Optimization and prospective evaluation of sensitive real-time PCR assays with an internal control for the diagnosis of melioidosis in Thailand

IMPORTANCE

Melioidosis is a serious infectious disease caused by Burkholderia pseudomallei, an environmental Gram-negative bacterium. Early detection of B. pseudomallei infection is crucial for successful antibiotic treatment and reducing mortality rates associated with melioidosis. Bacteria culture is currently used to identify B. pseudomallei in clinical samples, but the method is slow. Therefore, there is a need for more accurate and sensitive molecular-based diagnostic methods that can detect B. pseudomallei in all sample types, including samples from blood. We developed an optimal DNA extraction method for B. pseudomallei from plasma samples and used an internal control for real-time PCR. We evaluated six PCR target genes and identified the most effective target for the early detection of B. pseudomallei infection in patients. To prevent delays in the treatment of melioidosis that can lead to fatal outcomes, we recommend implementing this new approach for routine early detection of B. pseudomallei in clinical settings.

Genetic diversity, determinants, and dissemination of Burkholderia pseudomallei lineages implicated in melioidosis in northeast Thailand

Melioidosis is an often-fatal neglected tropical disease caused by an environmental bacterium Burkholderia pseudomallei. However, our understanding of the disease-causing bacterial lineages, their dissemination, and adaptive mechanisms remains limited. To address this, we conducted a comprehensive genomic analysis of 1,391 B. pseudomallei isolates collected from nine hospitals in northeast Thailand between 2015 and 2018, and contemporaneous isolates from neighbouring countries, representing the most densely sampled collection to date. Our study identified three dominant lineages with unique gene sets enhancing bacterial fitness, indicating lineage-specific adaptation strategies. Crucially, recombination was found to drive lineage-specific gene flow. Transcriptome analyses of representative clinical isolates from each dominant lineage revealed heightened expression of lineage-specific genes in environmental versus infection conditions, notably under nutrient depletion, highlighting environmental persistence as a key factor in the success of dominant lineages. The study also revealed the role of environmental factors - slope of terrain, altitude, direction of rivers, and the northeast monsoons - in shaping B. pseudomallei geographical dispersal. Collectively, our findings highlight persistence in the environment as a pivotal element facilitating B. pseudomallei spread, and as a prelude to exposure and infection, thereby providing useful insights for informing melioidosis prevention and control strategies.

Type VI Secretion System Accessory Protein TagAB-5 Promotes Burkholderia pseudomallei Pathogenicity in Human Microglia

Central nervous system (CNS) melioidosis caused by Burkholderia pseudomallei is being increasingly reported. Because of the high mortality associated with CNS melioidosis, understanding the underlying mechanism of B. pseudomallei pathogenesis in the CNS needs to be intensively investigated to develop better therapeutic strategies against this deadly disease. The type VI secretion system (T6SS) is a multiprotein machine that uses a spring-like mechanism to inject effectors into target cells to benefit the infection process. In this study, the role of the T6SS accessory protein TagAB-5 in B. pseudomallei pathogenicity was examined using the human microglial cell line HCM3, a unique resident immune cell of the CNS acting as a primary mediator of inflammation. We constructed B. pseudomallei tagAB-5 mutant and complementary strains by the markerless allele replacement method. The effects of tagAB-5 deletion on the pathogenicity of B. pseudomallei were studied by bacterial infection assays of HCM3 cells. Compared with the wild type, the tagAB-5 mutant exhibited defective pathogenic abilities in intracellular replication, multinucleated giant cell formation, and induction of cell damage. Additionally, infection by the tagAB-5 mutant elicited a decreased production of interleukin 8 (IL-8) in HCM3, suggesting that efficient pathogenicity of B. pseudomallei is required for IL-8 production in microglia. However, no significant differences in virulence in the Galleria mellonella model were observed between the tagAB-5 mutant and the wild type. Taken together, this study indicated that microglia might be an important intracellular niche for B. pseudomallei, particularly in CNS infection, and TagAB-5 confers B. pseudomallei pathogenicity in these cells.

Antibiotic susceptibility of clinical Burkholderia pseudomallei isolates in northeast Thailand during 2015-2018 and the genomic characterization of β-lactam-resistant isolates

Melioidosis is an often fatal infection in tropical regions caused by an environmental bacterium, Burkholderia pseudomallei Current recommended melioidosis treatment requires intravenous β-lactam antibiotics such as ceftazidime (CAZ), meropenem (MEM) or amoxicillin-clavulanic acid (AMC) and oral trimethoprim-sulfamethoxazole. Emerging antibiotic resistance could lead to therapy failure and high mortality. We performed a prospective multicentre study in northeast Thailand during 2015-2018 to evaluate antibiotic susceptibility and characterize β-lactam resistance in clinical B. pseudomallei isolates. Collection of 1,317 B. pseudomallei isolates from patients with primary and relapse infections were evaluated for susceptibility to CAZ, imipenem (IPM), MEM and AMC. β-lactam resistant isolates were confirmed by broth microdilution method and characterized by whole genome sequence analysis, penA expression and β-lactamase activity. The resistant phenotype was verified via penA mutagenesis. All primary isolates were IPM-susceptible but we observed two CAZ-resistant and one CAZ-intermediate resistant isolates, two MEM-less susceptible isolates, one AMC-resistant and two AMC-intermediate resistant isolates. One of 13 relapse isolates was resistant to both CAZ and AMC. Two isolates were MEM-less susceptible. Strains DR10212A (primary) and DR50054E (relapse) were multi-drug resistant. Genomic and mutagenesis analyses supplemented with gene expression and β-lactamase analyses demonstrated that CAZ-resistant phenotype was caused by PenA variants: P167S (N=2) and penA amplification (N=1). Despite the high mortality rate in melioidosis, our study revealed that B. pseudomallei isolates had a low frequency of β-lactam resistance caused by penA alterations. Clinical data suggest that resistant variants may emerge in patients during antibiotic therapy and be associated with poor response to treatment.

Entomopathogenic nematodes isolated from agricultural areas of Thailand and their activity against the larvae of Aedes aegypti, Aedes albopictus and Culex quinquefasciatus (Diptera: Culicidae)

Entomopathogenic nematodes (EPNs) of the genera Steinernema and Heterorhabditis have been considered to be effective biological control agents for several insects. In this study, we isolated and identified EPNs from soil samples in agricultural areas of northern Thailand and evaluated their efficacy for controlling larvae of three mosquito vector species, Aedes aegypti, Ae. albopictus and Culex quinquefasciatus. A total of 51 of 1,000 soil samples were positive (5.1% prevalence) for EPNs, which were identified through sequencing of the rDNA and ITS to 37 Steinernema isolates (3.7%) and 14 Heterorhabditis isolates (1.4%). For the bioassay, the larvae of mosquitoes were exposed to Steinernema surkhetense (eALN6.3_TH), Steinernema lamjungense (eALN11.5_TH), Heterorhabditis indica (eACM14.2_TH) and Heterorhabditis bacteriophora (eALN18.2_TH). Heterorhabditis bacteriophora showed the highest efficacy against Ae. aegypti and Cx. quinquefasciatus. At 96 h after exposure, the mortality rates were 60.0 and 91.7%, respectively. The EPNs were observed in the dead mosquito larvae, which were mostly found in the thorax followed by the head and abdomen. Some EPNs were dead with melanization, and some were able to survive in the cavity of mosquito larvae. Our results show the low prevalence of EPN in agricultural areas of Thailand. Moreover, H. bacteriophora may be considered an alternative biocontrol agent for managing and controlling these vector mosquitoes.

Abundance of ACVR1B transcript is elevated during septic conditions: Perspectives obtained from a hands-on reductionist investigation

Sepsis is a complex heterogeneous condition, and the current lack of effective risk and outcome predictors hinders the improvement of its management. Using a reductionist approach leveraging publicly available transcriptomic data, we describe a knowledge gap for the role of ACVR1B (activin A receptor type 1B) in sepsis. ACVR1B, a member of the transforming growth factor-beta (TGF-beta) superfamily, was selected based on the following: 1) induction upon in vitro exposure of neutrophils from healthy subjects with the serum of septic patients (GSE49755), and 2) absence or minimal overlap between ACVR1B, sepsis, inflammation, or neutrophil in published literature. Moreover, ACVR1B expression is upregulated in septic melioidosis, a widespread cause of fatal sepsis in the tropics. Key biological concepts extracted from a series of PubMed queries established indirect links between ACVR1B and "cancer", "TGF-beta superfamily", "cell proliferation", "inhibitors of activin", and "apoptosis". We confirmed our observations by measuring ACVR1B transcript abundance in buffy coat samples obtained from healthy individuals (n=3) exposed to septic plasma (n = 26 melioidosis sepsis cases)ex vivo. Based on our re-investigation of publicly available transcriptomic data and newly generated ex vivo data, we provide perspective on the role of ACVR1B during sepsis. Additional experiments for addressing this knowledge gap are discussed.

Entomopathogenic Nematodes and Their Symbiotic Bacteria from the National Parks of Thailand and Larvicidal Property of Symbiotic Bacteria against Aedes aegypti and Culex quinquefasciatus

Simple Summary

Entomopathogenic nematodes (EPNs) are insect-parasitizing nematodes of the genera Heterorhabditis and Steinernema that are symbiotically associated with the symbiotic bacteria Photorhabdus and Xenorhabdus, respectively. Heterorhabditis indica, H. baujardi, Heterorhabditis SGmg3, Steinernema guangdongense, S. surkhetense, S. minutum, and S. longicaudum were isolated from soil samples in the national parks of Thailand. For symbiotic bacterial isolates, P. luminescens subsp. akhurstii, P. luminescens subsp. hainanensis, P. luminescens subsp. australis, Xenorhabdus stockiae, X. indica, X. griffiniae, X. japonica, and X. hominickii were isolated from those EPNs. In mosquito larvicidal bioassays, Photorhabdus isolates were effective against both Aedes aegypti and Culex quinquefasciatus. In conclusion, a wide diversity of entomopathogenic nematodes and symbiotic bacteria was found in the national parks of Thailand. Moreover, isolated Photorhabdus bacteria were shown to have potential as biocontrol agents to control culicine mosquitoes.

Abstract

Entomopathogenic nematodes (EPNs) are insect parasitic nematodes of the genera Het-erorhabditis and Steinernema. These nematodes are symbiotically associated with the bacteria, Photorhabdus and Xenorhabdus, respectively. National parks in Thailand are a potentially rich resource for recovering native EPNs and their symbiotic bacteria. The objectives of this study are to isolate and identify EPNs and their bacterial flora from soil samples in four national parks in Thailand and to evaluate their efficacy for controlling mosquito larvae. Using a baiting method with a Galleria mellonella moth larvae and a White trap technique, 80 out of 840 soil samples (9.5%) from 168 field sites were positive for EPNs. Sequencing of an internal transcribed spacer resulted in the molecular identification of Heterorhabditis nematode isolates as H. indica, H. baujardi and Heterorhabditis SGmg3, while using 28S rDNA sequencing, Steinernema nematode species were identified as S. guang-dongense, S. surkhetense, S. minutum, S. longicaudum and one closely related to S. yirgalemense. For the symbiotic bacterial isolates, based on recA sequencing, the Photorhabdus spp. were identified as P. luminescens subsp. akhurstii, P. luminescens subsp. hainanensis and P. luminescens subsp. australis. Xenorhabdus isolates were identified as X. stockiae, X. indica, X. griffiniae, X. japonica and X. hominickii. Results of bioassays demonstrate that Photorhabdus isolates were effective on both Aedes aegypti and Culex quinquefasciatus. Therefore, we conclude that soil from Thailand’s national parks contain a high diversity of entomopathogenic nematodes and their symbiotic bacteria. Photorhabdus bacteria are larvicidal against culicine mosquitoes and may serve as effective biocontrol agents.

Genome analysis of secondary metabolite‑biosynthetic gene clusters of Photorhabdus akhurstii subsp. akhurstii and its antibacterial activity against antibiotic-resistant bacteria

Xenorhabdus and Photorhabdus can produce a variety of secondary metabolites with broad spectrum bioactivity against microorganisms. We investigated the antibacterial activity of Xenorhabdus and Photorhabdus against 15 antibiotic-resistant bacteria strains. Photorhabdus extracts had strong inhibitory the growth of Methicillin-resistant Staphylococcus aureus (MRSA) by disk diffusion. The P. akhurstii s subsp. akhurstii (bNN168.5_TH) extract showed lower minimum inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC). The interaction between either P. akhurstii subsp. akhurstii (bNN141.3_TH) or P. akhurstii subsp. akhurstii (bNN168.5_TH) or P. hainanensis (bNN163.3_TH) extract in combination with oxacillin determined by checkerboard assay exhibited partially synergistic interaction with fractional inhibitory concentration index (FICI) of 0.53. Time-killing assay for P. akhurstii subsp. akhurstii (bNN168.5_TH) extract against S. aureus strain PB36 significantly decreased cell viability from 10⁵ CFU/ml to 10³ CFU/ml within 30 min (P < 0.001, t-test). Transmission electron microscopic investigation elucidated that the bNN168.5_TH extract caused treated S. aureus strain PB36 (MRSA) cell membrane damage. The biosynthetic gene clusters of the bNN168.5_TH contained non-ribosomal peptide synthetase cluster (NRPS), hybrid NRPS-type l polyketide synthase (PKS) and siderophore, which identified potentially interesting bioactive products: xenematide, luminmide, xenortide A-D, luminmycin A, putrebactin/avaroferrin and rhizomide A-C. This study demonstrates that bNN168.5_TH showed antibacterial activity by disrupting bacterial cytoplasmic membrane and the draft genome provided insights into the classes of bioactive products. This also provides a potential approach in developing a novel antibacterial agent.

Molecular identification and phylogeny of Steinernema and Heterorhabditis nematodes and their efficacy in controlling the larvae of Aedes aegypti, a major vector of the dengue virus

Aedes aegypti is the mosquito vector of several arboviruses, especially the dengue virus. Aedes aegypti strain resistant to chemical insecticides have been reported worldwide. To tackle this, an entomopathogenic nematode (EPN) may be an alternative bio-control agent. To this end, this study aims to isolate, identify, and analyze the phylogeny of EPNs in Thailand and evaluate their efficacy for controlling the Ae. aegypti larvae. From 12 provinces in Thailand, soil samples were randomly collected, with 118 out of 1,100 them being positive for EPNs (10.73% prevalence) in genera Steinernema (4.46%) and Heterorhabditis (6.27%). Then, molecular discrimination of these two genus was performed based on the sequencing and phylogenetic analysis of the 28S rDNA and internal transcribed spacer regions. The most abundant species of EPN were Heterorhabditis indica, with minor species of Heterorhabditis sp. SGmg3, H. baujardi, S. surkhetense, S. kushidai, S. siamkayai, Steinernema sp. YNd80, Steinernema sp. YNc215, S. guangdongense, and S. huense. The larvicidal activity of five selected EPN isolates were tested against Ae. aegypti. Ten larvae of Ae. aegypti were incubated with different concentration (80, 160, 320, and 640 IJs/larva) of the infective juveniles of EPN in a 24-well and 6-well plates for 4 days. The mortality rates of the larvae were observed daily. Steinernema surkhetense (ePYO8.5_TH) showed the potential to kill mosquito larvae, with the highest mortality rate of 92 ± 9.37% and 89 ± 9.91% after it was treated with 640 IJs/larva in a 24-well plate and 1600 IJs/larva in a 6-well plate, respectively. There is an abundant distribution of EPNs across the country, and S. surkhetense ePYO8.5_TH may be used as a biocontrol agent against Ae. aegypti larvae.

Identification of entomopathogenic nematodes and their symbiotic bacteria in national parks of Thailand, and mosquitocidal activity of Xenorhabdus griffiniae against Aedes aegypti larvae

Entomopathogenic nematodes (EPN) Steinernema and Heterorhabditis with symbionts with Xenorhabdus and Photorhabdus bacteria, respectively, are reported as biocontrol agents for insect control. The objectives of this study were to identify EPN and their symbiotic bacteria in national parks of Phitsanulok Province, Thailand, and to test how Xenorhabdus isolates attack Aedes aegypti larvae. We collected 810 soil samples from four national parks. The juvenile stage of EPN was isolated from soil samples using a baiting technique with Galleria mellonella followed by a White trap. Partial regions of 28S rDNA and internal transcript spacer were sequenced to identify EPN, and recA sequencing was used to discriminate between Xenorhabdus and Photorhabdus. We found that 74 of the 810 soil samples (9.1%) were positive for the EPN. The EPN were molecularly identified as S. surkhetense, S. longicaudum, H. indica and Heterorhabditis sp. SGmg3, and their symbiotic bacteria were identified as X. stockiae, X. griffiniae, X. indica, X. vietnamensis, P. luminescens subsp. akhurstii, and P. temperata subsp. temperata. Xenorhabdus griffiniae showed potential larvicidal activity against A. aegypti (91% mortality at 72 and 96 h after exposure). This study demonstrates the diversity of EPN and symbiotic bacteria in national parks of Thailand and the potential to use X. griffiniae as a biocontrol agent to kill A. aegypti larvae.

Longitudinal analysis to characterize classes and subclasses of antibody responses to recombinant receptor-binding protein (RBD) of SARS-CoV-2 in COVID-19 patients in Thailand

Serological assays to detect antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) might contribute to confirming the suspected coronavirus disease 2019 (COVID-19) in patients not detected with molecular assays. Human antibodies that target the host angiotensin-converting enzyme 2-binding domain of the viral spike protein are a target for serodiagnosis and therapeutics. This study aimed to characterize the classes and subclasses of antibody responses to a recombinant receptor-binding protein (RBD) of SARS-CoV-2 in COVID-19 patients and investigated the reactivity of these antibodies in patients with other tropical infections and healthy individuals in Thailand. ELISAs for IgM, IgA, IgG and IgG subclasses based on RBD antigen were developed and tested with time series of 27 serum samples from 15 patients with COVID-19 and 60 samples from pre-COVID-19 outbreaks including acute dengue fever, murine typhus, influenza, leptospirosis and healthy individuals. Both RBD-specific IgA and IgG were detected in only 21% of the COVID-19 patients in the acute phase. The median IgA and IgG levels were significantly higher in the convalescent serum sample compared to the acute serum sample (P < 0.05). We observed the highest correlation between levels of IgG and IgA (rho = 0. 92). IgG1 and IgG3 were the major IgG subclasses detected in SARS-CoV-2 infection. Only acute IgG3 level was negatively associated with viral detection based on RT-PCR of ORF1ab gene (rho = -0.57). The median IgA and IgG levels in convalescence sera of COVID-19 patients were significantly higher than healthy individuals and convalescent sera of other febrile infectious patients. The analyses of antibody classes and subclasses provide insights into human immune responses against SARS-CoV-2 during natural infection and interpretation of antibody assays.

Genomic loss in environmental and isogenic morphotype isolates of Burkholderia pseudomallei is associated with intracellular survival and plaque-forming efficiency

BACKGROUND

Burkholderia pseudomallei is an environmental bacterium that causes melioidosis. A facultative intracellular pathogen, B. pseudomallei can induce multinucleated giant cells (MNGCs) leading to plaque formation in vitro. B. pseudomallei can switch colony morphotypes under stress conditions. In addition, different isolates have been reported to have varying virulence in vivo, but genomic evolution and the relationship with plaque formation is poorly understood.

METHODOLOGY/PRINCIPLE FINDINGS

To gain insights into genetic underpinnings of virulence of B. pseudomallei, we screened plaque formation of 52 clinical isolates and 11 environmental isolates as well as 4 isogenic morphotype isolates of B. pseudomallei strains K96243 (types II and III) and 153 (types II and III) from Thailand in A549 and HeLa cells. All isolates except one environmental strain (A4) and K96243 morphotype II were able to induce plaque formation in both cell lines. Intracellular growth assay and confocal microscopy analyses demonstrated that the two plaque-forming-defective isolates were also impaired in intracellular replication, actin polymerization and MNGC formation in infected cells. Whole genome sequencing analysis and PCR revealed that both isolates had a large genomic loss on the same region in chromosome 2, which included Bim cluster, T3SS-3 and T6SS-5 genes.

CONCLUSIONS/SIGNIFICANCE

Our plaque screening and genomic studies revealed evidence of impairment in plaque formation in environmental isolates of B. pseudomallei that is associated with large genomic loss of genes important for intracellular multiplication and MNGC formation. These findings suggest that the genomic and phenotypic differences of environmental isolates may be associated with clinical infection.

Lactoferrin is a dynamic protein in human melioidosis and is a TLR4-dependent driver of TNF-α release in Burkholderia thailandensis infection in vitro

Melioidosis is an often-severe tropical infection caused by Burkholderia pseudomallei (Bp) with high associated morbidity and mortality. Burkholderia thailandensis (Bt) is a closely related surrogate that does not require BSL-3 conditions for study. Lactoferrin is an iron-binding glycoprotein that can modulate the innate inflammatory response. Here we investigated the impact of lactoferrin on the host immune response in melioidosis. Lactoferrin concentrations were measured in plasma from patients with melioidosis and following ex vivo stimulation of blood from healthy individuals. Bt growth was quantified in liquid media in the presence of purified and recombinant human lactoferrin. Differentiated THP-1 cells and human blood monocytes were infected with Bt in the presence of purified and recombinant human lactoferrin, and bacterial intracellular replication and cytokine responses (tumor necrosis factor-α (TNF-α), interleukin-1β and interferon-γ) were measured. In a cohort of 49 melioidosis patients, non-survivors to 28 days had significantly higher plasma lactoferrin concentrations compared to survivors (median (interquartile range (IQR)): 326 ng/ml (230–748) vs 144 ng/ml (99–277), p<0.001). In blood stimulated with heat-killed Bp, plasma lactoferrin concentration significantly increased compared to unstimulated blood (median (IQR): 424 ng/ml (349–479) vs 130 ng/ml (91–214), respectively; p<0.001). Neither purified nor recombinant human lactoferrin impaired growth of Bt in media. Lactoferrin significantly increased TNF-α production by differentiated THP-1 cells and blood monocytes after Bt infection. This phenotype was largely abrogated when Toll-like receptor 4 (TLR4) was blocked with a monoclonal antibody. In sum, lactoferrin is produced by blood cells after exposure to Bp and lactoferrin concentrations are higher in 28-day survivors in melioidosis. Lactoferrin induces proinflammatory cytokine production after Bt infection that may be TLR4 dependent.

Melioidosis is a severe tropical infection caused by the bacterium Burkholderia pseudomallei. Despite antibiotics, mortality in some regions remains very high, necessitating the need for alternative treatment strategies, including targeting the immune system. Lactoferrin is an iron-binding protein with a variety of different functions. In this study, we wanted to test whether lactoferrin alters how the immune system responds during melioidosis. To achieve this, we first tested the blood of melioidosis patients and found that patients who later died had higher lactoferrin levels compared to those who survived. We also stimulated blood obtained from healthy individuals with B. pseudomallei and found that lactoferrin levels increase. We next analyzed whether lactoferrin impaired how the bacteria grows and found that the growth of Burkholderia thailandensis, a closely related bacterium, was not affected by the addition of lactoferrin to the media. When human immune cells, called monocytes, were infected with B. thailandensis, we found that levels of a specific inflammatory protein, TNF-α, increased after adding lactoferrin and that this effect was related to a specific immune recognition pathway called Toll-like receptor 4. These findings provide new data about the role of lactoferrin in modulating the immune response in melioidosis.

Predictive Validity of the qSOFA Score for Sepsis in Adults with Community-Onset Staphylococcal Infection in Thailand

The quick sequential organ failure assessment (qSOFA) score has had limited validation in lower resource settings and was developed using data from high-income countries. We sought to evaluate the predictive validity of the qSOFA score for sepsis within a low- and middle-income country (LMIC) population with culture-proven staphylococcal infection. This was a secondary analysis of a prospective multicenter cohort in Thailand with culture-positive infection due to Staphylococcus aureus or S. argenteus within 24 h of admission and positive (≥2/4) systemic inflammatory response syndrome (SIRS) criteria. Primary exposure was maximum qSOFA score within 48 h of culture collection and primary outcome was mortality at 28 days. Baseline risk of mortality was determined using a multivariable logistic regression model with age, gender, and co-morbidities significantly associated with the outcome. Predictive validity was assessed by discrimination of mortality using area under the receiver operating characteristic (AUROC) curve compared to a model using baseline risk factors alone. Of 253 patients (mean age 54 years (SD 16)) included in the analysis, 23 (9.1%) died by 28 days after enrollment. Of those who died, 0 (0%) had a qSOFA score of 0, 8 (35%) had a score of 1, and 15 (65%) had a score ≥2. The AUROC of qSOFA plus baseline risk was significantly greater than for the baseline risk model alone (AUROC_qSOFA = 0.80 (95% CI, 0.70–0.89), AUROC_baseline = 0.62 (95% CI, 0.49–0.75); p < 0.001). Among adults admitted to four Thai hospitals with community-onset coagulase-positive staphylococcal infection and SIRS, the qSOFA score had good predictive validity for sepsis.

Identification of entomopathogenic nematodes and symbiotic bacteria from Nam Nao National Park in Thailand and larvicidal activity of symbiotic bacteria against Aedes aegypti and Aedes albopictus

Entomopathogenic nematodes (EPNs) that are symbiotically associated with Xenorhabdus and Photorhabdus bacteria can kill target insects via direct infection and toxin action. There are limited reports identifying such organisms in the National Park of Thailand. Therefore, the objectives of this study were to identify EPNs and symbiotic bacteria from Nam Nao National Park, Phetchabun Province, Thailand and to evaluate the larvicidal activity of bacteria against Aedes aegypti and Ae. albopictus. A total of 12 EPN isolates belonging to Steinernema and Heterorhabditis were obtained form 940 soil samples between February 2014 and July 2016. EPNs were molecularly identified as S. websteri (10 isolates) and H. baujardi (2 isolates). Symbiotic bacteria were isolated from EPNs and molecularly identified as P. luminescens subsp. akhurstii (13 isolates), X. stockiae (11 isolates), X. vietnamensis (2 isolates) and X. japonica (1 isolate). For the bioassay, bacterial suspensions were evaluated for toxicity against third to early fourth instar larvae of Aedes spp. The larvae of both Aedes species were orally susceptible to symbiotic bacteria. The highest larval mortality of Ae. aegypti was 99% after exposure to X. stockiae (bNN112.3_TH) at 96 h, and the highest mortality of Ae. albopictus was 98% after exposure to P. luminescens subsp. akhurstii (bNN121.4_TH) at 96 h. In contrast to the control groups (Escherichia coli and distilled water), the mortality rate of both mosquito larvae ranged between 0 and 7% at 72 h. Here, we report the first observation of X. vietnamensis in Thailand. Additionally, we report the first observation of P. luminescens subsp. akhurstii associated with H. baujardi in Thailand. X. stockiae has potential to be a biocontrol agent for mosquitoes. This investigation provides a survey of the basic diversity of EPNs and symbiotic bacteria in the National Park of Thailand, and it is a bacterial resource for further studies of bioactive compounds.

Isolation and identification of Xenorhabdus and Photorhabdus bacteria associated with entomopathogenic nematodes and their larvicidal activity against Aedes aegypti

BACKGROUND

Aedes aegypti is a potential vector of West Nile, Japanese encephalitis, chikungunya, dengue and Zika viruses. Alternative control measurements of the vector are needed to overcome the problems of environmental contamination and chemical resistance. Xenorhabdus and Photorhabdus are symbionts in the intestine of entomopathogenic nematodes (EPNs) Steinernema spp. and Heterorhabditis spp. These bacteria are able to produce a broad range of bioactive compounds including antimicrobial, antiparasitic, cytotoxic and insecticidal compounds. The objectives of this study were to identify Xenorhabdus and Photorhabdus isolated from EPNs in upper northern Thailand and to study their larvicidal activity against Ae. aegypti larvae.

RESULTS

A total of 60 isolates of symbiotic bacteria isolated from EPNs consisted of Xenorhabdus (32 isolates) and Photorhabdus (28 isolates). Based on recA gene sequencing, BLASTN and phylogenetic analysis, 27 isolates of Xenorhabdus were identical and closely related to X. stockiae, 4 isolates were identical to X. miraniensis, and one isolate was identical to X. ehlersii. Twenty-seven isolates of Photorhabdus were closely related to P. luminescens akhurstii and P. luminescens hainanensis, and only one isolate was identical and closely related to P. luminescens laumondii. Xenorhabdus and Photorhabdus were lethal to Ae aegypti larvae. Xenorhabdus ehlersii bMH9.2_TH showed 100% efficiency for killing larvae of both fed and unfed conditions, the highest for control of Ae. aegypti larvae and X. stockiae (bLPA18.4_TH) was likely to be effective in killing Ae. aegypti larvae given the mortality rates above 60% at 72 h and 96 h.

CONCLUSIONS

The common species in the study area are X. stockiae, P. luminescens akhurstii, and P. luminescens hainanensis. Three symbiotic associations identified included P. luminescens akhurstii-H. gerrardi, P. luminescens hainanensis-H. gerrardi and X. ehlersii-S. Scarabaei which are new observations of importance to our knowledge of the biodiversity of, and relationships between, EPNs and their symbiotic bacteria. Based on the biological assay, X. ehlersii bMH9.2_TH begins to kill Ae. aegypti larvae within 48 h and has the most potential as a pathogen to the larvae. These data indicate that X. ehlersii may be an alternative biological control agent for Ae. aegypti and other mosquitoes.

Patient Characteristics, Management, and Predictors of Outcome from Severe Community-Onset Staphylococcal Sepsis in Northeast Thailand: A Prospective Multicenter Study

Staphylococcus aureus infection is a persistent threat in resource-restricted settings in southeast Asia but informative data about this disease remain limited. We analyzed characteristics, management, and predictors of outcome in severely septic patients with community-onset S. aureus infection in northeast Thailand. We performed a prospective, multicenter observational cohort study of community-onset S. aureus sepsis in four referral hospitals recruiting patients at least 14 years of age admitted between March 2010 and December 2013. One hundred and nineteen patients with severe staphylococcal sepsis were enrolled. Diabetes was the most common underlying condition. Methicillin-resistant infection was rare. Twenty-eight-day mortality was 20%. Ninety-two percent of patients received appropriate antibiotic therapy and 82% were administered intravenous fluids on the first hospital day, although only 14% were managed in an intensive care unit (ICU). On univariable analysis, clinical variables at enrollment significantly associated with death at 28 days were coagulopathy or respiratory failure. Plasma interleukin (IL)-8 concentration alone accurately predicted mortality (area under the receiver operating curve = 0.82, 95% confidence interval = 0.73–0.90). In multivariable analysis, addition of IL-8 concentration to a mortality prediction model containing clinical variables further improved the predictive ability of the model. We conclude that severe staphylococcal sepsis in northeast Thailand causes significant mortality. Diabetes is a common preexisting condition and most patients are managed outside the ICU even if they receive vasoactive/inotropic agents or mechanical ventilation. While clinical factors apparent on presentation including coagulopathy and respiratory failure predict death, plasma IL-8 improves this prediction.

TLR4 genetic variation is associated with inflammatory responses in Gram-positive sepsis

OBJECTIVES

To identify important pathogen recognition receptor (PRR) pathways regulating innate immune responses and outcome in Staphylococcus aureus sepsis.

METHODS

We analysed whether candidate PRR pathway genetic variants were associated with killed S. aureus-induced cytokine responses ex vivo and performed follow-up in vitro studies. We tested the association of our top-ranked variant with cytokine responses and clinical outcomes in a prospective multicentre cohort of patients with staphylococcal sepsis.

RESULTS

An intronic TLR4 polymorphism and expression quantitative trait locus, rs1927907, was highly associated with cytokine release induced by stimulation of blood from healthy Thai subjects with S. aureus ex vivo. S. aureus did not induce TLR4-dependent NF-κB activation in transfected HEK293 cells. In monocytes, tumor necrosis factor (TNF)-α release induced by S. aureus was not blunted by a TLR4/MD-2 neutralizing antibody, but in a monocyte cell line, TNF-α was reduced by knockdown of TLR4. In Thai patients with staphylococcal sepsis, rs1927907 was associated with higher interleukin (IL)-6 and IL-8 levels as well as with respiratory failure. S. aureus-induced responses in blood were most highly correlated with responses to Gram-negative stimulants whole blood.

CONCLUSIONS

A genetic variant in TLR4 is associated with cytokine responses to S. aureus ex vivo and plasma cytokine levels and respiratory failure in staphylococcal sepsis. While S. aureus does not express lipopolysaccharide or activate TLR4 directly, the innate immune response to S. aureus does appear to be modulated by TLR4 and shares significant commonality with that induced by Gram-negative pathogens and lipopolysaccharide.

Competition between Burkholderia pseudomallei and B. thailandensis

Background

Burkholderia pseudomallei is a Gram-negative bacterium that causes melioidosis, an often fatal disease in tropical countries. Burkholderia thailandensis is a non-virulent but closely related species. Both species are soil saprophytes but are almost never isolated together.

Results

We identified two mechanisms by which B. pseudomallei affects the growth of B. thailandensis. First, we found that six different isolates of B. pseudomallei inhibited the growth of B. thailandensis on LB agar plates. Second, our results indicated that 55% of isolated strains of B. pseudomallei produced a secreted compound that inhibited the motility but not the viability of B. thailandensis. Analysis showed that the active compound was a pH-sensitive and heat-labile compound, likely a protein, which may affect flagella processing or facilitate their degradation. Analysis of bacterial sequence types (STs) demonstrated an association between this and motility inhibition. The active compound was produced from B. pseudomallei during the stationary growth phase.

Conclusion

Taken together, our results indicate that B. pseudomallei inhibits both the growth and motility of its close relative B. thailandensis. The latter phenomenon appears to occur via a previously unreported mechanism involving flagellar processing or degradation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0395-7) contains supplementary material, which is available to authorized users.

Evaluation of a latex agglutination assay for the identification of Burkholderia pseudomallei and Burkholderia mallei

Cases of melioidosis and glanders are rare in the United States, but the etiologic agents of each disease (Burkholderia pseudomallei and Burkholderia mallei, respectively) are classified as Tier 1 select agents because of concerns about their potential use as bioterrorism agents. A rapid, highly sensitive, and portable assay for clinical laboratories and field use is required. Our laboratory has further evaluated a latex agglutination assay for its ability to identify B. pseudomallei and B. mallei isolates. This assay uses a monoclonal antibody that specifically recognizes the capsular polysaccharide produced by B. pseudomallei and B. mallei, but is absent in closely related Burkholderia species. A total of 110 B. pseudomallei and B. mallei were tested, and 36 closely related Burkholderia species. The latex agglutination assay was positive for 109 of 110 (99.1% sensitivity) B. pseudomallei and B. mallei isolates tested.

Common TLR1 genetic variation is not associated with death from melioidosis, a common cause of sepsis in rural Thailand

Melioidosis, infection caused by the Gram-negative bacterium Burkholderia pseudomallei, is a common cause of sepsis in northeast Thailand. In white North Americans, common functional genetic variation in TLR1 is associated with organ failure and death from sepsis. We hypothesized that TLR1 variants would be associated with outcomes in Thais with melioidosis. We collated the global frequencies of three TLR1 variants that are common in white North American populations: rs5743551 (-7202A/G), rs4833095 (742A/G), and rs5743618 (1804G/T). We noted a reversal of the minor allele from white North American subjects to Asian populations that was particularly pronounced for rs5743618. In the Utah residents of European ancestry, the frequency of the rs5743618 T allele was 17% whereas in Vietnamese subjects the frequency was >99%. We conducted a genetic association study in 427 patients with melioidosis to determine the association of TLR1 variation with organ failure or death. We genotyped rs5743551 and rs4833095. The variants were in high linkage disequilibrium but neither variant was associated with organ failure or in-hospital death. In 300 healthy Thai individuals we further tested the association of TLR1 variation with ex vivo blood responses to Pam3CSK4, a TLR1 agonist. Neither variant was robustly associated with blood cytokine responses induced by Pam3CSK4. We identified additional common variation in TLR1 by searching public databases and the published literature and screened three additional TLR1 variants for associations with Pam3CSK4-induced responses but found none. We conclude that the genetic architecture of TLR1 variation differs substantially in southeast Asians compared to other populations and common variation in TLR1 in Thais is not associated with outcome from melioidosis or with altered blood responses to Pam3CSK4. Our findings highlight the need for additional studies of TLR1 and other innate immune genetic modulators of the inflammatory host response and determinants of sepsis in southeast Asian populations.

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

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

Rapid detection of Burkholderia pseudomallei in blood cultures using a monoclonal antibody-based immunofluorescent assay

Melioidosis is a severe bacterial infection caused by Burkholderia pseudomallei. Rapid antimicrobial therapy is necessary to improve patient outcome, which is aided by direct detection of B. pseudomallei in clinical samples. A drawback for all antigen assays is that the number of B. pseudomallei in blood usually falls below the achievable level of detection. We performed a prospective cohort study of 461 patients with 541 blood cultures to evaluate the utility of a pre-incubation step prior to detection of B. pseudomallei using a monoclonal antibody-based immunofluorescent assay (Mab-IFA). The Mab-IFA was positive in 74 of 76 patients with melioidosis (sensitivity = 97.4%), and negative in 385 patients who did not have blood cultures containing B. pseudomallei (specificity = 100%). The Mab-IFA could be a valuable supplementary tool for rapid detection. We recommend the use of the Mab-IFA to test blood cultures that flag positive in regions where melioidosis is endemic.

Monoclonal antibody-based immunofluorescence microscopy for the rapid identification of Burkholderia pseudomallei in clinical specimens

The diagnosis of melioidosis depends on the culture of Burkholderia pseudomallei, which takes at least 48 hours. We used a polyclonal-FITC-based immunofluorescence microscopic assay (Pab-IFA) on clinical samples to provide a rapid presumptive diagnosis. This has limitations including photobleaching and batch-to-batch variability. This study evaluated an IFA based on a monoclonal antibody specific to B. pseudomallei (Mab-IFA) and Alexa Fluor 488. A diagnostic evaluation was performed on a prospective cohort of 951 consecutive patients with suspected melioidosis. A total of 1,407 samples were tested. Test accuracy was defined against culture as the gold standard, and was also compared against Pab-IFA. A total of 88 samples from 64 patients were culture positive for B. pseudomallei. The diagnostic sensitivity and specificity of the Mab-IFA was comparable to the Pab-IFA (48.4% versus 45.3% for sensitivity, and 99.8% versus 98.8% for specificity). We have incorporated the Mab-IFA into our routine practice.

Diversity of Xenorhabdus and Photorhabdus spp. and their symbiotic entomopathogenic nematodes from Thailand

Xenorhabdus and Photorhabdus spp. are bacterial symbionts of entomopathogenic nematodes (EPNs). In this study, we isolated and characterized Xenorhabdus and Photorhabdus spp. from across Thailand together with their associated nematode symbionts, and characterized their phylogenetic diversity. EPNs were isolated from soil samples using a Galleria-baiting technique. Bacteria from EPNs were cultured and genotyped based on recA sequence. The nematodes were identified based on sequences of 28S rDNA and internal transcribed spacer regions. A total of 795 soil samples were collected from 159 sites in 13 provinces across Thailand. A total of 126 EPNs isolated from samples taken from 10 provinces were positive for Xenorhabdus (n = 69) or Photorhabdus spp. (n = 57). Phylogenetic analysis separated the 69 Xenorhabdus isolates into 4 groups. Groups 1, 2 and 3 consisting of 52, 13 and 1 isolates related to X. stockiae, and group 4 consisting of 3 isolates related to X. miraniensis. The EPN host for isolates related to X. stockiae was S. websteri, and for X. miraniensis was S. khoisanae. The Photorhabdus species were identified as P. luminescens (n = 56) and P. asymbiotica (n = 1). Phylogenenic analysis divided P. luminescens into five groups. Groups 1 and 2 consisted of 45 and 8 isolates defined as subspecies hainanensis and akhurstii, respectively. One isolate was related to hainanensis and akhurstii, two isolates were related to laumondii, and one isolate was the pathogenic species P. asymbiotica subsp. australis. H. indica was the major EPN host for Photorhabdus. This study reveals the genetic diversity of Xenorhabdus and Photorhabdus spp. and describes new associations between EPNs and their bacterial symbionts in Thailand.

Effect of colony morphology variation of Burkholderia pseudomallei on intracellular survival and resistance to antimicrobial environments in human macrophages in vitro

Background

Primary diagnostic cultures from patients with melioidosis demonstrate variation in colony morphology of the causative organism, Burkholderia pseudomallei. Variable morphology is associated with changes in the expression of a range of putative virulence factors. This study investigated the effect of B. pseudomallei colony variation on survival in the human macrophage cell line U937 and under laboratory conditions simulating conditions within the macrophage milieu. Isogenic colony morphology types II and III were generated from 5 parental type I B. pseudomallei isolates using nutritional limitation. Survival of types II and III were compared with type I for all assays.

Results

Morphotype was associated with survival in the presence of H₂O₂ and antimicrobial peptide LL-37, but not with susceptibility to acid, acidified sodium nitrite, or resistance to lysozyme, lactoferrin, human neutrophil peptide-1 or human beta defensin-2. Incubation under anaerobic conditions was a strong driver for switching of type III to an alternative morphotype. Differences were noted in the survival and replication of the three types following uptake by human macrophages, but marked strain-to strain-variability was observed. Uptake of type III alone was associated with colony morphology switching.

Conclusions

Morphotype is associated with phenotypes that alter the ability of B. pseudomallei to survive in adverse environmental conditions.