseudomallei, the T3SS-3 and T6SS-1 clusters have been implicated in bacterial virulence in mammalian hosts. We recently discovered a regulatory cascade that coordinately controls the expression of T3SS-3 and T6SS-1. BsaN is a central regulator located within T3SS-3 for the expression of T3SS-3 effectors and regulators for T6SS-1 such as VirA-VirG (VirAG) and BprC. Whereas T6SS-1 gene expression was completely dependent on BprC when bacteria were grown in medium, the expression inside host cells was dependent on the two-component sensor-regulator VirAG, with the exception of the tssAB operon, which was dependent primarily on BprC. VirAG and BprC initiate different transcriptional start sites within T6SS-1, and VirAG is able to activate the hcp1 promoter directly. We also provided novel evidence that virAG, bprC, and tssAB are critical for T6SS-1 function in macrophages. Furthermore, virAG and bprC regulator mutants were avirulent in mice, demonstrating the absolute dependence of T6SS-1 expression on these regulators in vivo.

Dictyostelium discoideum as a model system for identification of Burkholderia pseudomallei virulence factors

Burkholderia pseudomallei is an emerging bacterial pathogen and category B biothreat. Human infections with B. pseudomallei (called melioidosis) present as a range of manifestations, including acute septicemia and pneumonia. Although melioidosis can be fatal, little is known about the molecular basis of B. pseudomallei pathogenicity, in part because of the lack of simple, genetically tractable eukaryotic models to facilitate en masse identification of virulence determinants or explore host-pathogen interactions. Two assays, one high-throughput and one quantitative, were developed to monitor levels of resistance of B. pseudomallei and the closely related nearly avirulent species Burkholderia thailandensis to predation by the phagocytic amoeba Dictyostelium discoideum. The quantitative assay showed that levels of resistance to, and survival within, amoeba by these bacteria and their known virulence mutants correlate well with their published levels of virulence in animals. Using the high-throughput assay, we screened a 1,500-member B. thailandensis transposon mutant library and identified 13 genes involved in resistance to predation by D. discoideum. Orthologs of these genes were disrupted in B. pseudomallei, and nearly all mutants had similarly decreased resistance to predation by D. discoideum. For some mutants, decreased resistance also correlated with reduced survival in and cytotoxicity toward macrophages, as well as attenuated virulence in mice. These observations suggest that some factors required by B. pseudomallei for resistance to environmental phagocytes also aid in resistance to phagocytic immune cells and contribute to disease in animals. Thus, D. discoideum provides a novel, high-throughput model system for facilitating inquiry into B. pseudomallei virulence.

Genome-wide analysis reveals loci encoding anti-macrophage factors in the human pathogen Burkholderia pseudomallei K96243

Burkholderia pseudomallei is an important human pathogen whose infection biology is still poorly understood. The bacterium is endemic to tropical regions, including South East Asia and Northern Australia, where it causes melioidosis, a serious disease associated with both high mortality and antibiotic resistance. B. pseudomallei is a Gram-negative facultative intracellular pathogen that is able to replicate in macrophages. However despite the critical nature of its interaction with macrophages, few anti-macrophage factors have been characterized to date. Here we perform a genome-wide gain of function screen of B. pseudomallei strain K96243 to identify loci encoding factors with anti-macrophage activity. We identify a total of 113 such loci scattered across both chromosomes, with positive gene clusters encoding transporters and secretion systems, enzymes/toxins, secondary metabolite, biofilm, adhesion and signal response related factors. Further phenotypic analysis of four of these regions shows that the encoded factors cause striking cellular phenotypes relevant to infection biology, including apoptosis, formation of actin 'tails' and multi-nucleation within treated macrophages. The detailed analysis of the remaining host of loci will facilitate genetic dissection of the interaction of this important pathogen with host macrophages and thus further elucidate this critical part of its infection cycle.

Molecular insights into Burkholderia pseudomallei and Burkholderia mallei pathogenesis

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

Effect of acidic pH on the invasion efficiency and the type III secretion system of Burkholderia thailandensis

Burkholderia thailandensis is a close relative of Burkholderia pseudomallei. These organisms are very similar, but B. thailandensis is far less virulent than B. pseudomallei. Nucleotide sequencing and analysis of 14 B. thailandensis isolates revealed variation in the regions coding for the type III secreted BipD protein. The degree of B. thailandensis BipD sequence variation was greater than that found in B. pseudomallei. Western blot analysis indicated that, unlike B. pseudomallei, B. thailandensis type III secreted proteins including BipD and BopE could not be detected in the supernatant of culture medium unless induced by acidic conditions. In addition, culturing B. thailandensis under acidic growth conditions (pH 4.5) can induce the ability of this bacterium to invade human respiratory epithelial cells A549. The identification of an environmental stimulus that increases the invasion capability of B. thailandensis invasion is of value for those who would like to use this bacterium as a model to study B. pseudomallei virulence.

Global transcriptional profiling of Burkholderia pseudomallei under salt stress reveals differential effects on the Bsa type III secretion system

Background

Burkholderia pseudomallei is the causative agent of melioidosis where the highest reported incidence world wide is in the Northeast of Thailand, where saline soil and water are prevalent. Moreover, recent reports indicate a potential pathogenic role for B. pseudomallei in cystic fibrosis lung disease, where an increased sodium chloride (NaCl) concentration in airway surface liquid has been proposed. These observations raise the possibility that high salinity may represent a favorable niche for B. pseudomallei. We therefore investigated the global transcriptional response of B. pseudomallei to increased salinity using microarray analysis.

Results

Transcriptome analysis of B. pseudomallei under salt stress revealed several genes significantly up-regulated in the presence of 320 mM NaCl including genes associated with the bsa-derived Type III secretion system (T3SS). Microarray data were verified by reverse transcriptase-polymerase chain reactions (RT-PCR). Western blot analysis confirmed the increased expression and secretion of the invasion-associated type III secreted proteins BipD and BopE in B. pseudomallei cultures at 170 and 320 mM NaCl relative to salt-free medium. Furthermore, salt-treated B. pseudomallei exhibited greater invasion efficiency into the lung epithelial cell line A549 in a manner partly dependent on a functional Bsa system.

Conclusions

B. pseudomallei responds to salt stress by modulating the transcription of a relatively small set of genes, among which is the bsa locus associated with invasion and virulence. Expression and secretion of Bsa-secreted proteins was elevated in the presence of exogenous salt and the invasion efficiency was enhanced. Our data indicate that salinity has the potential to influence the virulence of B. pseudomallei.