Identification and characterization of a two-component regulatory system involved in invasion of eukaryotic cells and heavy-metal resistance in Burkholderia pseudomallei

TetR and OmpR family regulators in natural product biosynthesis and resistance

This article provides a comprehensive review and sequence-structure analysis of transcription regulator (TR) families, TetR and OmpR/PhoB, involved in specialized secondary metabolite (SSM) biosynthesis and resistance. Transcription regulation is a fundamental process, playing a crucial role in orchestrating gene expression to confer a survival advantage in response to frequent environmental stress conditions. This process, coupled with signal sensing, enables bacteria to respond to a diverse range of intra and extracellular signals. Thus, major bacterial signaling systems use a receptor domain to sense chemical stimuli along with an output domain responsible for transcription regulation through DNA-binding. Sensory and output domains on a single polypeptide chain (one component system, OCS) allow response to stimuli by allostery, that is, DNA-binding affinity modulation upon signal presence/absence. On the other hand, two component systems (TCSs) allow cross-talk between the sensory and output domains as they are disjoint and transmit information by phosphorelay to mount a response. In both cases, however, TRs play a central role. Biosynthesis of SSMs, which includes antibiotics, is heavily regulated by TRs as it diverts the cell's resources towards the production of these expendable compounds, which also have clinical applications. These TRs have evolved to relay information across specific signals and target genes, thus providing a rich source of unique mechanisms to explore towards addressing the rapid escalation in antimicrobial resistance (AMR). Here, we focus on the TetR and OmpR family TRs, which belong to OCS and TCS, respectively. These TR families are well-known examples of regulators in secondary metabolism and are ubiquitous across different bacteria, as they also participate in a myriad of cellular processes apart from SSM biosynthesis and resistance. As a result, these families exhibit higher sequence divergence, which is also evident from our bioinformatic analysis of 158 389 and 77 437 sequences from TetR and OmpR family TRs, respectively. The analysis of both sequence and structure allowed us to identify novel motifs in addition to the known motifs responsible for TR function and its structural integrity. Understanding the diverse mechanisms employed by these TRs is essential for unraveling the biosynthesis of SSMs. This can also help exploit their regulatory role in biosynthesis for significant pharmaceutical, agricultural, and industrial applications.

A virulence activator of a surface attachment protein in Burkholderia pseudomallei acts as a global regulator of other membrane-associated virulence factors

Burkholderia pseudomallei (Bp), causing a highly fatal disease called melioidosis, is a facultative intracellular pathogen that attaches and invades a variety of cell types. We previously identified BP1026B_I0091 as a surface attachment protein (Sap1) and an essential virulence factor, contributing to Bp pathogenesis in vitro and in vivo. The expression of sap1 is regulated at different stages of Bp intracellular lifecycle by unidentified regulator(s). Here, we identified SapR (BP1026B_II1046) as a transcriptional regulator that activates sap1, using a high-throughput transposon mutagenesis screen in combination with Tn-Seq. Consistent with phenotypes of the Δsap1 mutant, the ΔsapR activator mutant exhibited a significant reduction in Bp attachment to the host cell, leading to subsequent decreased intracellular replication. RNA-Seq analysis further revealed that SapR regulates sap1. The regulation of sap1 by SapR was confirmed quantitatively by qRT-PCR, which also validated the RNA-Seq data. SapR globally regulates genes associated with the bacterial membrane in response to diverse environments, and some of the genes regulated by SapR are virulence factors that are required for Bp intracellular infection (e.g., type III and type VI secretion systems). This study has identified the complex SapR regulatory network and its importance as an activator of an essential Sap1 attachment factor.

A DUF4148 family protein produced inside RAW264.7 cells is a critical Burkholderia pseudomallei virulence factor

Burkholderia pseudomallei: is the etiological agent of the disease melioidosis and is a Tier 1 select agent. It survives and replicates inside phagocytic cells by escaping from the endocytic vacuole, replicating in the cytosol, spreading to other cells via actin polymerization and promoting the fusion of infected and uninfected host cells to form multinucleated giant cells. In this study, we utilized a proteomics approach to identify bacterial proteins produced inside RAW264.7 murine macrophages and host proteins produced in response to B. pseudomallei infection. Cells infected with B. pseudomallei strain K96243 were lysed and the lysate proteins digested and analyzed using nanoflow reversed-phase liquid chromatography and tandem mass spectrometry. Approximately 160 bacterial proteins were identified in the infected macrophages, including BimA, TssA, TssB, Hcp1 and TssM. Several previously uncharacterized B. pseudomallei proteins were also identified, including BPSS1996 and BPSL2748. Mutations were constructed in the genes encoding these novel proteins and their relative virulence was assessed in BALB/c mice. The 50% lethal dose for the BPSS1996 mutant was approximately 55-fold higher than that of the wild type, suggesting that BPSS1996 is required for full virulence. Sera from B. pseudomallei-infected animals reacted with BPSS1996 and it was found to localize to the bacterial surface using indirect immunofluorescence. Finally, we identified 274 host proteins that were exclusively present or absent in infected RAW264.7 cells, including chemokines and cytokines involved in controlling the initial stages of infection.

Regulation of Virulence by Two-Component Systems in Pathogenic Burkholderia

The regulation and timely expression of bacterial genes during infection is critical for a pathogen to cause an infection. Bacteria have multiple mechanisms to regulate gene expression in response to their environment, one of which is two-component systems (TCS). TCS have two components. One component is a sensory histidine kinase (HK) that autophosphorylates when activated by a signal. The activated sensory histidine kinase then transfers the phosphoryl group to the second component, the response regulator, which activates transcription of target genes. The genus Burkholderia contains members that cause human disease and are often extensively resistant to many antibiotics. The Burkholderia cepacia complex (BCC) can cause severe lung infections in patients with cystic fibrosis (CF) or chronic granulomatous disease (CGD). BCC members have also recently been associated with several outbreaks of bacteremia from contaminated pharmaceutical products. Separate from the BCC is Burkholderia pseudomallei, which is the causative agent of melioidosis, a serious disease that occurs in the tropics, and a potential bioterrorism weapon. Bioinformatic analysis of sequenced Burkholderia isolates predicts that most strains have at least 40 TCS. The vast majority of these TCS are uncharacterized both in terms of the signals that activate them and the genes that are regulated by them. This review will highlight TCS that have been described to play a role in virulence in either the BCC or B. pseudomallei Since many of these TCS are involved in virulence, TCS are potential novel therapeutic targets, and elucidating their function is critical for understanding Burkholderia pathogenesis.

Reclassification of the Specialized Metabolite Producer Pseudomonas mesoacidophila ATCC 31433 as a Member of the Burkholderia cepacia Complex

Pseudomonas mesoacidophila ATCC 31433 is a Gram-negative bacterium, first isolated from Japanese soil samples, that produces the monobactam isosulfazecin and the β-lactam-potentiating bulgecins. To characterize the biosynthetic potential of P. mesoacidophila ATCC 31433, its complete genome was determined using single-molecule real-time DNA sequence analysis. The 7.8-Mb genome comprised four replicons, three chromosomal (each encoding rRNA) and one plasmid. Phylogenetic analysis demonstrated that P. mesoacidophila ATCC 31433 was misclassified at the time of its deposition and is a member of the Burkholderia cepacia complex, most closely related to Burkholderia ubonensis The sequenced genome shows considerable additional biosynthetic potential; known gene clusters for malleilactone, ornibactin, isosulfazecin, alkylhydroxyquinoline, and pyrrolnitrin biosynthesis and several uncharacterized biosynthetic gene clusters for polyketides, nonribosomal peptides, and other metabolites were identified. Furthermore, P. mesoacidophila ATCC 31433 harbors many genes associated with environmental resilience and antibiotic resistance and was resistant to a range of antibiotics and metal ions. In summary, this bioactive strain should be designated B. cepacia complex strain ATCC 31433, pending further detailed taxonomic characterization.IMPORTANCE This work reports the complete genome sequence of Pseudomonas mesoacidophila ATCC 31433, a known producer of bioactive compounds. Large numbers of both known and novel biosynthetic gene clusters were identified, indicating that P. mesoacidophila ATCC 31433 is an untapped resource for discovery of novel bioactive compounds. Phylogenetic analysis demonstrated that P. mesoacidophila ATCC 31433 is in fact a member of the Burkholderia cepacia complex, most closely related to the species Burkholderia ubonensis Further investigation of the classification and biosynthetic potential of P. mesoacidophila ATCC 31433 is warranted.

Towards the complete proteinaceous regulome of Acinetobacter baumannii

The emergence of Acinetobacter baumannii strains, with broad multidrug-resistance phenotypes and novel virulence factors unique to hypervirulent strains, presents a major threat to human health worldwide. Although a number of studies have described virulence-affecting entities for this organism, very few have identified regulatory elements controlling their expression. Previously, our group has documented the global identification and curation of regulatory RNAs in A. baumannii. As such, in the present study, we detail an extension of this work, the performance of an extensive bioinformatic analysis to identify regulatory proteins in the recently annotated genome of the highly virulent AB5075 strain. In so doing, 243 transcription factors, 14 two-component systems (TCSs), 2 orphan response regulators, 1 hybrid TCS and 5 σ factors were found. A comparison of these elements between AB5075 and other clinical isolates, as well as a laboratory strain, led to the identification of several conserved regulatory elements, whilst at the same time uncovering regulators unique to hypervirulent strains. Lastly, by comparing regulatory elements compiled in this study to genes shown to be essential for AB5075 infection, we were able to highlight elements with a specific importance for pathogenic behaviour. Collectively, our work offers a unique insight into the regulatory network of A. baumannii strains, and provides insight into the evolution of hypervirulent lineages.

Burkholderia pseudomallei Biofilm Promotes Adhesion, Internalization and Stimulates Proinflammatory Cytokines in Human Epithelial A549 Cells

Burkholderia pseudomallei is a Gram-negative bacterium that causes melioidosis. Inhalational exposure leading to pulmonary melioidosis is the most common clinical manifestation with significant mortality. However, the role of B. pseudomallei biofilm phenotype during bacterial-host interaction remains unclear. We hypothesize that biofilm phenotype may play a role in such interactions. In this study, B. pseudomallei H777 (biofilm wild type), B. pseudomallei M10 (biofilm mutant) and B. pseudomallei C17 (biofilm-complemented) strains were used to assess the contribution of biofilm to adhesion to human lung epithelial cells (A549), intracellular interactions, apoptosis/necrosis and impact on proinflammatory responses. Confocal laser scanning microscopy demonstrated that B. pseudomallei H777 and C17 produced biofilm, whereas M10 did not. To determine the role of biofilm in host interaction, we assessed the ability of each of the three strains to interact with the A549 cells at MOI 10. Strain H777 exhibited higher levels of attachment and invasion compared to strain M10 (p < 0.05). In addition, the biofilm-complemented strain, C17 exhibited restored bacterial invasion ability. Flow cytometry combined with a double-staining assay using annexin V and propidium iodide revealed significantly higher numbers of early apoptotic and late apoptotic A549 cells when these were infected with strain H777 (1.52%) and C17 (1.43%) compared to strain M10 (0.85%) (p < 0.05). Strains H777 and C17 were able to stimulate significant secretion of IL-6 and IL-8 compared with the biofilm mutant (p < 0.05). Together, these findings demonstrated the role of biofilm-associated phenotypes of B. pseudomallei in cellular pathogenesis of human lung epithelial cells with respect to initial attachment and invasion, apoptosis and proinflammatory responses.

Perturbation of the two-component signal transduction system, BprRS, results in attenuated virulence and motility defects in Burkholderia pseudomallei

Background

Burkholderia pseudomallei is the causative agent of melioidosis, a severe invasive disease of humans and animals. Initial screening of a B. pseudomallei signature-tagged mutagenesis library identified an attenuated mutant with a transposon insertion in a gene encoding the sensor component of an uncharacterised two-component signal transduction system (TCSTS), which we designated BprRS.

Results

Single gene inactivation of either the response regulator gene (bprR) or the sensor histidine kinase gene (bprS) resulted in mutants with reduced swarming motility and reduced virulence in mice. However, a bprRS double mutant was not attenuated for virulence and displayed wild-type levels of motility. The transcriptomes of the bprS, bprR and bprRS mutants were compared with the transcriptome of the parent strain K96243. Inactivation of the entire BprRS TCSTS (bprRS double mutant) resulted in altered expression of only nine genes, including both bprR and bprS, five phage-related genes and bpss0686, encoding a putative 5, 10-methylene tetrahydromethanopterin reductase involved in one carbon metabolism. In contrast, the transcriptomes of each of the bprR and bprS single gene mutants revealed more than 70 differentially expressed genes common to both mutants, including regulatory genes and those required for flagella assembly and for the biosynthesis of the cytotoxic polyketide, malleilactone.

Conclusions

Inactivation of the entire BprRS TCSTS did not alter virulence or motility and very few genes were differentially expressed indicating that the definitive BprRS regulon is relatively small. However, loss of a single component, either the sensor histidine kinase BprS or its cognate response regulator BprR, resulted in significant transcriptomic and phenotypic differences from the wild-type strain. We hypothesize that the dramatically altered phenotypes of these single mutants are the result of cross-regulation with one or more other TCSTSs and concomitant dysregulation of other key regulatory genes.

Melioidosis in New Caledonia: a dominant strain in a transmission hotspot

Melioidosis is an infectious disease caused by Burkholderia pseudomallei, a bacterium endemic in Southeast Asia and northern Australia. In New Caledonia, sporadic cases were first described in 2005; since then, more cases have been identified. To improve our understanding of melioidosis epidemiology in New Caledonia, we compared the local cases and B. pseudomallei isolates with those from endemic areas. Nineteen melioidosis cases have been diagnosed in New Caledonia since 1999, mostly severe and with frequent bacteraemia, leading to three (16%) fatalities. All but one occurred in the North Province. Besides sporadic cases caused by non-clonal strains, we also identified a hotspot of transmission related to a clonal group of B. pseudomallei that is phylogenetically related to Australian strains.

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.

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.

Caspase-1-dependent and -independent cell death pathways in Burkholderia pseudomallei infection of macrophages

The cytosolic pathogen Burkholderia pseudomallei and causative agent of melioidosis has been shown to regulate IL-1β and IL-18 production through NOD-like receptor NLRP3 and pyroptosis via NLRC4. Downstream signalling pathways of those receptors and other cell death mechanisms induced during B. pseudomallei infection have not been addressed so far in detail. Furthermore, the role of B. pseudomallei factors in inflammasome activation is still ill defined. In the present study we show that caspase-1 processing and pyroptosis is exclusively dependent on NLRC4, but not on NLRP3 in the early phase of macrophage infection, whereas at later time points caspase-1 activation and cell death is NLRC4- independent. In the early phase we identified an activation pathway involving caspases-9, -7 and PARP downstream of NLRC4 and caspase-1. Analyses of caspase-1/11-deficient infected macrophages revealed a strong induction of apoptosis, which is dependent on activation of apoptotic initiator and effector caspases. The early activation pathway of caspase-1 in macrophages was markedly reduced or completely abolished after infection with a B. pseudomallei flagellin FliC or a T3SS3 BsaU mutant. Studies using cells transfected with the wild-type and mutated T3SS3 effector protein BopE indicated also a role of this protein in caspase-1 processing. A T3SS3 inner rod protein BsaK mutant failed to activate caspase-1, revealed higher intracellular counts, reduced cell death and IL-1β secretion during early but not during late macrophage infection compared to the wild-type. Intranasal infection of BALB/c mice with the BsaK mutant displayed a strongly decreased mortality, lower bacterial loads in organs, and reduced levels of IL-1β, myeloperoxidase and neutrophils in bronchoalveolar lavage fluid. In conclusion, our results indicate a major role for a functional T3SS3 in early NLRC4-mediated caspase-1 activation and pyroptosis and a contribution of late caspase-1-dependent and -independent cell death mechanisms in the pathogenesis of B. pseudomallei infection.

Inflammasome activation is important for host defence against bacterial infection. Many gram-negative pathogens use secretion systems to inject bacterial proteins such as flagellin or structural components of the secretion machinery itself into the host cytosol leading to caspase-1 activation and pyroptotic cell death. However, little is known about the B. pseudomallei factors that trigger caspase-1 activation as well as downstream signalling pathways and effector mechanisms of caspase-1. Here, we identified the B. pseudomallei T3SS3 inner rod protein BsaK as an early activator of caspase-1-dependent cell death and IL-1β secretion in primary macrophages and as a virulence factor in murine melioidosis. We could show that upon infection of macrophages, caspase-7 is activated downstream of the NLRC4/caspase-1 inflammasome and requires caspase-9 processing. Although caspase-7 was essential for cleavage of the DNA damage sensor PARP during pyroptosis, it did neither contribute to cytokine production nor B. pseudomallei growth restriction by promoting early macrophage death. In addition to a rapid NLRC4/caspase-1- dependent induction of pyroptosis in wild-type macrophages, we observed a delayed activation of classical apoptosis in macrophages lacking caspase-1/11. Thus, initiation of different cell death pathways seems to be an effective strategy to limit intracellular B. pseudomallei infection.

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.

Development of vaccines against burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative bacterium which is the causative agent of melioidosis, a disease which carries a high mortality and morbidity rate in endemic areas of South East Asia and Northern Australia. At present there is no available human vaccine that protects against B. pseudomallei, and with the current limitations of antibiotic treatment, the development of new preventative and therapeutic interventions is crucial. This review considers the multiple elements of melioidosis vaccine research including: (i) the immune responses required for protective immunity, (ii) animal models available for preclinical testing of potential candidates, (iii) the different experimental vaccine strategies which are being pursued, and (iv) the obstacles and opportunities for eventual registration of a licensed vaccine in humans.

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.

Evaluation of surrogate animal models of melioidosis

Burkholderia pseudomallei is the Gram-negative bacterial pathogen responsible for the disease melioidosis. B. pseudomallei establishes disease in susceptible individuals through multiple routes of infection, all of which may proceed to a septicemic disease associated with a high mortality rate. B. pseudomallei opportunistically infects humans and a wide range of animals directly from the environment, and modeling of experimental melioidosis has been conducted in numerous biologically relevant models including mammalian and invertebrate hosts. This review seeks to summarize published findings related to established animal models of melioidosis, with an aim to compare and contrast the virulence of B. pseudomallei in these models. The effect of the route of delivery on disease is also discussed for intravenous, intraperitoneal, subcutaneous, intranasal, aerosol, oral, and intratracheal infection methodologies, with a particular focus on how they relate to modeling clinical melioidosis. The importance of the translational validity of the animal models used in B. pseudomallei research is highlighted as these studies have become increasingly therapeutic in nature.

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 pseudomallei: animal models of infection

A range of animal models of Burkholderia pseudomallei infection have been reported, and the host species differ widely both in their susceptibility to infection and in the pathogenesis of disease. In mice, and depending on the route of infection, dose, and mouse strain, the disease can range from a chronic, and in some cases, an apparently latent infection to an acute fulminant disease. Alternative small animal models of infection include diabetic rats or hamsters. Larger animal models of disease have not yet been fully developed. It is not clear which of the small animal models of melioidosis most accurately reflect disease in humans. However, the findings that diabetic rats are susceptible to infection, that some strains of mice can develop persistent subclinical infections that can spontaneously reactivate, and that inhalation exposure generally results in more acute disease suggest that these different models mimic different aspects of human melioidosis.

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

Melioidosis, a febrile illness with disease states ranging from acute pneumonia or septicaemia to chronic abscesses, was first documented by Whitmore & Krishnaswami (1912). The causative agent, Burkholderia pseudomallei, was subsequently identified as a motile, gram-negative bacillus, which is principally an environmental saprophyte. Melioidosis has become an increasingly important disease in endemic areas such as northern Thailand and Australia (Currie et al., 2000). This health burden, plus the classification of B. pseudomallei as a category B biological agent (Rotz et al., 2002), has resulted in an escalation of research interest. This review focuses on the molecular and cellular basis of pathogenesis in melioidosis, with a comprehensive overview of the current knowledge on how B. pseudomallei can cause disease. The process of B. pseudomallei movement from the environmental reservoir to attachment and invasion of epithelial and macrophage cells and the subsequent intracellular survival and spread is outlined. Furthermore, the diverse assortment of virulence factors that allow B. pseudomallei to become an effective opportunistic pathogen, as well as to avoid or subvert the host immune response, is discussed. With the recent increase in genomic and molecular studies, the current understanding of the infection process of melioidosis has increased substantially, yet, much still remains to be elucidated.

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

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

Inflammation patterns induced by different Burkholderia species in mice

Burkholderia pseudomallei, which causes melioidosis, a severe, mainly pulmonary disease endemic in South-East Asia, is considered to be the most pathogenic of the Burkholderia genus. B. thailandensis, however, is considered avirulent. We determined differences in patterns of inflammation of B. pseudomallei 1026b (clinical virulent isolate), B. pseudomallei AJ1D8 (an in vitro invasion-deficient mutant generated from strain 1026b by Tn5-OT182 mutagenesis) and B. thailandensis by intranasally inoculating C57BL/6 mice with each strain. Mice infected with B. thailandensis showed a markedly decreased bacterial outgrowth from lungs, spleen and blood 24 h after inoculation, compared with infection with B. pseudomallei and the invasion mutant AJ1D8. Forty-eight hours after inoculation, B. thailandensis was no longer detectable. This was consistent with elevated pulmonary cytokine and chemokine concentrations after infection with B. pseudomallei 1026b and AJ1D8, and the absence of these mediators 48 h, but not 24 h, after inoculation with B. thailandensis. Histological examination, however, did show marked pulmonary inflammation in the mice infected with B. thailandensis, corresponding with substantial granulocyte influx and raised myeloperoxidase levels. Survival experiments showed that infection with 1 x 10(3) cfu B. thailandensis was not lethal, whereas inoculation with 1 x 10(6) cfu B. thailandensis was equally lethal as 1 x 10(3) cfu B. pseudomallei 1026b or AJ1D8. These data show that B. pseudomallei AJ1D8 is just as lethal as wild-type B. pseudomallei in an in vivo mouse model, and B. thailandensis is perhaps more virulent than is often recognized.

Blue–white selection of regulatory genes that affect the expression of dehalogenase IVa of Burkholderia cepacia MBA4

We have developed a method for rapid screening of genes that affected the expression of dehalogenase IVa of Burkholderia cepacia MBA4. The promoter region of the dehalogenase gene was used to drive the expression of a beta-galactosidase gene. A plasmid containing this reporter was first electroporated into MBA4, and a Tn5 containing suicidal plasmid was introduced subsequently. The use of electroporation was necessary because Escherichia coli mediated transconjugation was ineffective in plasmid-carrying MBA4. The number of integrants generated was directly proportional to the amount of plasmid DNA used. Integrants with an elevated beta-galactosidase activity were isolated. Mutants with a disruption in a putative iron-transporter gene and in a putative response regulator receiver gene were identified. The basal dehalogenase transcript levels of these mutants were higher than the wild type. These mutants also grow faster than the wild type in chloroacetate-containing medium. This methodology of isolating regulatory mutants is theoretically feasible and convenient for any kinds of bacteria.

Genome-wide expression analysis of Burkholderia pseudomallei infection in a hamster model of acute melioidosis

Burkholderia pseudomallei is the causative agent of melioidosis and represents a potential bioterrorism threat. In the current studies we have examined gene expression in B. pseudomallei in an animal model of acute melioidosis using whole-genome microarrays. Gene expression profiles were generated by comparing transcriptional levels of B. pseudomallei-expressed genes in infected hamster organs including liver, lung, and spleen following intraperitoneal and intranasal routes of infection to those from bacteria grown in vitro. Differentially expressed genes were similar in infected livers irrespective of the route of infection. Reduced expression of a number of housekeeping genes suggested a lower bacterial growth rate during infection. Energy production during growth in vivo involved specific biochemical pathways such as isomerization of 3-phosphoglycerate, catabolism of d-glucosamine and inositol, and biosynthesis of particular amino acids. In addition, the induction of genes known to be involved in oxidative phosphorylation including ubiquinol oxidase, ferredoxin oxidoreductase, and formate dehydrogenase enzymes suggested the use of alternative pathways for energy production, while the expression of genes coding for ATP-synthase and NADH-dehydrogenase enzymes was reduced. Our studies have identified differentially expressed genes which include potential virulence genes such as those for a putative phospholipase C and a putative two-component regulatory system, and they have also provided a better understanding of bacterial metabolism in response to the host environment during acute melioidosis.

Characterization of the mrgRS locus of the opportunistic pathogen Burkholderia pseudomallei: temperature regulates the expression of a two-component signal transduction system

Background

Burkholderia pseudomallei is a saprophyte in tropical environments and an opportunistic human pathogen. This versatility requires a sensing mechanism that allows the bacterium to respond rapidly to altered environmental conditions. We characterized a two-component signal transduction locus from B. pseudomallei 204, mrgR and mrgS, encoding products with extensive homology with response regulators and histidine protein kinases of Escherichia coli, Bordetella pertussis, and Vibrio cholerae.

Results

The locus was present and expressed in a variety of B. pseudomallei human and environmental isolates but was absent from other Burkholderia species, B. cepacia, B. cocovenenans, B. plantarii, B. thailandensis, B. vandii, and B. vietnamiensis. A 2128 bp sequence, including the full response regulator mrgR, but not the sensor kinase mrgS, was present in the B. mallei genome. Restriction fragment length polymorphism downstream from mrgRS showed two distinct groups were present among B. pseudomallei isolates. Our analysis of the open reading frames in this region of the genome revealed that transposase and bacteriophage activity may help explain this variation. MrgR and MrgS proteins were expressed in B. pseudomallei 204 cultured at different pH, salinity and temperatures and the expression was substantially reduced at 25°C compared with 37°C or 42°C but was mostly unaffected by pH or salinity, although at 25°C and 0.15% NaCl a small increase in MrgR expression was observed at pH 5. MrgR was recognized by antibodies in convalescent sera pooled from melioidosis patients.

Conclusion

The results suggest that mrgRS regulates an adaptive response to temperature that may be essential for pathogenesis, particularly during the initial phases of infection. B. pseudomallei and B. mallei are very closely related species that differ in their capacity to adapt to changing environmental conditions. Modifications in this region of the genome may assist our understanding of the reasons for this difference.

Protection against heterologous Burkholderia pseudomallei strains by dendritic cell immunization

Burkholderia pseudomallei, the causative agent of melioidosis, is a gram-negative bacterium which can cause either chronic infections or acute lethal sepsis in infected individuals. The disease is endemic in Southeast Asia and northern Australia, but little is known about the mechanisms of protective immunity to the bacterium. In this study, we have developed a procedure to utilize dendritic cells in combination with CpG oligodeoxynucleotides as a vaccine delivery vector to induce protective immune responses to various strains of B. pseudomallei. Our results show that strong cell-mediated immune responses were generated, while antibody responses, although low, were detectable. Upon virulent challenge with B. pseudomallei strain K96243, NCTC 4845, or 576, animals immunized with dendritic cells that were pulsed with heat-killed K96243 and matured in the presence of CpG 1826 showed significant levels of protection. These results show that a vaccine strategy that actively targets dendritic cells can evoke protective immune responses.

RaxH/RaxR: a two-component regulatory system in Xanthomonas oryzae pv. oryzae required for AvrXa21 activity

Xanthomonas oryzae pv. oryzae is the causal agent of bacterial leaf blight, one of the most serious diseases in rice. X. oryzae pv. oryzae Philippine race 6 (PR6) strains are unable to establish infection in rice lines expressing the resistance gene Xa21. Although the pathogen-associated molecule that triggers the Xa21-mediated defense response (AvrXa21) is unknown, six rax (required for AvrXa21 activity) genes encoding proteins involved in sulfur metabolism and Type I secretion were recently identified. Here, we report on the identification of two additional rax genes, raxR and raxH, which encode a response regulator and a histidine protein kinase of two-component regulatory systems, respectively. Null mutants of PR6 strain PXO99 that are impaired in either raxR, raxH, or both cause lesions significantly longer and grow to significantly higher levels than does the wild-type strain in Xa21-rice leaves. Both raxR and raxH mutants are complemented to wild-type levels of AvrXa21 activity by introduction of expression vectors carrying raxR and raxH, respectively. These null mutants do not affect AvrXa7 and AvrXa10 activities, as observed in inoculation experiments with Xa7- and Xa10-rice lines. Western blot and raxR/gfp promoter-reporter analyses confirmed RaxR expression in X. oryzae pv. oryzae. The results of promoter-reporter studies also suggest that the previously identified raxSTAB operon is a target for RaxH/RaxR regulation. Characterization of the RaxH/RaxR system provides new opportunities for understanding the specificity of the X. oryzae pv. oryzae-Xa21 interaction and may contribute to the identification of AvrXa21.

Role of two novel two-component regulatory systems in development and phosphatase expression in Myxococcus xanthus

We have cloned a two-component regulatory system (phoR2-phoP2) of Myxococcus xanthus while searching for genes that encode proteins with phosphatase activity, where phoR2 encodes the histidine kinase and phoP2 encodes the response regulator. A second system, phoR3-phoP3, was identified and isolated by using phoP2 as a probe. These two systems are quite similar, sharing identities along the full-length proteins of 52% on the histidine kinases and 64% on the response regulators. The predicted structures of both kinases suggest that they are anchored to the membrane, with the sensor domains being located in the periplasmic space and the kinase domains in the cytoplasm. The response regulators (PhoP2 and PhoP3) exhibit a helix-loop-helix motif typical of DNA-binding proteins in the effector domains located in the C-terminal region. Studies on two single-deletion mutants and one double-deletion mutant have revealed that these systems are involved in development. Mutant fruiting bodies are not well packed, originating loose and flat aggregates where some myxospores do not reshape properly, and they remain as elongated cells. These systems are also involved in the expression of Mg-independent acid and neutral phosphatases, which are expressed during development. The neutral phosphatase gene is especially dependent on PhoP3. Neither PhoP2 nor PhoP3 regulates the expression of alkaline phosphatases and the pph1 gene.

An Inv/Mxi-Spa-like type III protein secretion system in Burkholderia pseudomallei modulates intracellular behaviour of the pathogen

Burkholderia pseudomallei is the causative agent of melioidosis, a serious infectious disease of humans and animals that is endemic in subtropical areas. B. pseudomallei is a facultative intracellular pathogen that may invade and survive within eukaryotic cells for prolonged periods. After internalization, the bacteria escape from endocytic vacuoles into the cytoplasm of infected cells and form membrane protrusions by inducing actin polymerization at one pole. It is believed that survival within phagocytic cells and cell-to-cell spread via actin protrusions is required for full virulence. We have studied the role of a putative type III protein secretion apparatus (Bsa) in the interaction between B. pseudomallei and host cells. The Bsa system is very similar to the Inv/Mxi-Spa type III secretion systems of Salmonella and Shigella. Moreover, B. pseudomallei encodes proteins that are very similar to Salmonella and Shigella Inv/Mxi-Spa secreted proteins required for invasion, escape from endocytic vacuoles, intercellular spread and pathogenesis. Antibodies to putative Bsa-secreted proteins were detected in convalescent serum from a melioidosis patient, suggesting that the system is functionally expressed in vivo. B. pseudomallei mutant strains lacking components of the Bsa secretion and translocation apparatus were constructed. The mutant strains exhibited reduced replication in J774.2 murine macrophage-like cells, an inability to escape from endocytic vacuoles and a complete absence of formation of membrane protrusions and actin tails. These findings indicate that the Bsa type III secretion system plays an essential role in modulating the intracellular behaviour of B. pseudomallei.

A mutation in an ompR-like gene on a Legionella longbeachae serogroup 1 plasmid attenuates virulence

Examination of a panel of Legionella longbeachae serogroup 1 strains using a guinea pig model of virulence determined that this clonal species of Legionella shows a remarkable variation in symptoms and disease outcome (Doyle et al., Infect. Immun. 69, 5335-5344, 2001). The presence of plasmids was investigated, as plasmid encoded functions may contribute to the virulence of genetically similar strains. Partial sequence analysis of a large native plasmid (approximately 120 kb), designated pA5H5, from a highly virulent Australian isolate revealed a putative two-component regulatory system with inferred identity to the OmpR family of two-component transcriptional regulatory proteins and EnvZ sensor kinases. An isogenic mutant was constructed in the transcriptional regulatory gene, designated lrpR (L. longbeachae sg 1 regulatory protein) and this strain was tested in Acanthamoeba, U937 cells and in a guinea pig animal model. The mutant was reduced in intracellular multiplication within Acanthamoeba but not U937 macrophage-like cells. However, the lrpR mutant did appear reduced in invasion at the early stages of infection of U937 cells. The lrpR mutant was also attenuated for virulence in a guinea pig animal model of infection. The importance of plasmid-encoded functions for the pathogenicity of Legionella longbeachae serogroup 1 strains is discussed.

Study on the pathophysiology of experimental Burkholderia pseudomallei infection in mice

Burkholderia pseudomallei is the etiological agent of melioidosis, a potentially fatal disease occurring in man and animals. The aim of this study was to investigate the pathophysiological course of experimental melioidosis, and to identify the target organs, in an animal model. For this purpose SWISS mice were infected intraperitoneally with the virulent strain B. pseudomallei 6068. The bacterial load of various organs was quantified daily by bacteriological analysis and by an enzyme-linked immunosorbent assay (ELISA) based on a monoclonal antibody specific to B. pseudomallei exopolysaccharide (EPS). Electron microscopic investigation of the spleen was performed to locate the bacteria at the cellular level. In this model of acute melioidosis, B. pseudomallei had a marked organ tropism for liver and spleen, and showed evidence of in vivo growth with a bacterial burden of 1.6x10(9) colony forming units (CFU) per gram of spleen 5 days after infection with 200 CFU. The highest bacterial loads were detected in the spleen at all time points, in a range from 2x10(6) to 2x10(9) CFU g(-1). They were still 50-80 times greater than the load of the liver at the time of peak burden. Other investigated organs such as lungs, kidneys, and bone marrow were 10(2)-10(4)-fold less infected than the spleen, with loads ranging from 3x10(2) to 3x10(6) CFU g(-1). The heart and the brain were sites of a delayed infection, with counts in a range from 10(3) to 10(7) times lower than bacterial counts in the spleen. The EPS-specific ELISA proved to be highly sensitive, particularly at the level of those tissues in which colony counting on agar revealed low contamination. In the blood, EPS was detected at concentrations corresponding to bacterial loads ranging from 8x10(3) to 6x10(4) CFU ml(-1). Electron microscopic examination of the spleen revealed figures of phagocytosis, and the presence of large numbers of intact bacteria, which occurred either as single cells or densely packed into vacuoles. Sparse figures suggesting bacterial replication were also observed. In addition, some bacteria could be seen in vacuoles that seemed to have lost their membrane. These observations provide a basis for further investigations on the pathogenesis of the disease.

A new two-component regulatory system involved in adhesion, autolysis, and extracellular proteolytic activity of Staphylococcus aureus

A transposition mutant of Staphylococcus aureus was selected from the parent strain MT23142, a derivative of strain 8325. The site of transposition was near the 5' terminus of the gene arlS. ArlS exhibits strong similarities with histidine protein kinases. Sequence analysis suggested that arlS forms an operon with upstream gene arlR. The predicted product of arlR is a member of the OmpR-PhoB family of response regulators. The arlS mutant formed a biofilm on a polystyrene surface unlike the parent strain and the complemented mutant. Biofilm formation was associated with increased primary adherence to polystyrene, whereas cellular adhesion was only slightly decreased. In addition, the arlS mutant exhibited increased autolysis and altered peptidoglycan hydrolase activity compared to the parental strain and to the complemented mutant. As it has been shown for coagulase-negative staphylococci that some autolysins are able to bind polymer surfaces, these data suggest that the two-component regulatory system ArlS-ArlR may control attachment to polymer surfaces by affecting secreted peptidoglycan hydrolase activity. Finally, the arlS mutant showed a dramatic decrease of extracellular proteolytic activity, including serine protease activity, in comparison to the wild-type strain and the complemented mutant, and cells grown in the presence of phenylmethylsulfonyl fluoride (a serine protease inhibitor) showed an increased autolysin activity. Since the locus arlR-arlS strikingly modifies extracellular proteolytic activity, this locus might also be involved in the virulence of S. aureus.

The dsbA-dsbB disulfide bond formation system of Burkholderia cepacia is involved in the production of protease and alkaline phosphatase, motility, metal resistance, and multi-drug resistance

In a previous study, we isolated a dsbB mutant of Burkholderia cepacia KF1 and showed that phenotypes of protease production and motility are dependent on DsbB, a membrane-bound disulfide bond oxidoreductase. We have now isolated a dsbA mutant by transposon mutagenesis, cloned the dsbA gene encoding a periplasmic disulfide bond oxidoreductase, and characterized the function of the DsbA-DsbB disulfide bond formation system in B. cepacia. The complementing DNA fragment had an open reading frame for a 212-amino acid polypeptide with a potential redox-active site sequence of Cys-Pro-His-Cys that is homologous to Escherichia coli DsbA. The dsbA mutant, as well as the previously isolated dsbB mutant, was defective in the production of extracellular protease and alkaline phosphatase, as well as in motility. In addition, mutation in the DsbA-DsbB system resulted in an increase in sensitivity to Cd2+ and Zn2+ as well as a variety of antibiotics including beta-lactams, kanamycin, erythromycin, novobiocin, ofloxacin and sodium dodecyl sulfate. These results suggested that the DsbA-DsbB system might be involved in the formation of a metal efflux system as well as a multi-drug resistance system.

Multiple replicons constitute the 6.5-megabase genome of Burkholderia pseudomallei

Burkholderia pseudomallei is a causative agent of melioidosis, a fatal tropical infectious disease endemic in Southeast Asia and Northern Australia. In order to determine the size and characteristics of the bacterial genome, the B. pseudomallei genome and genes were analyzed by pulsed field gel electrophoresis of the undigested, intact megabase DNA, and by computational analysis of nucleotide sequences of B. pseudomallei genes which have been sequenced by several investigators and already deposited in a public database. The results showed that the B. pseudomallei genome consists of two large replicons, and that both contain ribosomal RNA gene sequences, indicating the presence of two chromosomes. The classical arabinose-negative B. pseudomallei isolate K96243 has chromosomes of approximately 3563 +/- 73 and 2974 +/- 40 kilobase-pairs in size, giving a total genome size of about 6.5 million base-pairs. The arabinose-positive nonvirulent biotype of B. pseudomallei also has two replicons which are smaller than those of the arabinose-negative biotype. Analysis of the publicly-available nucleotide sequences showed that the average B. pseudomallei gene is approximately 1031 base-pairs in size, with an average G + C content of 65.7%. The genome is gene-rich and about 89% of the coding capacity is used as coding sequences. It can therefore be estimated that the entire B. pseudomallei genome encodes about 5600 genes.

Identification of regions of the chromosome of Neisseria meningitidis and Neisseria gonorrhoeae which are specific to the pathogenic Neisseria species

Neisseria meningitidis and Neisseria gonorrhoeae give rise to dramatically different diseases. Their interactions with the host, however, do share common characteristics: they are both human pathogens which do not survive in the environment and which colonize and invade mucosa at their port of entry. It is therefore likely that they have common properties that might not be found in nonpathogenic bacteria belonging to the same genetically related group, such as Neisseria lactamica. Their common properties may be determined by chromosomal regions found only in the pathogenic Neisseria species. To address this issue, we used a previously described technique (C. R. Tinsley and X. Nassif, Proc. Natl. Acad. Sci. USA 93:11109-11114, 1996) to identify sequences of DNA specific for pathogenic neisseriae and not found in N. lactamica. Sequences present in N. lactamica were physically subtracted from the N. meningitidis Z2491 sequence and also from the N. gonorrhoeae FA1090 sequence. The clones obtained from each subtraction were tested by Southern blotting for their reactivity with the three species, and only those which reacted with both N. meningitidis and N. gonorrhoeae (i.e., not specific to either one of the pathogens) were further investigated. In a first step, these clones were mapped onto the chromosomes of both N. meningitidis and N. gonorrhoeae. The majority of the clones were arranged in clusters extending up to 10 kb, suggesting the presence of chromosomal regions common to N. meningitidis and N. gonorrhoeae which distinguish these pathogens from the commensal N. lactamica. The sequences surrounding these clones were determined from the N. meningitidis genome-sequencing project. Several clones corresponded to previously described factors required for colonization and survival at the port of entry, such as immunoglobulin A protease and PilC. Others were homologous to virulence-associated proteins in other bacteria, demonstrating that the subtractive clones are capable of pinpointing chromosomal regions shared by N. meningitidis and N. gonorrhoeae which are involved in common aspects of the host interaction of both pathogens.

Multiple mechanisms of action for inhibitors of histidine protein kinases from bacterial two-component systems

Many pathogenic bacteria utilize two-component systems consisting of a histidine protein kinase (HPK) and a response regulator (RR) for signal transduction. During the search for novel inhibitors, several chemical series, including benzoxazines, benzimidazoles, bis-phenols, cyclohexenes, trityls, and salicylanilides, were identified that inhibited the purified HPK-RR pairs KinA-Spo0F and NRII-NRI, with 50% inhibitory concentrations (IC50s) ranging from 1.9 to >500 microM and MICs ranging from 0.5 to >16 microg/ml for gram-positive bacteria. However, additional observations suggested that mechanisms other than HPK inhibition might contribute to antibacterial activity. In the present work, representative compounds from the six different series of inhibitors were analyzed for their effects on membrane integrity and macromolecular synthesis. At 4x MIC, 17 of 24 compounds compromised the integrity of the bacterial cell membrane within 10 min, as measured by uptake of propidium iodide. In this set, compounds with lower IC50s tended to cause greater membrane disruption. Eleven of 12 compounds inhibited cellular incorporation of radiolabeled thymidine and uridine >97% in 5 min and amino acids >80% in 15 min. The HPK inhibitor that allowed >25% precursor incorporation had no measurable MIC (>16 microg/ml). Fifteen of 24 compounds also caused hemolysis of equine erythrocytes. Thus, the antibacterial HPK inhibitors caused a rapid decrease in cellular incorporation of RNA, DNA, and protein precursors, possibly as a result of the concomitant disruption of the cytoplasmic membrane. Bacterial killing by these HPK inhibitors may therefore be due to multiple mechanisms, independent of HPK inhibition.

Efflux-mediated aminoglycoside and macrolide resistance in Burkholderia pseudomallei

Burkholderia pseudomallei, the causative agent of melioidosis, is intrinsically resistant to a wide range of antimicrobial agents including beta-lactams, aminoglycosides, macrolides, and polymyxins. We used Tn5-OT182 to mutagenize B. pseudomallei to identify the genes involved in aminoglycoside resistance. We report here on the identification of AmrAB-OprA, a multidrug efflux system in B. pseudomallei which is specific for both aminoglycoside and macrolide antibiotics. We isolated two transposon mutants, RM101 and RM102, which had 8- to 128-fold increases in their susceptibilities to the aminoglycosides streptomycin, gentamicin, neomycin, tobramycin, kanamycin, and spectinomycin. In addition, both mutants, in contrast to the parent, were susceptible to the macrolides erythromycin and clarithromycin but not to the lincosamide clindamycin. Sequencing of the DNA flanking the transposon insertions revealed a putative operon consisting of a resistance, nodulation, division-type transporter, a membrane fusion protein, an outer membrane protein, and a divergently transcribed regulatorprotein. Consistent with the presence of an efflux system, both mutants accumulated [3H] dihydro streptomycin, whereas the parent strain did not. We constructed an amr deletion strain, B. pseudomallei DD503, which was hypersusceptible to aminoglycosides and macrolides and which was used successfully in allelic exchange experiments. These results suggest that an efflux system is a major contributor to the inherent high-level aminoglycoside and macrolide resistance found in B. pseudomallei.

Current studies on the pathogenesis of melioidosis

Burkholderia pseudomallei is a major cause of bacterial septicemias in many parts of the world, particularly Thailand; the known geographic range of the organism appears to be enlarging as awareness of the organism and the disease it causes--melioidosis--increases. B. pseudomallei is intrinsically resistant to most antibiotics, and our knowledge of B. pseudomallei pathogenesis is lacking. Thus, the long-term objective of our research is to define at a molecular level the pathogenesis by combining genetic, immunologic, and biochemical approaches with animal model studies. Basic studies on B. pseudomallei pathogenesis are acutely needed to provide a knowledge base to rationally design new modes of therapy directed against this organism.

The type II O-antigenic polysaccharide moiety of Burkholderia pseudomallei lipopolysaccharide is required for serum resistance and virulence

Melioidosis, an infection caused by the gram-negative bacterial pathogen Burkholderia pseudomallei, is endemic in south-east Asia and northern Australia. Acute septicaemic melioidosis is a major cause of morbidity and mortality, especially in north-east Thailand. B. pseudomallei is highly resistant to the bactericidal activity of normal human serum (NHS), and we have found that B. pseudomallei 1026b multiplies in 10-30% NHS. We developed a simple screen for the identification of serum-sensitive mutants based on this novel phenotype. Approximately 1200 Tn5-OT182 mutants were screened, and three serum-sensitive mutants were identified. The type II O-antigenic polysaccharide (O-PS) moiety of lipopolysaccharide was not present in the serum-sensitive mutants. A representative serum-sensitive mutant, SRM117, was killed by the alternative pathway of complement and was less virulent than 1026b in three animal models of melioidosis. The Tn5-OT182 integrations in the serum-sensitive mutants were physically linked on the B. pseudomallei chromosome, and further genetic analysis of this locus revealed a cluster of 15 genes required for type II O-PS production. The proteins encoded by these genes were similar to proteins involved in bacterial polysaccharide biosynthesis. The results presented here demonstrate that type II O-PS is essential for B. pseudomallei serum resistance and virulence.