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1.  Development and Application of a Cellular, Gain-of-Signal, Bioluminescent Reporter Screen for Inhibitors of Type II Secretion in Pseudomonas aeruginosa and Burkholderia pseudomallei 
Journal of biomolecular screening  2011;16(7):694-705.
The type II secretion (T2S) system in Gram-negative bacteria is comprised of the Sec and Tat pathways for translocating proteins into the periplasm and an outer membrane secretin for transporting proteins into the extracellular space. To discover Sec/Tat/T2S pathway inhibitors as potential new therapeutics, we used a Pseudomonas aeruginosa bioluminescent reporter strain responsive to SecA depletion and inhibition to screen compound libraries and characterize the hits. The reporter strain placed a luxCDABE operon under regulation of a SecA depletion-responsive up-regulated promoter in a secA deletion background complemented with an ectopic lac-regulated secA copy. Bioluminescence was indirectly proportional to the IPTG concentration and stimulated by azide, a known SecA ATPase inhibitor. A total of 96 compounds (0.1% of 73,000) were detected as primary hits due to stimulation of luminescence with a z-score ≥5. Direct secretion assays of the 9 most potent hits, representing 5 chemical scaffolds, revealed that they do not inhibit SecA-mediated secretion of β-lactamase into the periplasm, but do inhibit T2S-mediated extracellular secretion of elastase with IC50 values from 5 – 25 μM. In addition, 7 of the 9 compounds also inhibited the T2S-mediated extracellular secretion of phospholipases C by P. aeruginosa and of protease activity by Burkholderia pseudomallei.
PMCID: PMC3195541  PMID: 21602485
P. aeruginosa; type II secretion; high throughput screening; inhibitors
2.  The Stringent Response Is Essential for Pseudomonas aeruginosa Virulence in the Rat Lung Agar Bead and Drosophila melanogaster Feeding Models of Infection▿† 
Infection and Immunity  2011;79(10):4094-4104.
The stringent response is a regulatory system that allows bacteria to sense and adapt to nutrient-poor environments. The central mediator of the stringent response is the molecule guanosine 3′,5′-bispyrophosphate (ppGpp), which is synthesized by the enzymes RelA and SpoT and which is also degraded by SpoT. Our laboratory previously demonstrated that a relA mutant of Pseudomonas aeruginosa, the principal cause of lung infections in cystic fibrosis patients, was attenuated in virulence in a Drosophila melanogaster feeding model of infection. In this study, we examined the role of spoT in P. aeruginosa virulence. We generated an insertion mutation in spoT within the previously constructed relA mutant, thereby producing a ppGpp-devoid strain. The relA spoT double mutant was unable to establish a chronic infection in D. melanogaster and was also avirulent in the rat lung agar bead model of infection, a model in which the relA mutant is fully virulent. Synthesis of the virulence determinants pyocyanin, elastase, protease, and siderophores was impaired in the relA spoT double mutant. This mutant was also defective in swarming and twitching, but not in swimming motility. The relA spoT mutant and, to a lesser extent, the relA mutant were less able to withstand stresses such as heat shock and oxidative stress than the wild-type strain PAO1, which may partially account for the inability of the relA spoT mutant to successfully colonize the rat lung. Our results indicate that the stringent response, and SpoT in particular, is a crucial regulator of virulence processes in P. aeruginosa.
PMCID: PMC3187270  PMID: 21788391
3.  Burkholderia thailandensis oacA Mutants Facilitate the Expression of Burkholderia mallei-Like O Polysaccharides▿  
Infection and Immunity  2010;79(2):961-969.
Previous studies have shown that the O polysaccharides (OPS) expressed by Burkholderia mallei are similar to those produced by Burkholderia thailandensis except that they lack the 4-O-acetyl modifications on their 6-deoxy-α-l-talopyranosyl residues. In the present study, we describe the identification and characterization of an open reading frame, designated oacA, expressed by B. thailandensis that accounts for this phenomenon. Utilizing the B. thailandensis and B. mallei lipopolysaccharide (LPS)-specific monoclonal antibodies Pp-PS-W and 3D11, Western immunoblot analyses demonstrated that the LPS antigens expressed by the oacA mutant, B. thailandensis ZT0715, were antigenically similar to those produced by B. mallei ATCC 23344. In addition, immunoblot analyses demonstrated that when B. mallei ATCC 23344 was complemented in trans with oacA, it synthesized B. thailandensis-like LPS antigens. To elucidate the structure of the OPS moieties expressed by ZT0715, purified samples were analyzed via nuclear magnetic resonance spectroscopy. As predicted, these studies demonstrated that the loss of OacA activity influenced the O acetylation phenotype of the OPS moieties. Unexpectedly, however, the results indicated that the O methylation status of the OPS antigens was also affected by the loss of OacA activity. Nonetheless, it was revealed that the LPS moieties expressed by the oacA mutant reacted strongly with the B. mallei LPS-specific protective monoclonal antibody 9C1-2. Based on these findings, it appears that OacA is required for the 4-O acetylation and 2-O methylation of B. thailandensis OPS antigens and that ZT0715 may provide a safe and cost-effective source of B. mallei-like OPS to facilitate the synthesis of glanders subunit vaccine candidates.
PMCID: PMC3028842  PMID: 21115721
4.  Identification of Burkholderia mallei and Burkholderia pseudomallei adhesins for human respiratory epithelial cells 
BMC Microbiology  2010;10:250.
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.
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.
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.
PMCID: PMC2955633  PMID: 20920184
5.  Burkholderia pseudomallei Isocitrate Lyase Is a Persistence Factor in Pulmonary Melioidosis: Implications for the Development of Isocitrate Lyase Inhibitors as Novel Antimicrobials ▿ †  
Infection and Immunity  2009;77(10):4275-4283.
Burkholderia pseudomallei, the causative agent of melioidosis, has often been called the great “mimicker,” and clinical disease due to this organism may include acute, chronic, and latent pulmonary infections. Interestingly, chronic pulmonary melioidosis is often mistaken for tuberculosis, and this can have significant consequences, as the treatments for these two infections are radically different. The recurrent misdiagnosis of melioidosis for tuberculosis has caused many to speculate that these two bacterial pathogens use similar pathways to produce latent infections. Here we show that isocitrate lyase is a persistence factor for B. pseudomallei, and inhibiting the activity of this enzyme during experimental chronic B. pseudomallei lung infection forces the infection into an acute state, which can then be treated with antibiotics. We found that if antibiotics are not provided in combination with isocitrate lyase inhibitors, the resulting B. pseudomallei infection overwhelms the host, resulting in death. These results suggest that the inhibition of isocitrate lyase activity does not necessarily attenuate virulence as previously observed for Mycobacterium tuberculosis infections but does force the bacteria into a replicating state where antibiotics are effective. Therefore, isocitrate lyase inhibitors could be developed for chronic B. pseudomallei infections but only for use in combination with effective antibiotics.
PMCID: PMC2747945  PMID: 19620343
6.  Burkholderia thailandensis harbors two identical rhl gene clusters responsible for the biosynthesis of rhamnolipids 
BMC Microbiology  2009;9:263.
Rhamnolipids are surface active molecules composed of rhamnose and β-hydroxydecanoic acid. These biosurfactants are produced mainly by Pseudomonas aeruginosa and have been thoroughly investigated since their early discovery. Recently, they have attracted renewed attention because of their involvement in various multicellular behaviors. Despite this high interest, only very few studies have focused on the production of rhamnolipids by Burkholderia species.
Orthologs of rhlA, rhlB and rhlC, which are responsible for the biosynthesis of rhamnolipids in P. aeruginosa, have been found in the non-infectious Burkholderia thailandensis, as well as in the genetically similar important pathogen B. pseudomallei. In contrast to P. aeruginosa, both Burkholderia species contain these three genes necessary for rhamnolipid production within a single gene cluster. Furthermore, two identical, paralogous copies of this gene cluster are found on the second chromosome of these bacteria. Both Burkholderia spp. produce rhamnolipids containing 3-hydroxy fatty acid moieties with longer side chains than those described for P. aeruginosa. Additionally, the rhamnolipids produced by B. thailandensis contain a much larger proportion of dirhamnolipids versus monorhamnolipids when compared to P. aeruginosa. The rhamnolipids produced by B. thailandensis reduce the surface tension of water to 42 mN/m while displaying a critical micelle concentration value of 225 mg/L. Separate mutations in both rhlA alleles, which are responsible for the synthesis of the rhamnolipid precursor 3-(3-hydroxyalkanoyloxy)alkanoic acid, prove that both copies of the rhl gene cluster are functional, but one contributes more to the total production than the other. Finally, a double ΔrhlA mutant that is completely devoid of rhamnolipid production is incapable of swarming motility, showing that both gene clusters contribute to this phenotype.
Collectively, these results add another Burkholderia species to the list of bacteria able to produce rhamnolipids and this, by the means of two identical functional gene clusters. Our results also demonstrate the very impressive tensio-active properties these long-chain rhamnolipids possess in comparison to the well-studied short-chain ones from P. aeruginosa.
PMCID: PMC2804600  PMID: 20017946
7.  Burkholderia pseudomallei, B. thailandensis, and B. ambifaria Produce 4-Hydroxy-2-Alkylquinoline Analogues with a Methyl Group at the 3 Position That Is Required for Quorum-Sensing Regulation ▿  
Journal of Bacteriology  2008;190(15):5339-5352.
4-Hydroxy-2-alkylquinolines (HAQs), especially 3,4-dihydroxy-2-heptylquinoline (Pseudomonas quinolone signal) and its precursor, 4-hydroxy-2-heptylquinoline, are attracting much attention, mainly because of their role as signaling molecules in Pseudomonas aeruginosa. The pqsABCDE operon is centrally involved in their biosynthesis. The presence of a homologous operon in Burkholderia pseudomallei and B. thailandensis was recently reported. Thus, we have investigated the abilities of 11 Burkholderia species to produce HAQ-like molecules by liquid chromatography/mass spectrometry. We have identified 29 different HAQ derivatives produced by the only three Burkholderia species where a pqsABCDE homologue was found among available sequenced Burkholderia species genomes, including B. ambifaria, a member of the Burkholderia cepacia complex. In contrast with those of P. aeruginosa, Burkholderia HAQs typically bear a methyl group, hence their designation as 4-hydroxy-3-methyl-2-alkylquinolines (HMAQs). We identified three families of HMAQs with a saturated or unsaturated alkyl chain at the 2′ position, in contrast with the 1′ position of P. aeruginosa, including one with an N-oxide group. Furthermore, the operon in these species contains two more genes downstream of the pqsE homologue, resulting in the hmqABCDEFG operon. While the inactivation of hmqA inhibits the production of HMAQs, the methylation of the quinoline ring requires a putative methyltransferase encoded by hmqG. Interestingly, hmqA or hmqG mutations increase the production of acyl homoserine lactones and, consequently, phenotypes under the control of quorum sensing in B. ambifaria: antifungal activity, siderophore production, and proteolytic activity. These results indicate that only HAQs bearing a methyl group (HMAQs) are involved in quorum-sensing regulation.
PMCID: PMC2493281  PMID: 18539738
8.  Burkholderia pseudomallei Type III Secretion System Mutants Exhibit Delayed Vacuolar Escape Phenotypes in RAW 264.7 Murine Macrophages▿  
Infection and Immunity  2008;76(7):2991-3000.
Burkholderia pseudomallei is a facultative intracellular pathogen capable of surviving and replicating within eukaryotic cells. Recent studies have shown that B. pseudomallei Bsa type III secretion system 3 (T3SS-3) mutants exhibit vacuolar escape and replication defects in J774.2 murine macrophages. In the present study, we characterized the interactions of a B. pseudomallei bsaZ mutant with RAW 264.7 murine macrophages. Following uptake, the mutant was found to survive and replicate within infected RAW 264.7 cells over an 18-h period. In addition, high levels of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), granulocyte-macrophage colony-stimulating factor (GM-CSF), and RANTES, but not IL-1α and IL-1β, were detected in culture supernatants harvested from infected monolayers. The subcellular location of B. pseudomallei within infected RAW 264.7 cells was determined, and as expected, the bsaZ mutant demonstrated early-vacuolar-escape defects. Interestingly, however, experiments also indicated that this mutant was capable of delayed vacuolar escape. Consistent with this finding, evidence of actin-based motility and multinucleated giant cell formation were observed between 12 and 18 h postinfection. Further studies demonstrated that a triple mutant defective in all three B. pseudomallei T3SSs exhibited the same phenotype as the bsaZ mutant, indicating that functional T3SS-1 and T3SS-2 did not appear to be responsible for the delayed escape phenotype in RAW 264.7 cells. Based upon these findings, it appears that B. pseudomallei may not require T3SS-1, -2, and -3 to facilitate survival, delayed vacuolar escape, and actin-based motility in activated RAW 264.7 macrophages.
PMCID: PMC2446725  PMID: 18443088
9.  Survival of Burkholderia pseudomallei in Water 
BMC Research Notes  2008;1:11.
The ability of Burkholderia pseudomallei to survive in water likely contributes to its environmental persistence in endemic regions. To determine the physiological adaptations which allow B. pseudomallei to survive in aqueous environments, we performed microarray analyses of B. pseudomallei cultures transferred from Luria broth (LB) to distilled water.
Increased expression of a gene encoding for a putative membrane protein (BPSL0721) was confirmed using a lux-based transcriptional reporter system, and maximal expression was noted at approximately 6 hrs after shifting cells from LB to water. A BPSL0721 deficient mutant of B. pseudomallei was able to survive in water for at least 90 days indicating that although involved, BPSL0721 was not essential for survival. BPSL2961, a gene encoding a putative phosphatidylglycerol phosphatase (PGP), was also induced when cells were shifted to water. This gene is likely involved in cell membrane biosynthesis. We were unable to construct a PGP mutant suggesting that the gene is not only involved in survival in water but is essential for cell viability. We also examined mutants of polyhydroxybutyrate synthase (phbC), lipopolysaccharide (LPS) oligosaccharide and capsule synthesis, and these mutations did not affect survival in water. LPS mutants lacking outer core were found to lose viability in water by 200 days indicating that an intact LPS core provides an outer membrane architecture which allows prolonged survival in water.
The results from these studies suggest that B. pseudomallei survival in water is a complex process that requires an LPS molecule which contains an intact core region.
PMCID: PMC2518269  PMID: 18710531
10.  Multilocus Sequence Typing of Historical Burkholderia pseudomallei Isolates Collected in Southeast Asia from 1964 to 1967 Provides Insight into the Epidemiology of Melioidosis 
Journal of Clinical Microbiology  2006;44(8):2951-2962.
A collection of 207 historically relevant Burkholderia pseudomallei isolates was analyzed by multilocus sequence typing (MLST). The strain collection contains environmental isolates obtained from a geographical distribution survey of B. pseudomallei isolates in Thailand (1964 to 1967), as well as stock cultures and colony variants from the U.S. Army Medical Research Unit (Malaysia), the Walter Reed Army Institute for Research, and the Pasteur Institute (Vietnam). The 207 isolates of the collection were resolved into 80 sequence types (STs); 56 of these were novel. eBURST diagrams predict that the historical-collection STs segregate into three complexes when analyzed separately. When added to the 760 isolates and 365 STs of the B. pseudomallei MLST database, the historical-collection STs cluster significantly within the main complex of the eBURST diagram in an ancestral pattern and alter the B. pseudomallei “population snapshot.” Differences in colony morphology among reference isolates were found not to affect the STs assigned, which were consistent with the original isolates. Australian ST84 is likely characteristic of B. pseudomallei isolates of Southeast Asia rather than Australia, since multiple environmental isolates from Thailand and Malaysia share this ST with the single Australian clinical isolate in the MLST database. Phylogenetic evidence is also provided suggesting that Australian isolates may not be distinct from those of Thailand, since ST60 is common to environmental isolates from both countries. MLST and eBURST are useful tools for the study of population biology and epidemiology, since they provide methods to elucidate new genetic relationships among bacterial isolates.
PMCID: PMC1594636  PMID: 16891516
11.  Genome-Wide Expression Analysis of Burkholderia pseudomallei Infection in a Hamster Model of Acute Melioidosis  
Infection and Immunity  2006;74(10):5465-5476.
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.
PMCID: PMC1594879  PMID: 16988221
12.  Antimicrobial Peptide Therapeutics for Cystic Fibrosis 
Greater than 90% of lung infections in cystic fibrosis (CF) patients are caused by Pseudomonas aeruginosa, and the majority of these patients subsequently die from lung damage. Current therapies are either targeted at reducing obstruction, reducing inflammation, or reducing infection. To identify potential therapeutic agents for the CF lung, 150 antimicrobial peptides consisting of three distinct structural classes were screened against mucoid and multidrug-resistant clinical isolates of P. aeruginosa, Stenotrophomonas maltophilia, Achromobacter xylosoxidans, and Staphylococcus aureus. Five peptides that retained potent antimicrobial activities in physiological salt and divalent cation environment were further characterized in vivo using a rat chronic lung infection model. All animals were inoculated intratracheally with 104 P. aeruginosa mucoid PAO1 cells in agar beads. Three days following inoculation treatment was initiated. Animals were treated daily for 3 days with 100 μl of peptide solution (1 mg/ml) in 10 mM sodium citrate, which was deposited via either intratracheal instillation or aerosolization. Control animals received daily exposure to vehicle alone. At the end of the treatment the lungs of the animals were removed for quantitative culture. Four peptides, HBCM2, HBCM3, HBCPα-2, and HB71, demonstrated significant reduction in Pseudomonas bioburden in the lung of rats. Further in vivo studies provided direct evidence that anti-inflammatory activity was associated with three of these peptides. Therefore, small bioactive peptides have the potential to attack two of the components responsible for the progression of lung damage in the CF disease: infection and inflammation.
PMCID: PMC1168697  PMID: 15980369
13.  The Capsular Polysaccharide of Burkholderia pseudomallei Contributes to Survival in Serum by Reducing Complement Factor C3b Deposition  
Infection and Immunity  2005;73(2):1106-1115.
Burkholderia pseudomallei produces an extracellular polysaccharide capsule -3)-2-O-acetyl-6-deoxy-β-d-manno-heptopyranose-(1- which has been shown to be an essential virulence determinant. The addition of purified capsule was shown to increase the virulence of a capsule mutant strain in the Syrian hamster model of acute melioidosis. An increase in the number of wild-type B. pseudomallei cells in the blood was seen by 48 h, while the number of capsule mutant cells in the blood declined by 48 h. Capsule expression was shown to be induced in the presence of serum using a lux reporter fusion to the capsule gene wcbB. The addition of purified B. pseudomallei capsule to serum bactericidal assays increased the survival of B. pseudomallei SLR5, a serum-sensitive strain, by 1,000-fold in normal human serum. Capsule production by B. pseudomallei contributed to reduced activation of the complement cascade by reducing the levels of complement factor C3b deposition. An increase in phagocytosis of the capsule mutant compared to the wild type was observed in the presence of normal human serum. These results suggest that the production of this capsule contributes to resistance to phagocytosis by reducing C3b deposition on the surface of the bacterium, thereby contributing to the persistence of bacteria in the blood of the infected host. Continued studies to characterize this capsule are essential for understanding the pathogenesis of B. pseudomallei infections and the development of preventive strategies for treatment of this disease.
PMCID: PMC547107  PMID: 15664954
14.  Contribution of Gene Loss to the Pathogenic Evolution of Burkholderia pseudomallei and Burkholderia mallei  
Infection and Immunity  2004;72(7):4172-4187.
Burkholderia pseudomallei is the causative agent of melioidosis. Burkholderia thailandensis is a closely related species that can readily utilize l-arabinose as a sole carbon source, whereas B. pseudomallei cannot. We used Tn5-OT182 mutagenesis to isolate an arabinose-negative mutant of B. thailandensis. Sequence analysis of regions flanking the transposon insertion revealed the presence of an arabinose assimilation operon consisting of nine genes. Analysis of the B. pseudomallei chromosome showed a deletion of the operon from this organism. This deletion was detected in all B. pseudomallei and Burkholderia mallei strains investigated. We cloned the B. thailandensis E264 arabinose assimilation operon and introduced the entire operon into the chromosome of B. pseudomallei 406e via homologous recombination. The resultant strain, B. pseudomallei SZ5028, was able to utilize l-arabinose as a sole carbon source. Strain SZ5028 had a significantly higher 50% lethal dose for Syrian hamsters compared to the parent strain 406e. Microarray analysis revealed that a number of genes in a type III secretion system were down-regulated in strain SZ5028 when cells were grown in l-arabinose, suggesting a regulatory role for l-arabinose or a metabolite of l-arabinose. These results suggest that the ability to metabolize l-arabinose reduces the virulence of B. pseudomallei and that the genes encoding arabinose assimilation may be considered antivirulence genes. The increase in virulence associated with the loss of these genes may have provided a selective advantage for B. pseudomallei as these organisms adapted to survival in animal hosts.
PMCID: PMC427422  PMID: 15213162
15.  Comparative Analysis of Plant and Animal Models for Characterization of Burkholderia cepacia Virulence  
Infection and Immunity  2003;71(9):5306-5313.
A simple alfalfa model was developed as an alternative infection model for virulence studies of the Burkholderia cepacia complex. Symptoms of disease were observed in wounded alfalfa seedlings within 7 days following inoculation of 101 to 105 CFU of most strains of the B. cepacia complex. Strains from seven genomovars of the B. cepacia complex were tested for virulence in the alfalfa model, and the degree of virulence was generally similar in strains belonging to the same genomovar. Strains of Burkholderia multivorans and some strains of Burkholderia stabilis did not cause symptoms of disease in alfalfa seedlings. Representative strains were also tested for virulence using the rat agar bead model. Most of the strains tested were able to establish chronic lung infections; B. stabilis strains were the exception. Most of the strains that were virulent in the alfalfa infection model were also virulent in the lung infection model. The B. cepacia genomovar III mutants K56pvdA::tp and K56-H15 were significantly less virulent in the alfalfa infection model than their parent strain. Therefore, this alfalfa infection model may be a useful tool for assessing virulence of strains of the B. cepacia complex and identifying new virulence-associated genes.
PMCID: PMC187319  PMID: 12933878
16.  Burkholderia pseudomallei Class A β-Lactamase Mutations That Confer Selective Resistance against Ceftazidime or Clavulanic Acid Inhibition 
Burkholderia pseudomallei, the causative agent of melioidosis, is inherently resistant to a variety of antibiotics including aminoglycosides, macrolides, polymyxins, and β-lactam antibiotics. Despite resistance to many β-lactams, ceftazidime and β-lactamase inhibitor-β-lactam combinations are commonly used for treatment of melioidosis. Here, we examine the enzyme kinetics of β-lactamase isolated from mutants resistant to ceftazidime and clavulanic acid inhibition and describe specific mutations within conserved motifs of the β-lactamase enzyme which account for these resistance patterns. Sequence analysis of regions flanking the B. pseudomallei penA gene revealed a putative regulator gene located downstream of penA. We have cloned and sequenced the penA gene from B. mallei and found it to be identical to penA from B. pseudomallei.
PMCID: PMC161859  PMID: 12821450
17.  Role of Phosphoglucomutase of Stenotrophomonas maltophilia in Lipopolysaccharide Biosynthesis, Virulence, and Antibiotic Resistance  
Infection and Immunity  2003;71(6):3068-3075.
A homologue of the algC gene, responsible for the production of a phosphoglucomutase (PGM) associated with LPS and alginate biosynthesis in Pseudomonas aeruginosa, spgM, was cloned from Stenotrophomonas maltophilia. The spgM gene was shown to encode a bifunctional enzyme with both PGM and phosphomannomutase activities. Mutants lacking spgM produced less LPS than the SpgM+ parent strain and had a tendency for shorter O polysaccharide chains. No changes in LPS chemistry were obvious as a result of the loss of spgM. Significantly, however, spgM mutants displayed a modest increase in susceptibility to several antimicrobial agents and were completely avirulent in an animal model of infection. The latter finding may relate to the resultant serum sensitivity of spgM mutants which, unlike the wild-type parent strain, were rapidly killed by human serum. These data highlight the contribution made by LPS to the antimicrobial resistance and virulence of S. maltophilia.
PMCID: PMC155759  PMID: 12761084
18.  Flagellum-Mediated Adhesion by Burkholderia pseudomallei Precedes Invasion of Acanthamoeba astronyxis  
Infection and Immunity  2003;71(4):2280-2282.
In this study we investigated the role of the bacterial flagellum in Burkholderia pseudomallei entry to Acanthamoeba astronyxis trophozoites. B. pseudomallei cells were tethered to the external amoebic surface via their flagella. MM35, the flagellum-lacking fliC knockout derivative of B. pseudomallei NCTC 1026b did not demonstrate flagellum-mediated endocytosis in timed coculture, confirming that an intact flagellar apparatus assists B. pseudomallei entry into A. astronyxis.
PMCID: PMC152052  PMID: 12654857
19.  Burkholderia thailandensis E125 Harbors a Temperate Bacteriophage Specific for Burkholderia mallei 
Journal of Bacteriology  2002;184(14):4003-4017.
Burkholderia thailandensis is a nonpathogenic gram-negative bacillus that is closely related to Burkholderia mallei and Burkholderia pseudomallei. We found that B. thailandensis E125 spontaneously produced a bacteriophage, termed φE125, which formed turbid plaques in top agar containing B. mallei ATCC 23344. We examined the host range of φE125 and found that it formed plaques on B. mallei but not on any other bacterial species tested, including B. thailandensis and B. pseudomallei. Examination of the bacteriophage by transmission electron microscopy revealed an isometric head and a long noncontractile tail. B. mallei NCTC 120 and B. mallei DB110795 were resistant to infection with φE125 and did not produce lipopolysaccharide (LPS) O antigen due to IS407A insertions in wbiE and wbiG, respectively. wbiE was provided in trans on a broad-host-range plasmid to B. mallei NCTC 120, and it restored LPS O-antigen production and susceptibility to φE125. The 53,373-bp φE125 genome contained 70 genes, an IS3 family insertion sequence (ISBt3), and an attachment site (attP) encompassing the 3′ end of a proline tRNA (UGG) gene. While the overall genetic organization of the φE125 genome was similar to λ-like bacteriophages and prophages, it also possessed a novel cluster of putative replication and lysogeny genes. The φE125 genome encoded an adenine and a cytosine methyltransferase, and purified bacteriophage DNA contained both N6-methyladenine and N4-methylcytosine. The results presented here demonstrate that φE125 is a new member of the λ supergroup of Siphoviridae that may be useful as a diagnostic tool for B. mallei.
PMCID: PMC135171  PMID: 12081973
20.  Molecular and Physical Characterization of Burkholderia mallei O Antigens 
Journal of Bacteriology  2002;184(3):849-852.
Burkholderia mallei lipopolysaccharide (LPS) has been previously shown to cross-react with polyclonal antibodies raised against B. pseudomallei LPS; however, we observed that B. mallei LPS does not react with a monoclonal antibody (Pp-PS-W) specific for B. pseudomallei O polysaccharide (O-PS). In this study, we identified the O-PS biosynthetic gene cluster from B. mallei ATCC 23344 and subsequently characterized the molecular structure of the O-PS produced by this organism.
PMCID: PMC139525  PMID: 11790757
21.  Detection of Bacterial Virulence Genes by Subtractive Hybridization: Identification of Capsular Polysaccharide of Burkholderia pseudomallei as a Major Virulence Determinant 
Infection and Immunity  2001;69(1):34-44.
Burkholderia pseudomallei, the etiologic agent of melioidosis, is responsible for a broad spectrum of illnesses in humans and animals particularly in Southeast Asia and northern Australia, where it is endemic. Burkholderia thailandensis is a nonpathogenic environmental organism closely related to B. pseudomallei. Subtractive hybridization was carried out between these two species to identify genes encoding virulence determinants in B. pseudomallei. Screening of the subtraction library revealed A-T-rich DNA sequences unique to B. pseudomallei, suggesting they may have been acquired by horizontal transfer. One of the subtraction clones, pDD1015, encoded a protein with homology to a glycosyltransferase from Pseudomonas aeruginosa. This gene was insertionally inactivated in wild-type B. pseudomallei to create SR1015. It was determined by enzyme-linked immunosorbent assay and immunoelectron microscopy that the inactivated gene was involved in the production of a major surface polysaccharide. The 50% lethal dose (LD50) for wild-type B. pseudomallei is <10 CFU; the LD50 for SR1015 was determined to be 3.5 × 105 CFU, similar to that of B. thailandensis (6.8 × 105 CFU). DNA sequencing of the region flanking the glycosyltransferase gene revealed open reading frames similar to capsular polysaccharide genes in Haemophilus influenzae, Escherichia coli, and Neisseria meningitidis. In addition, DNA from Burkholderia mallei and Burkholderia stabilis hybridized to a glycosyltransferase fragment probe, and a capsular structure was identified on the surface of B. stabilis via immunoelectron microscopy. Thus, the combination of PCR-based subtractive hybridization, insertional inactivation, and animal virulence studies has facilitated the identification of an important virulence determinant in B. pseudomallei.
PMCID: PMC97852  PMID: 11119486
22.  Pseudomonas aeruginosa Exoenzyme S Induces Transcriptional Expression of Proinflammatory Cytokines and Chemokines 
Infection and Immunity  2000;68(8):4811-4814.
Pseudomonas aeruginosa infection of cystic fibrosis patients causes lung damage that is substantially orchestrated by cytokines. In this study, multi-gene probe analysis was used to characterize the ability of the P. aeruginosa mitogen, exoenzyme S, to induce proinflammatory and immunoregulatory cytokines and chemokines. Exoenzyme S strongly induced transcription of proinflammatory cytokines and chemokines (tumor necrosis factor alpha, interleukin-1α [IL-1α], IL-1β, IL-6, IL-8, MIP-1α, MIP-1β, MCP-1, RANTES, and I-309), modest transcription of immunoregulatory cytokines (IL-10 and IL-12p40), and weak transcription of Th1 cytokines (IL-2 and gamma interferon). The response occurred early and subsided without evolving over time. These data suggest that cells responding to exoenzyme S would rapidly express proinflammatory cytokines and chemokines that may contribute to pulmonary inflammation in cystic fibrosis.
PMCID: PMC98444  PMID: 10899895
23.  Species versus Biotype Status 
Journal of Clinical Microbiology  1999;37(11):3786-3787.
PMCID: PMC85768  PMID: 10610379
24.  In Vivo-Induced Genes in Pseudomonas aeruginosa 
Infection and Immunity  2000;68(4):2359-2362.
In vivo expression technology was used for testing Pseudomonas aeruginosa in the rat lung model of chronic infection and in a mouse model of systemic infection. Three of the eight ivi proteins found showed sequence identity to known virulence factors involved in iron acquisition via an open reading frame (called pvdI) implicated in pyoverdine biosynthesis, membrane biogenesis (FtsY), and adhesion (Hag2).
PMCID: PMC97428  PMID: 10722644
25.  Isolation of Polymyxin B-Susceptible Mutants of Burkholderia pseudomallei and Molecular Characterization of Genetic Loci Involved in Polymyxin B Resistance 
Antimicrobial Agents and Chemotherapy  1999;43(11):2648-2656.
Burkholderia pseudomallei is a gram-negative bacterium that causes the disease known as melioidosis. This pathogen is endemic to Southeast Asia and northern Australia and is particularly problematic in northeastern Thailand. It has been previously reported that B. pseudomallei is resistant to the killing action of cationic antimicrobial peptides, including human neutrophil peptide, protamine sulfate, poly-l-lysine, magainins, and polymyxins. Recently, we have also found that the virulent clinical isolate B. pseudomallei 1026b is capable of replicating in media containing polymyxin B at concentrations of >100 mg/ml. In order to identify genetic loci that are associated with this particular resistance phenotype, we employed a Tn5-OT182 mutagenesis system in coordination with a replica plating screen to isolate polymyxin B-susceptible mutants. Of the 17,000 Tn5-OT182 mutants screened via this approach, five polymyxin B-susceptible mutants were obtained. Three of these mutants harbored Tn5-OT182 insertions within a genetic locus demonstrating strong homology to the lytB gene present in other gram-negative bacteria. Of the remaining two mutants, one contained a transposon insertion in a locus involved in lipopolysaccharide core biosynthesis (waaF), while the other contained an insertion in an open reading frame homologous to UDP-glucose dehydrogenase genes. Isogenic mutants were also constructed via allelic exchange and used in complementation analysis studies to further characterize the relative importance of each of the various genetic loci with respect to the polymyxin B resistance phenotype exhibited by B. pseudomallei 1026b.
PMCID: PMC89538  PMID: 10543742

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